[cfe] Add basic parsing support for extension show clause

Change-Id: I63847a06ff7370dd1b92fc6b4b706c684f15638e Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/209909 Reviewed-by: Konstantin Shcheglov <scheglov@google.com> Reviewed-by: Johnni Winther <johnniwinther@google.com> Commit-Queue: Dmitry Stefantsov <dmitryas@google.com>
2024-09-16 00:39:49 +00:00 · 2021-09-07 10:35:12 +00:00 · 2021-09-07 10:35:12 +00:00 · ab508bb2d1
parent ccfe2d65a9
commit ab508bb2d1
89 changed files with 2614 additions and 960 deletions
--- a/pkg/_fe_analyzer_shared/lib/src/parser/forwarding_listener.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/parser/forwarding_listener.dart
@ -651,9 +651,9 @@ class ForwardingListener implements Listener {

  @override
  void endExtensionDeclaration(Token extensionKeyword, Token? typeKeyword,
-      Token onKeyword, Token endToken) {
-    listener?.endExtensionDeclaration(
-        extensionKeyword, typeKeyword, onKeyword, endToken);
+      Token onKeyword, Token? showKeyword, Token? hideKeyword, Token endToken) {
+    listener?.endExtensionDeclaration(extensionKeyword, typeKeyword, onKeyword,
+        showKeyword, hideKeyword, endToken);
  }

  @override
@ -1159,6 +1159,13 @@ class ForwardingListener implements Listener {
    listener?.handleClassOrMixinImplements(implementsKeyword, interfacesCount);
  }

+  @override
+  void handleExtensionShowHide(Token? showKeyword, int showElementCount,
+      Token? hideKeyword, int hideElementCount) {
+    listener?.handleExtensionShowHide(
+        showKeyword, showElementCount, hideKeyword, hideElementCount);
+  }
+
  @override
  void handleClassWithClause(Token withKeyword) {
    listener?.handleClassWithClause(withKeyword);
@ -1280,6 +1287,11 @@ class ForwardingListener implements Listener {
    listener?.handleFunctionBodySkipped(token, isExpressionBody);
  }

+  @override
+  void handleShowHideIdentifier(Token? modifier, Token identifier) {
+    listener?.handleShowHideIdentifier(modifier, identifier);
+  }
+
  @override
  void handleIdentifier(Token token, IdentifierContext context) {
    listener?.handleIdentifier(token, context);
--- a/pkg/_fe_analyzer_shared/lib/src/parser/identifier_context.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/parser/identifier_context.dart
@ -226,6 +226,30 @@ abstract class IdentifierContext {
  static const ExpressionIdentifierContext expressionContinuation =
      const ExpressionIdentifierContext.continuation();

+  /// Identifier appears in a show or a hide clause of an extension type
+  /// declaration preceded by 'get'.
+  static const ExtensionShowHideElementIdentifierContext
+      extensionShowHideElementGetter =
+      const ExtensionShowHideElementIdentifierContext.getter();
+
+  /// Identifier appears in a show or a hide clause of an extension type
+  /// declaration, not preceded by 'get', 'set', or 'operator'.
+  static const ExtensionShowHideElementIdentifierContext
+      extensionShowHideElementMemberOrType =
+      const ExtensionShowHideElementIdentifierContext.memberOrType();
+
+  /// Identifier appears in a show or a hide clause of an extension type
+  /// declaration preceded by 'operator'.
+  static const ExtensionShowHideElementIdentifierContext
+      extensionShowHideElementOperator =
+      const ExtensionShowHideElementIdentifierContext.operator();
+
+  /// Identifier appears in a show or a hide clause of an extension type
+  /// declaration preceded by 'set'.
+  static const ExtensionShowHideElementIdentifierContext
+      extensionShowHideElementSetter =
+      const ExtensionShowHideElementIdentifierContext.setter();
+
  /// Identifier is a reference to a named argument of a function or method
  /// invocation (e.g. `foo` in `f(foo: 0);`.
  static const NamedArgumentReferenceIdentifierContext namedArgumentReference =
--- a/pkg/_fe_analyzer_shared/lib/src/parser/identifier_context_impl.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/parser/identifier_context_impl.dart
@ -371,6 +371,84 @@ class ExpressionIdentifierContext extends IdentifierContext {
  }
 }

+/// See [IdentifierContext.extensionShowHideElementGetter],
+/// [IdentifierContext.extensionShowHideElementMemberOrType],
+/// [IdentifierContext.extensionShowHideElementOperator],
+/// [IdentifierContext.extensionShowHideElementSetter].
+class ExtensionShowHideElementIdentifierContext extends IdentifierContext {
+  static const int _getterKind = 0;
+  static const int _memberOrTypeKind = 1;
+  static const int _operator = 2;
+  static const int _setterKind = 3;
+
+  final int _kind;
+
+  const ExtensionShowHideElementIdentifierContext.getter()
+      : _kind = _getterKind,
+        super('extensionShowHideElementGetter', inDeclaration: true);
+
+  const ExtensionShowHideElementIdentifierContext.memberOrType()
+      : _kind = _memberOrTypeKind,
+        super('extensionShowHideElementMemberOrType', inDeclaration: true);
+
+  const ExtensionShowHideElementIdentifierContext.operator()
+      : _kind = _operator,
+        super('extensionShowHideElementOperator', inDeclaration: true);
+
+  const ExtensionShowHideElementIdentifierContext.setter()
+      : _kind = _setterKind,
+        super('extensionShowHideElementSetter', inDeclaration: true);
+
+  @override
+  Token ensureIdentifier(Token token, Parser parser) {
+    Token identifier = token.next!;
+    if (identifier.isIdentifier ||
+        _kind == _operator && identifier.isOperator) {
+      return identifier;
+    }
+
+    // Recovery
+    const List<String> afterIdentifier = const [
+      '<',
+      '{',
+      'extends',
+      'with',
+      'implements',
+      'on',
+      '=',
+    ];
+    if (identifier.isEof ||
+        (looksLikeStartOfNextTopLevelDeclaration(identifier) &&
+            (identifier.next == null ||
+                !isOneOfOrEof(identifier.next!, afterIdentifier))) ||
+        (isOneOfOrEof(identifier, afterIdentifier) &&
+            (identifier.next == null ||
+                !isOneOfOrEof(identifier.next!, afterIdentifier)))) {
+      identifier = parser.insertSyntheticIdentifier(token, this,
+          message: codes.templateExpectedIdentifier.withArguments(identifier));
+    } else {
+      if (!identifier.isKeywordOrIdentifier) {
+        parser.reportRecoverableErrorWithToken(
+            identifier, codes.templateExpectedIdentifier);
+        // When in doubt, consume the token to ensure we make progress
+        // but insert a synthetic identifier to satisfy listeners.
+        identifier = parser.rewriter.insertSyntheticIdentifier(identifier);
+      } else {
+        // Use the keyword as the identifier.
+        parser.reportRecoverableErrorWithToken(
+            identifier, codes.templateExpectedIdentifierButGotKeyword);
+      }
+    }
+    return identifier;
+  }
+
+  @override
+  bool operator ==(dynamic other) {
+    return other is ExtensionShowHideElementIdentifierContext &&
+        _kind == other._kind;
+  }
+}
+
 /// See [IdentifierContext.fieldDeclaration].
 class FieldDeclarationIdentifierContext extends IdentifierContext {
  const FieldDeclarationIdentifierContext()
--- a/pkg/_fe_analyzer_shared/lib/src/parser/listener.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/parser/listener.dart
@ -144,6 +144,14 @@ class Listener implements UnescapeErrorListener {
    logEvent("ClassImplements");
  }

+  /// Handle a show clause in an extension declaration.
+  /// Substructures:
+  /// - shown types and instance members
+  void handleExtensionShowHide(Token? showKeyword, int showElementCount,
+      Token? hideKeyword, int hideElementCount) {
+    logEvent("ExtensionShowHide");
+  }
+
  /// Handle the header of a class declaration.  Substructures:
  /// - metadata
  /// - modifiers
@ -238,7 +246,7 @@ class Listener implements UnescapeErrorListener {
  /// - on type
  /// - body
  void endExtensionDeclaration(Token extensionKeyword, Token? typeKeyword,
-      Token onKeyword, Token endToken) {
+      Token onKeyword, Token? showKeyword, Token? hideKeyword, Token endToken) {
    logEvent('ExtensionDeclaration');
  }

@ -1445,6 +1453,13 @@ class Listener implements UnescapeErrorListener {
    logEvent("Identifier");
  }

+  /// Handle an identifier token in a show or hide clause.
+  ///
+  /// [context] indicates what kind of construct the identifier appears in.
+  void handleShowHideIdentifier(Token? modifier, Token identifier) {
+    logEvent("ShowHideIdentifier");
+  }
+
  void handleIndexedExpression(
      Token? question, Token openSquareBracket, Token closeSquareBracket) {
    logEvent("IndexedExpression");
--- a/pkg/_fe_analyzer_shared/lib/src/parser/parser_impl.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/parser/parser_impl.dart
@ -2391,6 +2391,63 @@ class Parser {
    }
    TypeInfo typeInfo = computeType(onKeyword, /* required = */ true);
    token = typeInfo.ensureTypeOrVoid(onKeyword, this);
+
+    int handleShowHideElements() {
+      int elementCount = 0;
+      do {
+        Token next = token.next!.next!;
+        if (optional('get', next)) {
+          token = IdentifierContext.extensionShowHideElementGetter
+              .ensureIdentifier(next, this);
+          listener.handleShowHideIdentifier(next, token);
+        } else if (optional('operator', next)) {
+          token = IdentifierContext.extensionShowHideElementOperator
+              .ensureIdentifier(next, this);
+          listener.handleShowHideIdentifier(next, token);
+        } else if (optional('set', next)) {
+          token = IdentifierContext.extensionShowHideElementSetter
+              .ensureIdentifier(next, this);
+          listener.handleShowHideIdentifier(next, token);
+        } else {
+          TypeInfo typeInfo = computeType(
+              token.next!,
+              /* required = */ true,
+              /* inDeclaration = */ true,
+              /* acceptKeywordForSimpleType = */ true);
+          final bool isUnambiguouslyType =
+              typeInfo.hasTypeArguments || typeInfo is PrefixedType;
+          if (isUnambiguouslyType) {
+            token = typeInfo.ensureTypeOrVoid(token.next!, this);
+          } else {
+            token = IdentifierContext.extensionShowHideElementMemberOrType
+                .ensureIdentifier(token.next!, this);
+            listener.handleShowHideIdentifier(null, token);
+          }
+        }
+        ++elementCount;
+      } while (optional(',', token.next!));
+      return elementCount;
+    }
+
+    Token? showKeyword = token.next!;
+    int showElementCount = 0;
+    if (optional('show', showKeyword)) {
+      showElementCount = handleShowHideElements();
+    } else {
+      showKeyword = null;
+    }
+
+    Token? hideKeyword = token.next!;
+    int hideElementCount = 0;
+    if (optional('hide', hideKeyword)) {
+      hideElementCount = handleShowHideElements();
+    } else {
+      hideKeyword = null;
+    }
+
+    listener.handleExtensionShowHide(
+        showKeyword, showElementCount, hideKeyword, hideElementCount);
+
    if (!optional('{', token.next!)) {
      // Recovery
      Token next = token.next!;
@ -2417,8 +2474,8 @@ class Parser {
    }
    token = parseClassOrMixinOrExtensionBody(
        token, DeclarationKind.Extension, name?.lexeme);
-    listener.endExtensionDeclaration(
-        extensionKeyword, typeKeyword, onKeyword, token);
+    listener.endExtensionDeclaration(extensionKeyword, typeKeyword, onKeyword,
+        showKeyword, hideKeyword, token);
    return token;
  }

--- a/pkg/_fe_analyzer_shared/lib/src/parser/stack_listener.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/parser/stack_listener.dart
@ -48,6 +48,7 @@ enum NullValue {
  FunctionBody,
  FunctionBodyAsyncToken,
  FunctionBodyStarToken,
+  HideClause,
  Identifier,
  IdentifierList,
  Initializers,
@ -55,8 +56,10 @@ enum NullValue {
  Metadata,
  Modifiers,
  Name,
+  OperatorList,
  ParameterDefaultValue,
  Prefix,
+  ShowClause,
  StringLiteral,
  SwitchScope,
  Token,
@ -345,6 +348,12 @@ abstract class StackListener extends Listener {
    debugEvent("ClassImplements");
  }

+  @override
+  void handleExtensionShowHide(Token? showKeyword, int showElementCount,
+      Token? hideKeyword, int hideElementCount) {
+    debugEvent("ExtensionShow");
+  }
+
  @override
  void handleNoTypeArguments(Token token) {
    debugEvent("NoTypeArguments");
--- a/pkg/_fe_analyzer_shared/lib/src/parser/type_info.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/parser/type_info.dart
@ -36,6 +36,9 @@ abstract class TypeInfo {
  /// void Function foo(int x);
  bool get isFunctionType;

+  /// Returns true if the type has type arguments.
+  bool get hasTypeArguments;
+
  /// Call this function when the token after [token] must be a type (not void).
  /// This function will call the appropriate event methods on the [Parser]'s
  /// listener to handle the type, inserting a synthetic type reference if
--- a/pkg/_fe_analyzer_shared/lib/src/parser/type_info_impl.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/parser/type_info_impl.dart
@ -98,6 +98,9 @@ class NoType implements TypeInfo {
  @override
  bool get couldBeExpression => false;

+  @override
+  bool get hasTypeArguments => false;
+
  @override
  bool get isNullable => false;

@ -142,6 +145,9 @@ class PrefixedType implements TypeInfo {
  @override
  bool get couldBeExpression => true;

+  @override
+  bool get hasTypeArguments => false;
+
  @override
  bool get isNullable => false;

@ -228,6 +234,9 @@ class SimpleTypeWith1Argument implements TypeInfo {
  @override
  bool get couldBeExpression => false;

+  @override
+  bool get hasTypeArguments => true;
+
  @override
  bool get isNullable => false;

@ -305,6 +314,9 @@ class SimpleType implements TypeInfo {
  @override
  bool get couldBeExpression => true;

+  @override
+  bool get hasTypeArguments => false;
+
  @override
  bool get isNullable => false;

@ -353,6 +365,9 @@ class VoidType implements TypeInfo {
  @override
  bool get couldBeExpression => false;

+  @override
+  bool get hasTypeArguments => false;
+
  @override
  bool get isNullable => false;

@ -492,6 +507,9 @@ class ComplexTypeInfo implements TypeInfo {
  bool get couldBeExpression =>
      typeArguments == noTypeParamOrArg && typeVariableStarters.isEmpty;

+  @override
+  bool get hasTypeArguments => typeArguments is! NoTypeParamOrArg;
+
  @override
  bool get isNullable => beforeQuestionMark != null;

@ -828,11 +846,8 @@ class SimpleTypeArgument1 extends TypeParamOrArgInfo {
    listener.beginTypeVariable(token);
    listener.handleTypeVariablesDefined(token, /* count = */ 1);
    listener.handleNoType(token);
-    listener.endTypeVariable(
-        endGroup,
-        /* index = */ 0,
-        /* extendsOrSuper = */ null,
-        /* variance = */ null);
+    listener.endTypeVariable(endGroup, /* index = */ 0,
+        /* extendsOrSuper = */ null, /* variance = */ null);
    listener.endTypeVariables(beginGroup, endGroup);
    return endGroup;
  }
--- a/pkg/analyzer/lib/dart/ast/ast.dart
+++ b/pkg/analyzer/lib/dart/ast/ast.dart
@ -469,6 +469,8 @@ abstract class AstVisitor<R> {

  R? visitGenericTypeAlias(GenericTypeAlias node);

+  R? visitHideClause(HideClause node);
+
  R? visitHideCombinator(HideCombinator node);

  R? visitIfElement(IfElement node);
@ -544,8 +546,12 @@ abstract class AstVisitor<R> {

  R? visitSetOrMapLiteral(SetOrMapLiteral node);

+  R? visitShowClause(ShowClause node);
+
  R? visitShowCombinator(ShowCombinator node);

+  R? visitShowHideElement(ShowHideElement node);
+
  R? visitSimpleFormalParameter(SimpleFormalParameter node);

  R? visitSimpleIdentifier(SimpleIdentifier node);
@ -1683,7 +1689,8 @@ abstract class ExtendsClause implements AstNode {
 ///
 ///    extension ::=
 ///        'extension' [SimpleIdentifier]? [TypeParameterList]?
-///        'on' [TypeAnnotation] '{' [ClassMember]* '}'
+///        'on' [TypeAnnotation] [ShowClause]? [HideClause]?
+///        '{' [ClassMember]* '}'
 ///
 /// Clients may not extend, implement or mix-in this class.
 abstract class ExtensionDeclaration implements CompilationUnitMember {
@ -1696,6 +1703,10 @@ abstract class ExtensionDeclaration implements CompilationUnitMember {
  /// Return the token representing the 'extension' keyword.
  Token get extensionKeyword;

+  /// Return the hide clause, or `null` if the extension does not have a hide
+  /// clause.
+  HideClause? get hideClause;
+
  /// Return the left curly bracket.
  Token get leftBracket;

@ -1712,6 +1723,10 @@ abstract class ExtensionDeclaration implements CompilationUnitMember {
  /// Return the right curly bracket.
  Token get rightBracket;

+  /// Return the show clause, or `null` if the extension does not have a show
+  /// clause.
+  ShowClause? get showClause;
+
  /// Return the token representing the 'type' keyword.
  Token? get typeKeyword;

@ -2437,6 +2452,20 @@ abstract class GenericTypeAlias implements TypeAlias {
  TypeParameterList? get typeParameters;
 }

+/// The "hide" clause in an extension declaration.
+///
+///    hideClause ::=
+///        'hide' [TypeName] (',' [TypeName])*
+///
+///  Clients may not extend, implement or mix-in this class.
+abstract class HideClause implements AstNode {
+  /// Return the list of the elements that are being shown.
+  NodeList<ShowHideClauseElement> get elements;
+
+  /// Return the token representing the 'hide' keyword.
+  Token get hideKeyword;
+}
+
 /// A combinator that restricts the names being imported to those that are not
 /// in a given list.
 ///
@ -3667,6 +3696,20 @@ abstract class SetOrMapLiteral implements TypedLiteral {
  Token get rightBracket;
 }

+/// The "show" clause in an extension declaration.
+///
+///    showClause ::=
+///        'show' [TypeName] (',' [TypeName])*
+///
+///  Clients may not extend, implement or mix-in this class.
+abstract class ShowClause implements AstNode {
+  /// Return the list of the elements that are being shown.
+  NodeList<ShowHideClauseElement> get elements;
+
+  /// Return the token representing the 'show' keyword.
+  Token get showKeyword;
+}
+
 /// A combinator that restricts the names being imported to those in a given list.
 ///
 ///    showCombinator ::=
@ -3679,6 +3722,29 @@ abstract class ShowCombinator implements Combinator {
  NodeList<SimpleIdentifier> get shownNames;
 }

+/// A node that can appear in the show or hide clauses.
+///
+/// Clients may not extend, implement or mix-in this class.
+abstract class ShowHideClauseElement implements AstNode {}
+
+/// A potentially non-type element of a show or a hide clause.
+///
+///    showHideElement ::=
+///        'get' [SimpleIdentifier] |
+///        'set' [SimpleIdentifier] |
+///        'operator' [SimpleIdentifier] |
+///        [SimpleIdentifier]
+///
+/// Clients may not extend, implement or mix-in this class.
+abstract class ShowHideElement implements AstNode, ShowHideClauseElement {
+  /// Return the 'get', 'set', or 'operator' modifier that appears before the
+  /// name, or `null` if there is no modifier.
+  Token? get modifier;
+
+  /// Return the name of the member the element refers to.
+  SimpleIdentifier get name;
+}
+
 /// A simple formal parameter.
 ///
 ///    simpleFormalParameter ::=
@ -4182,7 +4248,7 @@ abstract class TypeLiteral implements Expression {
 ///        [Identifier] typeArguments?
 ///
 /// Clients may not extend, implement or mix-in this class.
-abstract class TypeName implements NamedType {}
+abstract class TypeName implements NamedType, ShowHideClauseElement {}

 /// A type parameter.
 ///
--- a/pkg/analyzer/lib/dart/ast/ast_factory.dart
+++ b/pkg/analyzer/lib/dart/ast/ast_factory.dart
@ -545,6 +545,11 @@ abstract class AstFactory {
      TypeAnnotation type,
      Token semicolon);

+  /// Returns a newly created hide clause.
+  HideClause hideClause(
+      {required Token hideKeyword,
+      required List<ShowHideClauseElement> elements});
+
  /// Returns a newly created import show combinator.
  HideCombinator hideCombinator(
      Token keyword, List<SimpleIdentifier> hiddenNames);
@ -792,10 +797,19 @@ abstract class AstFactory {
      required List<CollectionElement> elements,
      required Token rightBracket});

+  /// Returns a newly created show clause.
+  ShowClause showClause(
+      {required Token showKeyword,
+      required List<ShowHideClauseElement> elements});
+
  /// Returns a newly created import show combinator.
  ShowCombinator showCombinator(
      Token keyword, List<SimpleIdentifier> shownNames);

+  /// Returns a newly created element of a show or hide clause.
+  ShowHideElement showHideElement(
+      {required Token? modifier, required SimpleIdentifier name});
+
  /// Returns a newly created formal parameter. Either or both of the
  /// [comment] and [metadata] can be `null` if the parameter does not have the
  /// corresponding attribute. The [keyword] can be `null` if a type was
--- a/pkg/analyzer/lib/dart/ast/visitor.dart
+++ b/pkg/analyzer/lib/dart/ast/visitor.dart
@ -365,6 +365,9 @@ class GeneralizingAstVisitor<R> implements AstVisitor<R> {
  @override
  R? visitGenericTypeAlias(GenericTypeAlias node) => visitTypeAlias(node);

+  @override
+  R? visitHideClause(HideClause node) => visitNode(node);
+
  @override
  R? visitHideCombinator(HideCombinator node) => visitCombinator(node);

@ -509,9 +512,15 @@ class GeneralizingAstVisitor<R> implements AstVisitor<R> {
  @override
  R? visitSetOrMapLiteral(SetOrMapLiteral node) => visitTypedLiteral(node);

+  @override
+  R? visitShowClause(ShowClause node) => visitNode(node);
+
  @override
  R? visitShowCombinator(ShowCombinator node) => visitCombinator(node);

+  @override
+  R? visitShowHideElement(ShowHideElement node) => visitNode(node);
+
  @override
  R? visitSimpleFormalParameter(SimpleFormalParameter node) =>
      visitNormalFormalParameter(node);
@ -990,6 +999,12 @@ class RecursiveAstVisitor<R> implements AstVisitor<R> {
    return null;
  }

+  @override
+  R? visitHideClause(HideClause node) {
+    node.visitChildren(this);
+    return null;
+  }
+
  @override
  R? visitHideCombinator(HideCombinator node) {
    node.visitChildren(this);
@ -1213,12 +1228,24 @@ class RecursiveAstVisitor<R> implements AstVisitor<R> {
    return null;
  }

+  @override
+  R? visitShowClause(ShowClause node) {
+    node.visitChildren(this);
+    return null;
+  }
+
  @override
  R? visitShowCombinator(ShowCombinator node) {
    node.visitChildren(this);
    return null;
  }

+  @override
+  R? visitShowHideElement(ShowHideElement node) {
+    node.visitChildren(this);
+    return null;
+  }
+
  @override
  R? visitSimpleFormalParameter(SimpleFormalParameter node) {
    node.visitChildren(this);
@ -1569,6 +1596,9 @@ class SimpleAstVisitor<R> implements AstVisitor<R> {
  @override
  R? visitGenericTypeAlias(GenericTypeAlias node) => null;

+  @override
+  R? visitHideClause(HideClause node) => null;
+
  @override
  R? visitHideCombinator(HideCombinator node) => null;

@ -1682,9 +1712,15 @@ class SimpleAstVisitor<R> implements AstVisitor<R> {
  @override
  R? visitSetOrMapLiteral(SetOrMapLiteral node) => null;

+  @override
+  R? visitShowClause(ShowClause node) => null;
+
  @override
  R? visitShowCombinator(ShowCombinator node) => null;

+  @override
+  R? visitShowHideElement(ShowHideElement node) => null;
+
  @override
  R? visitSimpleFormalParameter(SimpleFormalParameter node) => null;

@ -1963,6 +1999,9 @@ class ThrowingAstVisitor<R> implements AstVisitor<R> {
  @override
  R? visitGenericTypeAlias(GenericTypeAlias node) => _throw(node);

+  @override
+  R? visitHideClause(HideClause node) => _throw(node);
+
  @override
  R? visitHideCombinator(HideCombinator node) => _throw(node);

@ -2077,9 +2116,15 @@ class ThrowingAstVisitor<R> implements AstVisitor<R> {
  @override
  R? visitSetOrMapLiteral(SetOrMapLiteral node) => _throw(node);

+  @override
+  R? visitShowClause(ShowClause node) => _throw(node);
+
  @override
  R? visitShowCombinator(ShowCombinator node) => _throw(node);

+  @override
+  R? visitShowHideElement(ShowHideElement node) => _throw(node);
+
  @override
  R? visitSimpleFormalParameter(SimpleFormalParameter node) => _throw(node);

@ -2661,6 +2706,14 @@ class TimedAstVisitor<T> implements AstVisitor<T> {
    return result;
  }

+  @override
+  T? visitHideClause(HideClause node) {
+    stopwatch.start();
+    T? result = _baseVisitor.visitHideClause(node);
+    stopwatch.stop();
+    return result;
+  }
+
  @override
  T? visitHideCombinator(HideCombinator node) {
    stopwatch.start();
@ -2958,6 +3011,14 @@ class TimedAstVisitor<T> implements AstVisitor<T> {
    return result;
  }

+  @override
+  T? visitShowClause(ShowClause node) {
+    stopwatch.start();
+    T? result = _baseVisitor.visitShowClause(node);
+    stopwatch.stop();
+    return result;
+  }
+
  @override
  T? visitShowCombinator(ShowCombinator node) {
    stopwatch.start();
@ -2966,6 +3027,14 @@ class TimedAstVisitor<T> implements AstVisitor<T> {
    return result;
  }

+  @override
+  T? visitShowHideElement(ShowHideElement node) {
+    stopwatch.start();
+    T? result = _baseVisitor.visitShowHideElement(node);
+    stopwatch.stop();
+    return result;
+  }
+
  @override
  T? visitSimpleFormalParameter(SimpleFormalParameter node) {
    stopwatch.start();
@ -3382,6 +3451,9 @@ class UnifyingAstVisitor<R> implements AstVisitor<R> {
  @override
  R? visitGenericTypeAlias(GenericTypeAlias node) => visitNode(node);

+  @override
+  R? visitHideClause(HideClause node) => visitNode(node);
+
  @override
  R? visitHideCombinator(HideCombinator node) => visitNode(node);

@ -3503,9 +3575,15 @@ class UnifyingAstVisitor<R> implements AstVisitor<R> {
  @override
  R? visitSetOrMapLiteral(SetOrMapLiteral node) => visitNode(node);

+  @override
+  R? visitShowClause(ShowClause node) => visitNode(node);
+
  @override
  R? visitShowCombinator(ShowCombinator node) => visitNode(node);

+  @override
+  R? visitShowHideElement(ShowHideElement node) => visitNode(node);
+
  @override
  R? visitSimpleFormalParameter(SimpleFormalParameter node) => visitNode(node);

--- a/pkg/analyzer/lib/src/dart/ast/ast.dart
+++ b/pkg/analyzer/lib/src/dart/ast/ast.dart
@ -3679,10 +3679,18 @@ class ExtensionDeclarationImpl extends CompilationUnitMemberImpl
  @override
  Token? typeKeyword;

+  /// The hide clause for the extension or `null` if the declaration does not
+  /// hide any elements.
+  HideClauseImpl? _hideClause;
+
  /// The name of the extension, or `null` if the extension does not have a
  /// name.
  SimpleIdentifierImpl? _name;

+  /// The show clause for the extension or `null` if the declaration does not
+  /// show any elements.
+  ShowClauseImpl? _showClause;
+
  /// The type parameters for the extension, or `null` if the extension does not
  /// have any type parameters.
  TypeParameterListImpl? _typeParameters;
@ -3713,6 +3721,8 @@ class ExtensionDeclarationImpl extends CompilationUnitMemberImpl
      this._typeParameters,
      this.onKeyword,
      this._extendedType,
+      this._showClause,
+      this._hideClause,
      this.leftBracket,
      List<ClassMember> members,
      this.rightBracket)
@ -3755,6 +3765,13 @@ class ExtensionDeclarationImpl extends CompilationUnitMemberImpl
  @override
  Token get firstTokenAfterCommentAndMetadata => extensionKeyword;

+  @override
+  HideClauseImpl? get hideClause => _hideClause;
+
+  set hideClause(HideClause? hideClause) {
+    _hideClause = _becomeParentOf(hideClause as HideClauseImpl?);
+  }
+
  @override
  NodeListImpl<ClassMember> get members => _members;

@ -3765,6 +3782,13 @@ class ExtensionDeclarationImpl extends CompilationUnitMemberImpl
    _name = _becomeParentOf(identifier as SimpleIdentifierImpl?);
  }

+  @override
+  ShowClauseImpl? get showClause => _showClause;
+
+  set showClause(ShowClause? showClause) {
+    _showClause = _becomeParentOf(showClause as ShowClauseImpl?);
+  }
+
  @override
  TypeParameterListImpl? get typeParameters => _typeParameters;

@ -5457,6 +5481,46 @@ class GenericTypeAliasImpl extends TypeAliasImpl implements GenericTypeAlias {
  }
 }

+/// The "hide" clause in an extension declaration.
+///
+///    hideClause ::=
+///        'hide' [TypeName] (',' [TypeName])*
+class HideClauseImpl extends AstNodeImpl implements HideClause {
+  /// The token representing the 'hide' keyword.
+  @override
+  Token hideKeyword;
+
+  /// The elements that are being shown.
+  final NodeListImpl<ShowHideClauseElement> _elements = NodeListImpl._();
+
+  /// Initialize a newly created show clause.
+  HideClauseImpl(this.hideKeyword, List<ShowHideClauseElement> elements) {
+    _elements._initialize(this, elements);
+  }
+
+  @override
+  Token get beginToken => hideKeyword;
+
+  @override
+  Iterable<SyntacticEntity> get childEntities => ChildEntities()
+    ..add(hideKeyword)
+    ..addAll(elements);
+
+  @override
+  NodeListImpl<ShowHideClauseElement> get elements => _elements;
+
+  @override
+  Token get endToken => _elements.endToken!;
+
+  @override
+  E? accept<E>(AstVisitor<E> visitor) => visitor.visitHideClause(this);
+
+  @override
+  void visitChildren(AstVisitor visitor) {
+    _elements.accept(visitor);
+  }
+}
+
 /// A combinator that restricts the names being imported to those that are not
 /// in a given list.
 ///
@ -8620,6 +8684,46 @@ class SetOrMapLiteralImpl extends TypedLiteralImpl implements SetOrMapLiteral {
  }
 }

+/// The "show" clause in an extension declaration.
+///
+///    showClause ::=
+///        'show' [TypeName] (',' [TypeName])*
+class ShowClauseImpl extends AstNodeImpl implements ShowClause {
+  /// The token representing the 'show' keyword.
+  @override
+  Token showKeyword;
+
+  /// The elements that are being shown.
+  final NodeListImpl<ShowHideClauseElement> _elements = NodeListImpl._();
+
+  /// Initialize a newly created show clause.
+  ShowClauseImpl(this.showKeyword, List<ShowHideClauseElement> elements) {
+    _elements._initialize(this, elements);
+  }
+
+  @override
+  Token get beginToken => showKeyword;
+
+  @override
+  Iterable<SyntacticEntity> get childEntities => ChildEntities()
+    ..add(showKeyword)
+    ..addAll(elements);
+
+  @override
+  NodeListImpl<ShowHideClauseElement> get elements => _elements;
+
+  @override
+  Token get endToken => _elements.endToken!;
+
+  @override
+  E? accept<E>(AstVisitor<E> visitor) => visitor.visitShowClause(this);
+
+  @override
+  void visitChildren(AstVisitor visitor) {
+    _elements.accept(visitor);
+  }
+}
+
 /// A combinator that restricts the names being imported to those in a given
 /// list.
 ///
@ -8657,6 +8761,45 @@ class ShowCombinatorImpl extends CombinatorImpl implements ShowCombinator {
  }
 }

+/// A potentially non-type element of a show or a hide clause.
+///
+///    showHideElement ::=
+///        'get' [SimpleIdentifier] |
+///        'set' [SimpleIdentifier] |
+///        'operator' [SimpleIdentifier] |
+///        [SimpleIdentifier]
+///
+/// Clients may not extend, implement or mix-in this class.
+class ShowHideElementImpl extends AstNodeImpl implements ShowHideElement {
+  @override
+  Token? modifier;
+
+  @override
+  SimpleIdentifier name;
+
+  ShowHideElementImpl(this.modifier, this.name) {
+    _becomeParentOf<SimpleIdentifierImpl>(name as SimpleIdentifierImpl);
+  }
+
+  @override
+  Token get beginToken => modifier ?? name.beginToken;
+
+  @override
+  Iterable<SyntacticEntity> get childEntities =>
+      ChildEntities()..addAll([if (modifier != null) modifier!, name]);
+
+  @override
+  Token get endToken => name.endToken;
+
+  @override
+  E? accept<E>(AstVisitor<E> visitor) => visitor.visitShowHideElement(this);
+
+  @override
+  void visitChildren(AstVisitor visitor) {
+    name.accept(visitor);
+  }
+}
+
 /// A simple formal parameter.
 ///
 ///    simpleFormalParameter ::=
--- a/pkg/analyzer/lib/src/dart/ast/ast_factory.dart
+++ b/pkg/analyzer/lib/src/dart/ast/ast_factory.dart
@ -446,6 +446,8 @@ class AstFactoryImpl extends AstFactory {
          TypeParameterList? typeParameters,
          required Token onKeyword,
          required TypeAnnotation extendedType,
+          ShowClause? showClause,
+          HideClause? hideClause,
          required Token leftBracket,
          required List<ClassMember> members,
          required Token rightBracket}) =>
@ -458,6 +460,8 @@ class AstFactoryImpl extends AstFactory {
          typeParameters as TypeParameterListImpl?,
          onKeyword,
          extendedType as TypeAnnotationImpl,
+          showClause as ShowClauseImpl?,
+          hideClause as HideClauseImpl?,
          leftBracket,
          members,
          rightBracket);
@ -729,6 +733,12 @@ class AstFactoryImpl extends AstFactory {
          type as TypeAnnotationImpl,
          semicolon);

+  @override
+  HideClauseImpl hideClause(
+          {required Token hideKeyword,
+          required List<ShowHideClauseElement> elements}) =>
+      HideClauseImpl(hideKeyword, elements);
+
  @override
  HideCombinatorImpl hideCombinator(
          Token keyword, List<SimpleIdentifier> hiddenNames) =>
@ -1076,11 +1086,22 @@ class AstFactoryImpl extends AstFactory {
      SetOrMapLiteralImpl(constKeyword, typeArguments as TypeArgumentListImpl?,
          leftBracket, elements, rightBracket);

+  @override
+  ShowClauseImpl showClause(
+          {required Token showKeyword,
+          required List<ShowHideClauseElement> elements}) =>
+      ShowClauseImpl(showKeyword, elements);
+
  @override
  ShowCombinatorImpl showCombinator(
          Token keyword, List<SimpleIdentifier> shownNames) =>
      ShowCombinatorImpl(keyword, shownNames);

+  @override
+  ShowHideElementImpl showHideElement(
+          {required Token? modifier, required SimpleIdentifier name}) =>
+      ShowHideElementImpl(modifier, name);
+
  @override
  SimpleFormalParameterImpl simpleFormalParameter2(
          {Comment? comment,
--- a/pkg/analyzer/lib/src/dart/ast/to_source_visitor.dart
+++ b/pkg/analyzer/lib/src/dart/ast/to_source_visitor.dart
@ -368,6 +368,8 @@ class ToSourceVisitor implements AstVisitor<void> {
    _visitToken(node.onKeyword);
    sink.write(' ');
    _visitNode(node.extendedType, suffix: ' ');
+    _visitNode(node.showClause, suffix: ' ');
+    _visitNode(node.hideClause, suffix: ' ');
    _visitToken(node.leftBracket);
    _visitNodeList(node.members, separator: ' ');
    _visitToken(node.rightBracket);
@ -564,6 +566,12 @@ class ToSourceVisitor implements AstVisitor<void> {
    _visitNode(node.type);
  }

+  @override
+  void visitHideClause(HideClause node) {
+    sink.write('hide ');
+    _visitNodeList(node.elements, separator: ', ');
+  }
+
  @override
  void visitHideCombinator(HideCombinator node) {
    sink.write('hide ');
@ -869,12 +877,24 @@ class ToSourceVisitor implements AstVisitor<void> {
    sink.write('}');
  }

+  @override
+  void visitShowClause(ShowClause node) {
+    sink.write('show ');
+    _visitNodeList(node.elements, separator: ', ');
+  }
+
  @override
  void visitShowCombinator(ShowCombinator node) {
    sink.write('show ');
    _visitNodeList(node.shownNames, separator: ', ');
  }

+  @override
+  void visitShowHideElement(ShowHideElement node) {
+    _visitToken(node.modifier, suffix: ' ');
+    _visitNode(node.name);
+  }
+
  @override
  void visitSimpleFormalParameter(SimpleFormalParameter node) {
    _visitNodeList(node.metadata, separator: ' ', suffix: ' ');
--- a/pkg/analyzer/lib/src/dart/ast/utilities.dart
+++ b/pkg/analyzer/lib/src/dart/ast/utilities.dart
@ -676,6 +676,13 @@ class AstComparator implements AstVisitor<bool> {
        isEqualNodes(node.type, other.type);
  }

+  @override
+  bool visitHideClause(HideClause node) {
+    HideClause other = _other as HideClause;
+    return isEqualTokens(node.hideKeyword, other.hideKeyword) &&
+        _isEqualNodeLists(node.elements, other.elements);
+  }
+
  @override
  bool visitHideCombinator(HideCombinator node) {
    HideCombinator other = _other as HideCombinator;
@ -1006,6 +1013,13 @@ class AstComparator implements AstVisitor<bool> {
        isEqualTokens(node.rightBracket, other.rightBracket);
  }

+  @override
+  bool visitShowClause(ShowClause node) {
+    ShowClause other = _other as ShowClause;
+    return isEqualTokens(node.showKeyword, other.showKeyword) &&
+        _isEqualNodeLists(node.elements, other.elements);
+  }
+
  @override
  bool visitShowCombinator(ShowCombinator node) {
    ShowCombinator other = _other as ShowCombinator;
@ -1013,6 +1027,13 @@ class AstComparator implements AstVisitor<bool> {
        _isEqualNodeLists(node.shownNames, other.shownNames);
  }

+  @override
+  bool visitShowHideElement(ShowHideElement node) {
+    ShowHideElement other = _other as ShowHideElement;
+    return isEqualTokens(node.modifier, other.modifier) &&
+        isEqualNodes(node.name, other.name);
+  }
+
  @override
  bool visitSimpleFormalParameter(SimpleFormalParameter node) {
    SimpleFormalParameter other = _other as SimpleFormalParameter;
@ -2256,6 +2277,14 @@ class NodeReplacer implements AstVisitor<bool> {
    return visitNode(node);
  }

+  @override
+  bool visitHideClause(covariant HideClauseImpl node) {
+    if (_replaceInList(node.elements)) {
+      return true;
+    }
+    return visitNode(node);
+  }
+
  @override
  bool visitHideCombinator(covariant HideCombinatorImpl node) {
    if (_replaceInList(node.hiddenNames)) {
@ -2645,6 +2674,14 @@ class NodeReplacer implements AstVisitor<bool> {
    return visitTypedLiteral(node);
  }

+  @override
+  bool visitShowClause(covariant ShowClauseImpl node) {
+    if (_replaceInList(node.elements)) {
+      return true;
+    }
+    return visitNode(node);
+  }
+
  @override
  bool visitShowCombinator(covariant ShowCombinatorImpl node) {
    if (_replaceInList(node.shownNames)) {
@ -2653,6 +2690,15 @@ class NodeReplacer implements AstVisitor<bool> {
    return visitNode(node);
  }

+  @override
+  bool visitShowHideElement(covariant ShowHideElementImpl node) {
+    if (identical(node.name, _oldNode)) {
+      node.name = _newNode as SimpleIdentifier;
+      return true;
+    }
+    return visitNode(node);
+  }
+
  @override
  bool visitSimpleFormalParameter(covariant SimpleFormalParameterImpl node) {
    if (identical(node.type, _oldNode)) {
--- a/pkg/analyzer/lib/src/fasta/ast_builder.dart
+++ b/pkg/analyzer/lib/src/fasta/ast_builder.dart
@ -258,6 +258,8 @@ class AstBuilder extends StackListener {
        _tmpSimpleIdentifier(),
        null,
      ), // extendedType is set in [endExtensionDeclaration]
+      showClause: null,
+      hideClause: null,
      leftBracket: Tokens.openCurlyBracket(),
      rightBracket: Tokens.closeCurlyBracket(),
      members: [],
@ -1192,7 +1194,7 @@ class AstBuilder extends StackListener {

  @override
  void endExtensionDeclaration(Token extensionKeyword, Token? typeKeyword,
-      Token onKeyword, Token token) {
+      Token onKeyword, Token? showKeyword, Token? hideKeyword, Token token) {
    if (typeKeyword != null && !enableExtensionTypes) {
      var feature = ExperimentalFeatures.extension_types;
      handleRecoverableError(
@ -1203,11 +1205,29 @@ class AstBuilder extends StackListener {
          typeKeyword,
          typeKeyword);
    }
+
+    if ((showKeyword != null || hideKeyword != null) && !enableExtensionTypes) {
+      var feature = ExperimentalFeatures.extension_types;
+      handleRecoverableError(
+          templateExperimentNotEnabled.withArguments(
+            feature.enableString,
+            _versionAsString(ExperimentStatus.currentVersion),
+          ),
+          (showKeyword ?? hideKeyword)!,
+          (showKeyword ?? hideKeyword)!);
+    }
+
+    ShowClause? showClause = pop(NullValue.ShowClause) as ShowClause?;
+    HideClause? hideClause = pop(NullValue.HideClause) as HideClause?;
+
    var type = pop() as TypeAnnotation;
+
    extensionDeclaration!
      ..extendedType = type
      ..onKeyword = onKeyword
-      ..typeKeyword = typeKeyword;
+      ..typeKeyword = typeKeyword
+      ..showClause = showClause
+      ..hideClause = hideClause;
    extensionDeclaration = null;
  }

@ -2705,6 +2725,29 @@ class AstBuilder extends StackListener {
    push(ast.expressionStatement(expression, semicolon));
  }

+  @override
+  void handleExtensionShowHide(Token? showKeyword, int showElementCount,
+      Token? hideKeyword, int hideElementCount) {
+    assert(optionalOrNull('hide', hideKeyword));
+    assert(optionalOrNull('show', showKeyword));
+    debugEvent("ExtensionShowHide");
+
+    HideClause? hideClause;
+    if (hideKeyword != null) {
+      var elements = popTypedList2<ShowHideClauseElement>(hideElementCount);
+      hideClause = ast.hideClause(hideKeyword: hideKeyword, elements: elements);
+    }
+
+    ShowClause? showClause;
+    if (showKeyword != null) {
+      var elements = popTypedList2<ShowHideClauseElement>(showElementCount);
+      showClause = ast.showClause(showKeyword: showKeyword, elements: elements);
+    }
+
+    push(hideClause ?? NullValue.HideClause);
+    push(showClause ?? NullValue.ShowClause);
+  }
+
  @override
  void handleFinallyBlock(Token finallyKeyword) {
    debugEvent("FinallyBlock");
@ -3478,6 +3521,22 @@ class AstBuilder extends StackListener {
    }
  }

+  @override
+  void handleShowHideIdentifier(Token? modifier, Token identifier) {
+    debugEvent("handleShowHideIdentifier");
+
+    assert(modifier == null ||
+        modifier.stringValue! == "get" ||
+        modifier.stringValue! == "set" ||
+        modifier.stringValue! == "operator");
+
+    SimpleIdentifier name = ast.simpleIdentifier(identifier);
+    ShowHideElement element =
+        ast.showHideElement(modifier: modifier, name: name);
+
+    push(element);
+  }
+
  @override
  void handleSpreadExpression(Token spreadToken) {
    var expression = pop() as Expression;
--- a/pkg/analyzer/lib/src/generated/testing/ast_test_factory.dart
+++ b/pkg/analyzer/lib/src/generated/testing/ast_test_factory.dart
@ -519,6 +519,8 @@ class AstTestFactory {
          required bool isExtensionTypeDeclaration,
          TypeParameterList? typeParameters,
          required TypeAnnotation extendedType,
+          ShowClause? showClause,
+          HideClause? hideClause,
          List<ClassMember> members = const []}) =>
      astFactory.extensionDeclaration(
          comment: null,
@ -531,6 +533,8 @@ class AstTestFactory {
          typeParameters: typeParameters,
          onKeyword: TokenFactory.tokenFromKeyword(Keyword.ON),
          extendedType: extendedType,
+          showClause: showClause,
+          hideClause: hideClause,
          leftBracket: TokenFactory.tokenFromType(TokenType.OPEN_CURLY_BRACKET),
          members: members,
          rightBracket:
@ -720,6 +724,11 @@ class AstTestFactory {
          functionType,
          TokenFactory.tokenFromType(TokenType.SEMICOLON));

+  static HideClauseImpl hideClause(List<ShowHideClauseElement> elements) =>
+      astFactory.hideClause(
+          hideKeyword: TokenFactory.tokenFromString("hide"),
+          elements: elements);
+
  static HideCombinatorImpl hideCombinator(
          List<SimpleIdentifier> identifiers) =>
      astFactory.hideCombinator(
@ -1205,6 +1214,11 @@ class AstTestFactory {
        rightBracket: TokenFactory.tokenFromType(TokenType.CLOSE_CURLY_BRACKET),
      );

+  static ShowClauseImpl showClause(List<ShowHideClauseElement> elements) =>
+      astFactory.showClause(
+          showKeyword: TokenFactory.tokenFromString("show"),
+          elements: elements);
+
  static ShowCombinatorImpl showCombinator(
          List<SimpleIdentifier> identifiers) =>
      astFactory.showCombinator(
@ -1214,6 +1228,24 @@ class AstTestFactory {
      astFactory.showCombinator(
          TokenFactory.tokenFromString("show"), identifierList(identifiers));

+  static ShowHideElementImpl showHideElement(String name) =>
+      astFactory.showHideElement(modifier: null, name: identifier3(name));
+
+  static ShowHideElementImpl showHideElementGetter(String name) =>
+      astFactory.showHideElement(
+          modifier: TokenFactory.tokenFromString("get"),
+          name: identifier3(name));
+
+  static ShowHideElementImpl showHideElementOperator(String name) =>
+      astFactory.showHideElement(
+          modifier: TokenFactory.tokenFromString("operator"),
+          name: identifier3(name));
+
+  static ShowHideElementImpl showHideElementSetter(String name) =>
+      astFactory.showHideElement(
+          modifier: TokenFactory.tokenFromString("set"),
+          name: identifier3(name));
+
  static SimpleFormalParameterImpl simpleFormalParameter(
          Keyword keyword, String parameterName) =>
      simpleFormalParameter2(keyword, null, parameterName);
--- a/pkg/analyzer/lib/src/summary2/ast_text_printer.dart
+++ b/pkg/analyzer/lib/src/summary2/ast_text_printer.dart
@ -342,6 +342,8 @@ class AstTextPrinter extends ThrowingAstVisitor<void> {
    node.typeParameters?.accept(this);
    _token(node.onKeyword);
    node.extendedType.accept(this);
+    node.showClause?.accept(this);
+    node.hideClause?.accept(this);
    _token(node.leftBracket);
    node.members.accept(this);
    _token(node.rightBracket);
--- a/pkg/analyzer/test/generated/parser_fasta_listener.dart
+++ b/pkg/analyzer/test/generated/parser_fasta_listener.dart
@ -750,9 +750,9 @@ class ForwardingTestListener extends ForwardingListener {

  @override
  void endExtensionDeclaration(Token extensionKeyword, Token? typeKeyword,
-      Token onKeyword, Token token) {
-    super.endExtensionDeclaration(
-        extensionKeyword, typeKeyword, onKeyword, token);
+      Token onKeyword, Token? showKeyword, Token? hideKeyword, Token token) {
+    super.endExtensionDeclaration(extensionKeyword, typeKeyword, onKeyword,
+        showKeyword, hideKeyword, token);
    end('ExtensionDeclaration');
  }

--- a/pkg/analyzer/test/src/dart/ast/to_source_visitor_test.dart
+++ b/pkg/analyzer/test/src/dart/ast/to_source_visitor_test.dart
@ -895,6 +895,201 @@ export 'foo.dart'
            isExtensionTypeDeclaration: false));
  }

+  void test_visitExtensionDeclarationHideClause_empty() {
+    _assertSource(
+        'extension type E on C hide B {}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            extendedType: AstTestFactory.typeName4('C'),
+            hideClause:
+                AstTestFactory.hideClause([AstTestFactory.typeName4("B")]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationHideClause_multipleMember() {
+    _assertSource(
+        'extension type E on C hide B {var a; var b;}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            extendedType: AstTestFactory.typeName4('C'),
+            members: [
+              AstTestFactory.fieldDeclaration2(false, Keyword.VAR,
+                  [AstTestFactory.variableDeclaration('a')]),
+              AstTestFactory.fieldDeclaration2(
+                  false, Keyword.VAR, [AstTestFactory.variableDeclaration('b')])
+            ],
+            hideClause:
+                AstTestFactory.hideClause([AstTestFactory.typeName4("B")]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationHideClause_parameters() {
+    _assertSource(
+        'extension type E<T> on C hide B {}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            typeParameters: AstTestFactory.typeParameterList(['T']),
+            extendedType: AstTestFactory.typeName4('C'),
+            hideClause:
+                AstTestFactory.hideClause([AstTestFactory.typeName4("B")]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationHideClause_singleMember() {
+    _assertSource(
+        'extension type E on C hide B {var a;}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            extendedType: AstTestFactory.typeName4('C'),
+            members: [
+              AstTestFactory.fieldDeclaration2(
+                  false, Keyword.VAR, [AstTestFactory.variableDeclaration('a')])
+            ],
+            hideClause:
+                AstTestFactory.hideClause([AstTestFactory.typeName4("B")]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationShowClause_ambiguousElement() {
+    _assertSource(
+        'extension type E on C show foo {}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            extendedType: AstTestFactory.typeName4('C'),
+            showClause: AstTestFactory.showClause(
+                [AstTestFactory.showHideElement("foo")]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationShowClause_empty() {
+    _assertSource(
+        'extension type E on C show B {}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            extendedType: AstTestFactory.typeName4('C'),
+            showClause:
+                AstTestFactory.showClause([AstTestFactory.typeName4("B")]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationShowClause_getterElement() {
+    _assertSource(
+        'extension type E on C show get foo {}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            extendedType: AstTestFactory.typeName4('C'),
+            showClause: AstTestFactory.showClause(
+                [AstTestFactory.showHideElementGetter("foo")]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationShowClause_multipleMember() {
+    _assertSource(
+        'extension type E on C show B {var a; var b;}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            extendedType: AstTestFactory.typeName4('C'),
+            members: [
+              AstTestFactory.fieldDeclaration2(false, Keyword.VAR,
+                  [AstTestFactory.variableDeclaration('a')]),
+              AstTestFactory.fieldDeclaration2(
+                  false, Keyword.VAR, [AstTestFactory.variableDeclaration('b')])
+            ],
+            showClause:
+                AstTestFactory.showClause([AstTestFactory.typeName4("B")]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationShowClause_operatorElement() {
+    _assertSource(
+        'extension type E on C show operator * {}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            extendedType: AstTestFactory.typeName4('C'),
+            showClause: AstTestFactory.showClause(
+                [AstTestFactory.showHideElementOperator("*")]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationShowClause_parameters() {
+    _assertSource(
+        'extension type E<T> on C show B {}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            typeParameters: AstTestFactory.typeParameterList(['T']),
+            extendedType: AstTestFactory.typeName4('C'),
+            showClause:
+                AstTestFactory.showClause([AstTestFactory.typeName4("B")]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationShowClause_qualifiedTypeElement() {
+    _assertSource(
+        'extension type E on C show prefix.B {}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            extendedType: AstTestFactory.typeName4('C'),
+            showClause: AstTestFactory.showClause([
+              AstTestFactory.typeName3(
+                  AstTestFactory.identifier5('prefix', 'B'))
+            ]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationShowClause_setterElement() {
+    _assertSource(
+        'extension type E on C show set foo {}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            extendedType: AstTestFactory.typeName4('C'),
+            showClause: AstTestFactory.showClause(
+                [AstTestFactory.showHideElementSetter("foo")]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationShowClause_singleMember() {
+    _assertSource(
+        'extension type E on C show B {var a;}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            extendedType: AstTestFactory.typeName4('C'),
+            members: [
+              AstTestFactory.fieldDeclaration2(
+                  false, Keyword.VAR, [AstTestFactory.variableDeclaration('a')])
+            ],
+            showClause:
+                AstTestFactory.showClause([AstTestFactory.typeName4("B")]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationShowClause_typeWithArgumentsElement() {
+    _assertSource(
+        'extension type E on C show B<int, String> {}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            extendedType: AstTestFactory.typeName4('C'),
+            showClause: AstTestFactory.showClause([
+              AstTestFactory.typeName3(AstTestFactory.identifier3('B'), [
+                AstTestFactory.typeName4('int'),
+                AstTestFactory.typeName4('String')
+              ])
+            ]),
+            isExtensionTypeDeclaration: true));
+  }
+
+  void test_visitExtensionDeclarationShowHideClause_empty() {
+    _assertSource(
+        'extension type E on C show B hide foo {}',
+        AstTestFactory.extensionDeclaration(
+            name: 'E',
+            extendedType: AstTestFactory.typeName4('C'),
+            showClause:
+                AstTestFactory.showClause([AstTestFactory.typeName4("B")]),
+            hideClause: AstTestFactory.hideClause(
+                [AstTestFactory.showHideElement("foo")]),
+            isExtensionTypeDeclaration: true));
+  }
+
  void test_visitExtensionOverride_prefixedName_noTypeArgs() {
    _assertSource(
        'p.E(o)',
--- a/pkg/front_end/lib/src/fasta/source/diet_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/diet_listener.dart
@ -861,6 +861,12 @@ class DietListener extends StackListenerImpl {
    // Do nothing
  }

+  @override
+  void handleShowHideIdentifier(Token? modifier, Token? identifier) {
+    debugEvent("");
+    // Do nothing
+  }
+
  @override
  void beginClassOrMixinBody(DeclarationKind kind, Token token) {
    assert(checkState(token, [
@ -928,7 +934,7 @@ class DietListener extends StackListenerImpl {

  @override
  void endExtensionDeclaration(Token extensionKeyword, Token? typeKeyword,
-      Token onKeyword, Token endToken) {
+      Token onKeyword, Token? showKeyword, Token? hideKeyword, Token endToken) {
    debugEvent("endExtensionDeclaration");
    checkEmpty(extensionKeyword.charOffset);
  }
--- a/pkg/front_end/lib/src/fasta/source/outline_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/outline_builder.dart
@ -327,6 +327,30 @@ class OutlineBuilder extends StackListenerImpl {
    push(token.charOffset);
  }

+  @override
+  void handleShowHideIdentifier(Token? modifier, Token identifier) {
+    debugEvent("ShowHideIdentifier");
+
+    assert(modifier == null ||
+        modifier.stringValue! == "get" ||
+        modifier.stringValue! == "set" ||
+        modifier.stringValue! == "operator");
+
+    if (modifier == null) {
+      handleIdentifier(
+          identifier, IdentifierContext.extensionShowHideElementMemberOrType);
+    } else if (modifier.stringValue! == "get") {
+      handleIdentifier(
+          identifier, IdentifierContext.extensionShowHideElementGetter);
+    } else if (modifier.stringValue! == "set") {
+      handleIdentifier(
+          identifier, IdentifierContext.extensionShowHideElementSetter);
+    } else if (modifier.stringValue! == "operator") {
+      handleIdentifier(
+          identifier, IdentifierContext.extensionShowHideElementOperator);
+    }
+  }
+
  @override
  void handleIdentifier(Token token, IdentifierContext context) {
    if (context == IdentifierContext.enumValueDeclaration) {
@ -337,6 +361,16 @@ class OutlineBuilder extends StackListenerImpl {
      } else {
        push(new EnumConstantInfo(metadata, token.lexeme, token.charOffset));
      }
+    } else if (context == IdentifierContext.extensionShowHideElementGetter ||
+        context == IdentifierContext.extensionShowHideElementMemberOrType ||
+        context == IdentifierContext.extensionShowHideElementSetter) {
+      push(context);
+      super.handleIdentifier(token, context);
+      push(token.charOffset);
+    } else if (context == IdentifierContext.extensionShowHideElementOperator) {
+      push(context);
+      push(operatorFromString(token.stringValue!));
+      push(token.charOffset);
    } else {
      super.handleIdentifier(token, context);
      push(token.charOffset);
@ -492,9 +526,22 @@ class OutlineBuilder extends StackListenerImpl {
  void beginClassOrMixinBody(DeclarationKind kind, Token token) {
    if (kind == DeclarationKind.Extension) {
      assert(checkState(token, [
+        /* hide type elements = */ ValueKinds.TypeBuilderListOrNull,
+        /* hide get elements = */ ValueKinds.NameListOrNull,
+        /* hide member or type elements = */ ValueKinds.NameListOrNull,
+        /* hide set elements = */ ValueKinds.NameListOrNull,
+        /* hide operator elements = */ ValueKinds.OperatorListOrNull,
+        /* show type elements = */ ValueKinds.TypeBuilderListOrNull,
+        /* show get elements = */ ValueKinds.NameListOrNull,
+        /* show member or type elements = */ ValueKinds.NameListOrNull,
+        /* show set elements */ ValueKinds.NameListOrNull,
+        /* show operator elements*/ ValueKinds.OperatorListOrNull,
        unionOfKinds([ValueKinds.ParserRecovery, ValueKinds.TypeBuilder])
      ]));
-      Object? extensionThisType = peek();
+
+      // We peek into 10th frame on the stack for the extension 'this' type.
+      Object? extensionThisType = stack[10];
+
      if (extensionThisType is TypeBuilder) {
        libraryBuilder.currentTypeParameterScopeBuilder
            .registerExtensionThisType(extensionThisType);
@ -532,6 +579,75 @@ class OutlineBuilder extends StackListenerImpl {
        NullValue.TypeBuilderList);
  }

+  @override
+  void handleExtensionShowHide(Token? showKeyword, int showElementCount,
+      Token? hideKeyword, int hideElementCount) {
+    debugEvent("ExtensionShow");
+
+    List<dynamic> toBePushed = <dynamic>[];
+    void handleShowHideElements(int elementCount) {
+      if (elementCount == 0) {
+        toBePushed.add(NullValue.TypeBuilderList);
+        toBePushed.add(NullValue.IdentifierList);
+        toBePushed.add(NullValue.IdentifierList);
+        toBePushed.add(NullValue.IdentifierList);
+        toBePushed.add(NullValue.OperatorList);
+      } else {
+        List<TypeBuilder> typeElements = <TypeBuilder>[];
+        List<String> getElements = <String>[];
+        List<String> ambiguousMemberOrTypeElements = <String>[];
+        List<String> setElements = <String>[];
+        List<Operator> operatorElements = <Operator>[];
+
+        for (int i = 0; i < elementCount; ++i) {
+          Object leadingElementPart = pop()!;
+          if (leadingElementPart is TypeBuilder) {
+            typeElements.add(leadingElementPart);
+          } else {
+            leadingElementPart as int; // Offset.
+            Object name = pop()!;
+            IdentifierContext context = pop() as IdentifierContext;
+
+            if (name is! ParserRecovery) {
+              assert(context ==
+                      IdentifierContext.extensionShowHideElementGetter ||
+                  context ==
+                      IdentifierContext.extensionShowHideElementMemberOrType ||
+                  context ==
+                      IdentifierContext.extensionShowHideElementOperator ||
+                  context == IdentifierContext.extensionShowHideElementSetter);
+
+              if (context == IdentifierContext.extensionShowHideElementGetter) {
+                getElements.add(name as String);
+              } else if (context ==
+                  IdentifierContext.extensionShowHideElementMemberOrType) {
+                ambiguousMemberOrTypeElements.add(name as String);
+              } else if (context ==
+                  IdentifierContext.extensionShowHideElementOperator) {
+                operatorElements.add(name as Operator);
+              } else if (context ==
+                  IdentifierContext.extensionShowHideElementSetter) {
+                setElements.add(name as String);
+              }
+            }
+          }
+        }
+
+        toBePushed.add(typeElements);
+        toBePushed.add(getElements);
+        toBePushed.add(ambiguousMemberOrTypeElements);
+        toBePushed.add(setElements);
+        toBePushed.add(operatorElements);
+      }
+    }
+
+    handleShowHideElements(hideElementCount);
+    handleShowHideElements(showElementCount);
+    for (int i = toBePushed.length - 1; i >= 0; --i) {
+      push(toBePushed[i]);
+    }
+  }
+
  @override
  void handleRecoverClassHeader() {
    debugEvent("handleRecoverClassHeader");
@ -774,8 +890,18 @@ class OutlineBuilder extends StackListenerImpl {

  @override
  void endExtensionDeclaration(Token extensionKeyword, Token? typeKeyword,
-      Token onKeyword, Token endToken) {
+      Token onKeyword, Token? showKeyword, Token? hideKeyword, Token endToken) {
    assert(checkState(extensionKeyword, [
+      /* hide type elements = */ ValueKinds.TypeBuilderListOrNull,
+      /* hide get elements = */ ValueKinds.NameListOrNull,
+      /* hide member or type elements = */ ValueKinds.NameListOrNull,
+      /* hide set elements = */ ValueKinds.NameListOrNull,
+      /* hide operator elements = */ ValueKinds.OperatorListOrNull,
+      /* show type elements = */ ValueKinds.TypeBuilderListOrNull,
+      /* show get elements = */ ValueKinds.NameListOrNull,
+      /* show member or type elements = */ ValueKinds.NameListOrNull,
+      /* show set elements = */ ValueKinds.NameListOrNull,
+      /* show operator elements = */ ValueKinds.OperatorListOrNull,
      unionOfKinds([ValueKinds.ParserRecovery, ValueKinds.TypeBuilder]),
      ValueKinds.TypeVariableListOrNull,
      ValueKinds.Integer,
@ -783,6 +909,23 @@ class OutlineBuilder extends StackListenerImpl {
      ValueKinds.MetadataListOrNull
    ]));
    debugEvent("endExtensionDeclaration");
+    pop() as List<TypeBuilder>?; // Type elements of the 'hide' clause.
+    pop() as List<String>?; // Getter elements of the 'hide' clause.
+    pop() as List<String>?; // Member or type elements of the 'hide' clause.
+    pop() as List<String>?; // Setter elements of the 'hide' clause.
+    pop() as List<Operator>?; // Operator elements of the 'hide' clause.
+    pop() as List<TypeBuilder>?; // Type elements of the 'show' clause.
+    pop() as List<String>?; // Getter elements of the 'show' clause.
+    pop() as List<String>?; // Member or type elements of the 'show' clause.
+    pop() as List<String>?; // Setter elements of the 'show' clause.
+    pop() as List<Operator>?; // Operator elements of the 'show' clause.
+    if (showKeyword != null && !libraryBuilder.enableExtensionTypesInLibrary) {
+      addProblem(
+          templateExperimentNotEnabled.withArguments('extension-types',
+              libraryBuilder.enableExtensionTypesVersionInLibrary.toText()),
+          showKeyword.charOffset,
+          showKeyword.length);
+    }
    Object? onType = pop();
    if (onType is ParserRecovery) {
      ParserRecovery parserRecovery = onType;
--- a/pkg/front_end/lib/src/fasta/source/value_kinds.dart
+++ b/pkg/front_end/lib/src/fasta/source/value_kinds.dart
@ -75,6 +75,8 @@ class ValueKinds {
  static const ValueKind ModifiersOrNull =
      const SingleValueKind<List<type.Modifier>>(NullValue.Modifiers);
  static const ValueKind Name = const SingleValueKind<String>();
+  static const ValueKind NameListOrNull =
+      const SingleValueKind<List<String>>(NullValue.IdentifierList);
  static const ValueKind NameOrNull =
      const SingleValueKind<String>(NullValue.Name);
  static const ValueKind NameOrOperator =
@ -87,6 +89,8 @@ class ValueKinds {
      const SingleValueKind<List<type.MetadataBuilder>>(NullValue.Metadata);
  static const ValueKind ObjectList = const SingleValueKind<List<Object>>();
  static const ValueKind Operator = const SingleValueKind<type.Operator>();
+  static const ValueKind OperatorListOrNull =
+      const SingleValueKind<List<type.Operator>>(NullValue.OperatorList);
  static const ValueKind ParserRecovery =
      const SingleValueKind<type.ParserRecovery>();
  static const ValueKind ProblemBuilder =
@ -94,8 +98,7 @@ class ValueKinds {
  static const ValueKind QualifiedName =
      const SingleValueKind<type.QualifiedName>();
  static const ValueKind Scope = const SingleValueKind<type.Scope>();
-  static const ValueKind Selector =
-      const SingleValueKind<type.Selector>();
+  static const ValueKind Selector = const SingleValueKind<type.Selector>();
  static const ValueKind SwitchScopeOrNull =
      const SingleValueKind<type.Scope>(NullValue.SwitchScope);
  static const ValueKind Statement = const SingleValueKind<type.Statement>();
@ -114,6 +117,8 @@ class ValueKinds {
      const SingleValueKind<type.TypeBuilder>();
  static const ValueKind TypeBuilderOrNull =
      const SingleValueKind<type.TypeBuilder>(NullValue.Type);
+  static const ValueKind TypeBuilderListOrNull =
+      const SingleValueKind<List<type.TypeBuilder>>(NullValue.TypeBuilderList);
  static const ValueKind TypeVariableListOrNull =
      const SingleValueKind<List<type.TypeVariableBuilder>>(
          NullValue.TypeVariables);
--- a/pkg/front_end/lib/src/fasta/util/direct_parser_ast_helper.dart
+++ b/pkg/front_end/lib/src/fasta/util/direct_parser_ast_helper.dart
@ -215,6 +215,19 @@ abstract class AbstractDirectParserASTListener implements Listener {
    seen(data);
  }

+  @override
+  void handleExtensionShowHide(Token? showKeyword, int showElementCount,
+      Token? hideKeyword, int hideElementCount) {
+    DirectParserASTContentExtensionShowHideHandle data =
+        new DirectParserASTContentExtensionShowHideHandle(
+            DirectParserASTType.HANDLE,
+            showKeyword: showKeyword,
+            showElementCount: showElementCount,
+            hideKeyword: hideKeyword,
+            hideElementCount: hideElementCount);
+    seen(data);
+  }
+
  @override
  void handleClassHeader(Token begin, Token classKeyword, Token? nativeToken) {
    DirectParserASTContentClassHeaderHandle data =
@ -311,13 +324,15 @@ abstract class AbstractDirectParserASTListener implements Listener {

  @override
  void endExtensionDeclaration(Token extensionKeyword, Token? typeKeyword,
-      Token onKeyword, Token endToken) {
+      Token onKeyword, Token? showKeyword, Token? hideKeyword, Token endToken) {
    DirectParserASTContentExtensionDeclarationEnd data =
        new DirectParserASTContentExtensionDeclarationEnd(
            DirectParserASTType.END,
            extensionKeyword: extensionKeyword,
            typeKeyword: typeKeyword,
            onKeyword: onKeyword,
+            showKeyword: showKeyword,
+            hideKeyword: hideKeyword,
            endToken: endToken);
    seen(data);
  }
@ -2313,6 +2328,16 @@ abstract class AbstractDirectParserASTListener implements Listener {
    seen(data);
  }

+  @override
+  void handleShowHideIdentifier(Token? modifier, Token identifier) {
+    DirectParserASTContentShowHideIdentifierHandle data =
+        new DirectParserASTContentShowHideIdentifierHandle(
+            DirectParserASTType.HANDLE,
+            modifier: modifier,
+            identifier: identifier);
+    seen(data);
+  }
+
  @override
  void handleIndexedExpression(
      Token? question, Token openSquareBracket, Token closeSquareBracket) {
@ -3157,6 +3182,29 @@ class DirectParserASTContentClassOrMixinImplementsHandle
      };
 }

+class DirectParserASTContentExtensionShowHideHandle
+    extends DirectParserASTContent {
+  final Token? showKeyword;
+  final int showElementCount;
+  final Token? hideKeyword;
+  final int hideElementCount;
+
+  DirectParserASTContentExtensionShowHideHandle(DirectParserASTType type,
+      {this.showKeyword,
+      required this.showElementCount,
+      this.hideKeyword,
+      required this.hideElementCount})
+      : super("ExtensionShowHide", type);
+
+  @override
+  Map<String, Object?> get deprecatedArguments => {
+        "showKeyword": showKeyword,
+        "showElementCount": showElementCount,
+        "hideKeyword": hideKeyword,
+        "hideElementCount": hideElementCount,
+      };
+}
+
 class DirectParserASTContentClassHeaderHandle extends DirectParserASTContent {
  final Token begin;
  final Token classKeyword;
@ -3317,12 +3365,16 @@ class DirectParserASTContentExtensionDeclarationEnd
  final Token extensionKeyword;
  final Token? typeKeyword;
  final Token onKeyword;
+  final Token? showKeyword;
+  final Token? hideKeyword;
  final Token endToken;

  DirectParserASTContentExtensionDeclarationEnd(DirectParserASTType type,
      {required this.extensionKeyword,
      this.typeKeyword,
      required this.onKeyword,
+      this.showKeyword,
+      this.hideKeyword,
      required this.endToken})
      : super("ExtensionDeclaration", type);

@ -3331,6 +3383,8 @@ class DirectParserASTContentExtensionDeclarationEnd
        "extensionKeyword": extensionKeyword,
        "typeKeyword": typeKeyword,
        "onKeyword": onKeyword,
+        "showKeyword": showKeyword,
+        "hideKeyword": hideKeyword,
        "endToken": endToken,
      };
 }
@ -6627,6 +6681,22 @@ class DirectParserASTContentIdentifierHandle extends DirectParserASTContent {
      };
 }

+class DirectParserASTContentShowHideIdentifierHandle
+    extends DirectParserASTContent {
+  final Token? modifier;
+  final Token identifier;
+
+  DirectParserASTContentShowHideIdentifierHandle(DirectParserASTType type,
+      {this.modifier, required this.identifier})
+      : super("ShowHideIdentifier", type);
+
+  @override
+  Map<String, Object?> get deprecatedArguments => {
+        "modifier": modifier,
+        "identifier": identifier,
+      };
+}
+
 class DirectParserASTContentIndexedExpressionHandle
    extends DirectParserASTContent {
  final Token? question;
--- a/pkg/front_end/lib/src/fasta/util/textual_outline.dart
+++ b/pkg/front_end/lib/src/fasta/util/textual_outline.dart
@ -805,7 +805,7 @@ class TextualOutlineListener extends Listener {

  @override
  void endExtensionDeclaration(Token extensionKeyword, Token? typeKeyword,
-      Token onKeyword, Token endToken) {
+      Token onKeyword, Token? showKeyword, Token? hideKeyword, Token endToken) {
    classStartToChunk[extensionKeyword] =
        new _ExtensionDeclarationChunk(extensionKeyword, endToken);
  }
--- a/pkg/front_end/parser_testcases/error_recovery/extension_member_contributor_test_completion.dart.expect
+++ b/pkg/front_end/parser_testcases/error_recovery/extension_member_contributor_test_completion.dart.expect
@ -31,6 +31,7 @@ beginCompilationUnit(extension)
        handleType(T, null)
      endTypeArguments(1, <, >)
      handleType(List, null)
+      handleExtensionShowHide(null, 0, null, 0)
      beginClassOrMixinBody(DeclarationKind.Extension, {)
        beginMetadataStar(bool)
        endMetadataStar(0)
@ -108,7 +109,7 @@ beginCompilationUnit(extension)
          endExtensionMethod(set, set, (, null, })
        endMember()
      endClassOrMixinBody(DeclarationKind.Extension, 3, {, })
-    endExtensionDeclaration(extension, null, on, })
+    endExtensionDeclaration(extension, null, on, null, null, })
  endTopLevelDeclaration(void)
  beginMetadataStar(void)
  endMetadataStar(0)
--- a/pkg/front_end/parser_testcases/error_recovery/extension_member_contributor_test_completion.dart.intertwined.expect
+++ b/pkg/front_end/parser_testcases/error_recovery/extension_member_contributor_test_completion.dart.intertwined.expect
@ -31,6 +31,7 @@ parseUnit(extension)
        listener: handleType(T, null)
        listener: endTypeArguments(1, <, >)
        listener: handleType(List, null)
+        listener: handleExtensionShowHide(null, 0, null, 0)
        parseClassOrMixinOrExtensionBody(>, DeclarationKind.Extension, E)
          listener: beginClassOrMixinBody(DeclarationKind.Extension, {)
          notEofOrValue(}, bool)
@ -172,7 +173,7 @@ parseUnit(extension)
            listener: endMember()
          notEofOrValue(}, })
          listener: endClassOrMixinBody(DeclarationKind.Extension, 3, {, })
-        listener: endExtensionDeclaration(extension, null, on, })
+        listener: endExtensionDeclaration(extension, null, on, null, null, })
  listener: endTopLevelDeclaration(void)
  parseTopLevelDeclarationImpl(}, Instance of 'DirectiveContext')
    parseMetadataStar(})
--- a/pkg/front_end/parser_testcases/extension_named_type.dart.expect
+++ b/pkg/front_end/parser_testcases/extension_named_type.dart.expect
@ -22,6 +22,7 @@ beginCompilationUnit(class)
      handleIdentifier(A, typeReference)
      handleNoTypeArguments({)
      handleType(A, null)
+      handleExtensionShowHide(null, 0, null, 0)
      beginClassOrMixinBody(DeclarationKind.Extension, {)
        beginMetadataStar(method)
        endMetadataStar(0)
@ -39,7 +40,7 @@ beginCompilationUnit(class)
          endExtensionMethod(null, method, (, null, })
        endMember()
      endClassOrMixinBody(DeclarationKind.Extension, 1, {, })
-    endExtensionDeclaration(extension, null, on, })
+    endExtensionDeclaration(extension, null, on, null, null, })
  endTopLevelDeclaration(test)
  beginMetadataStar(test)
  endMetadataStar(0)
--- a/pkg/front_end/parser_testcases/extension_named_type.dart.intertwined.expect
+++ b/pkg/front_end/parser_testcases/extension_named_type.dart.intertwined.expect
@ -43,6 +43,7 @@ parseUnit(class)
        listener: handleIdentifier(A, typeReference)
        listener: handleNoTypeArguments({)
        listener: handleType(A, null)
+        listener: handleExtensionShowHide(null, 0, null, 0)
        parseClassOrMixinOrExtensionBody(A, DeclarationKind.Extension, type)
          listener: beginClassOrMixinBody(DeclarationKind.Extension, {)
          notEofOrValue(}, method)
@ -79,7 +80,7 @@ parseUnit(class)
            listener: endMember()
          notEofOrValue(}, })
          listener: endClassOrMixinBody(DeclarationKind.Extension, 1, {, })
-        listener: endExtensionDeclaration(extension, null, on, })
+        listener: endExtensionDeclaration(extension, null, on, null, null, })
  listener: endTopLevelDeclaration(test)
  parseTopLevelDeclarationImpl(}, Instance of 'DirectiveContext')
    parseMetadataStar(})
--- a/pkg/front_end/parser_testcases/extension_type.dart.expect
+++ b/pkg/front_end/parser_testcases/extension_type.dart.expect
@ -22,8 +22,9 @@ beginCompilationUnit(class)
      handleIdentifier(A, typeReference)
      handleNoTypeArguments({)
      handleType(A, null)
+      handleExtensionShowHide(null, 0, null, 0)
      beginClassOrMixinBody(DeclarationKind.Extension, {)
      endClassOrMixinBody(DeclarationKind.Extension, 0, {, })
-    endExtensionDeclaration(extension, type, on, })
+    endExtensionDeclaration(extension, type, on, null, null, })
  endTopLevelDeclaration()
 endCompilationUnit(2, )
--- a/pkg/front_end/parser_testcases/extension_type.dart.intertwined.expect
+++ b/pkg/front_end/parser_testcases/extension_type.dart.intertwined.expect
@ -43,11 +43,12 @@ parseUnit(class)
        listener: handleIdentifier(A, typeReference)
        listener: handleNoTypeArguments({)
        listener: handleType(A, null)
+        listener: handleExtensionShowHide(null, 0, null, 0)
        parseClassOrMixinOrExtensionBody(A, DeclarationKind.Extension, E)
          listener: beginClassOrMixinBody(DeclarationKind.Extension, {)
          notEofOrValue(}, })
          listener: endClassOrMixinBody(DeclarationKind.Extension, 0, {, })
-        listener: endExtensionDeclaration(extension, type, on, })
+        listener: endExtensionDeclaration(extension, type, on, null, null, })
  listener: endTopLevelDeclaration()
  reportAllErrorTokens(class)
  listener: endCompilationUnit(2, )
--- a/pkg/front_end/parser_testcases/extensions/covariant.dart.expect
+++ b/pkg/front_end/parser_testcases/extensions/covariant.dart.expect
@ -45,6 +45,7 @@ beginCompilationUnit(class)
      handleIdentifier(C, typeReference)
      handleNoTypeArguments({)
      handleType(C, null)
+      handleExtensionShowHide(null, 0, null, 0)
      beginClassOrMixinBody(DeclarationKind.Extension, {)
        beginMetadataStar(addChild)
        endMetadataStar(0)
@ -72,6 +73,6 @@ beginCompilationUnit(class)
          endExtensionMethod(null, addChild, (, null, })
        endMember()
      endClassOrMixinBody(DeclarationKind.Extension, 1, {, })
-    endExtensionDeclaration(extension, null, on, })
+    endExtensionDeclaration(extension, null, on, null, null, })
  endTopLevelDeclaration()
 endCompilationUnit(3, )
--- a/pkg/front_end/parser_testcases/extensions/covariant.dart.intertwined.expect
+++ b/pkg/front_end/parser_testcases/extensions/covariant.dart.intertwined.expect
@ -74,6 +74,7 @@ parseUnit(class)
        listener: handleIdentifier(C, typeReference)
        listener: handleNoTypeArguments({)
        listener: handleType(C, null)
+        listener: handleExtensionShowHide(null, 0, null, 0)
        parseClassOrMixinOrExtensionBody(C, DeclarationKind.Extension, null)
          listener: beginClassOrMixinBody(DeclarationKind.Extension, {)
          notEofOrValue(}, addChild)
@ -124,7 +125,7 @@ parseUnit(class)
            listener: endMember()
          notEofOrValue(}, })
          listener: endClassOrMixinBody(DeclarationKind.Extension, 1, {, })
-        listener: endExtensionDeclaration(extension, null, on, })
+        listener: endExtensionDeclaration(extension, null, on, null, null, })
  listener: endTopLevelDeclaration()
  reportAllErrorTokens(class)
  listener: endCompilationUnit(3, )
--- a/pkg/front_end/parser_testcases/extensions/not_covariant.dart.expect
+++ b/pkg/front_end/parser_testcases/extensions/not_covariant.dart.expect
@ -39,6 +39,7 @@ beginCompilationUnit(class)
      handleIdentifier(C, typeReference)
      handleNoTypeArguments({)
      handleType(C, null)
+      handleExtensionShowHide(null, 0, null, 0)
      beginClassOrMixinBody(DeclarationKind.Extension, {)
        beginMetadataStar(addChild)
        endMetadataStar(0)
@ -65,6 +66,6 @@ beginCompilationUnit(class)
          endExtensionMethod(null, addChild, (, null, })
        endMember()
      endClassOrMixinBody(DeclarationKind.Extension, 1, {, })
-    endExtensionDeclaration(extension, null, on, })
+    endExtensionDeclaration(extension, null, on, null, null, })
  endTopLevelDeclaration()
 endCompilationUnit(3, )
--- a/pkg/front_end/parser_testcases/extensions/not_covariant.dart.intertwined.expect
+++ b/pkg/front_end/parser_testcases/extensions/not_covariant.dart.intertwined.expect
@ -74,6 +74,7 @@ parseUnit(class)
        listener: handleIdentifier(C, typeReference)
        listener: handleNoTypeArguments({)
        listener: handleType(C, null)
+        listener: handleExtensionShowHide(null, 0, null, 0)
        parseClassOrMixinOrExtensionBody(C, DeclarationKind.Extension, null)
          listener: beginClassOrMixinBody(DeclarationKind.Extension, {)
          notEofOrValue(}, addChild)
@ -122,7 +123,7 @@ parseUnit(class)
            listener: endMember()
          notEofOrValue(}, })
          listener: endClassOrMixinBody(DeclarationKind.Extension, 1, {, })
-        listener: endExtensionDeclaration(extension, null, on, })
+        listener: endExtensionDeclaration(extension, null, on, null, null, })
  listener: endTopLevelDeclaration()
  reportAllErrorTokens(class)
  listener: endCompilationUnit(3, )
--- a/pkg/front_end/parser_testcases/extensions/static.dart.expect
+++ b/pkg/front_end/parser_testcases/extensions/static.dart.expect
@ -39,6 +39,7 @@ beginCompilationUnit(class)
      handleIdentifier(C, typeReference)
      handleNoTypeArguments({)
      handleType(C, null)
+      handleExtensionShowHide(null, 0, null, 0)
      beginClassOrMixinBody(DeclarationKind.Extension, {)
        beginMetadataStar(static)
        endMetadataStar(0)
@ -65,6 +66,6 @@ beginCompilationUnit(class)
          endExtensionMethod(null, static, (, null, })
        endMember()
      endClassOrMixinBody(DeclarationKind.Extension, 1, {, })
-    endExtensionDeclaration(extension, null, on, })
+    endExtensionDeclaration(extension, null, on, null, null, })
  endTopLevelDeclaration()
 endCompilationUnit(3, )
--- a/pkg/front_end/parser_testcases/extensions/static.dart.intertwined.expect
+++ b/pkg/front_end/parser_testcases/extensions/static.dart.intertwined.expect
@ -74,6 +74,7 @@ parseUnit(class)
        listener: handleIdentifier(C, typeReference)
        listener: handleNoTypeArguments({)
        listener: handleType(C, null)
+        listener: handleExtensionShowHide(null, 0, null, 0)
        parseClassOrMixinOrExtensionBody(C, DeclarationKind.Extension, null)
          listener: beginClassOrMixinBody(DeclarationKind.Extension, {)
          notEofOrValue(}, static)
@ -121,7 +122,7 @@ parseUnit(class)
            listener: endMember()
          notEofOrValue(}, })
          listener: endClassOrMixinBody(DeclarationKind.Extension, 1, {, })
-        listener: endExtensionDeclaration(extension, null, on, })
+        listener: endExtensionDeclaration(extension, null, on, null, null, })
  listener: endTopLevelDeclaration()
  reportAllErrorTokens(class)
  listener: endCompilationUnit(3, )
--- a/pkg/front_end/parser_testcases/extensions/static_covariant.dart.expect
+++ b/pkg/front_end/parser_testcases/extensions/static_covariant.dart.expect
@ -45,6 +45,7 @@ beginCompilationUnit(class)
      handleIdentifier(C, typeReference)
      handleNoTypeArguments({)
      handleType(C, null)
+      handleExtensionShowHide(null, 0, null, 0)
      beginClassOrMixinBody(DeclarationKind.Extension, {)
        beginMetadataStar(static)
        endMetadataStar(0)
@ -72,6 +73,6 @@ beginCompilationUnit(class)
          endExtensionMethod(null, static, (, null, })
        endMember()
      endClassOrMixinBody(DeclarationKind.Extension, 1, {, })
-    endExtensionDeclaration(extension, null, on, })
+    endExtensionDeclaration(extension, null, on, null, null, })
  endTopLevelDeclaration()
 endCompilationUnit(3, )
--- a/pkg/front_end/parser_testcases/extensions/static_covariant.dart.intertwined.expect
+++ b/pkg/front_end/parser_testcases/extensions/static_covariant.dart.intertwined.expect
@ -74,6 +74,7 @@ parseUnit(class)
        listener: handleIdentifier(C, typeReference)
        listener: handleNoTypeArguments({)
        listener: handleType(C, null)
+        listener: handleExtensionShowHide(null, 0, null, 0)
        parseClassOrMixinOrExtensionBody(C, DeclarationKind.Extension, null)
          listener: beginClassOrMixinBody(DeclarationKind.Extension, {)
          notEofOrValue(}, static)
@ -123,7 +124,7 @@ parseUnit(class)
            listener: endMember()
          notEofOrValue(}, })
          listener: endClassOrMixinBody(DeclarationKind.Extension, 1, {, })
-        listener: endExtensionDeclaration(extension, null, on, })
+        listener: endExtensionDeclaration(extension, null, on, null, null, })
  listener: endTopLevelDeclaration()
  reportAllErrorTokens(class)
  listener: endCompilationUnit(3, )
--- a/pkg/front_end/parser_testcases/no-triple-shift/triple_shift_not_triple_shift.dart.expect
+++ b/pkg/front_end/parser_testcases/no-triple-shift/triple_shift_not_triple_shift.dart.expect
@ -7,6 +7,7 @@ beginCompilationUnit(extension)
      handleIdentifier(Symbol, typeReference)
      handleNoTypeArguments({)
      handleType(Symbol, null)
+      handleExtensionShowHide(null, 0, null, 0)
      beginClassOrMixinBody(DeclarationKind.Extension, {)
        beginMetadataStar(String)
        endMetadataStar(0)
@ -59,7 +60,7 @@ beginCompilationUnit(extension)
          endExtensionMethod(null, String, (, null, ;)
        endMember()
      endClassOrMixinBody(DeclarationKind.Extension, 2, {, })
-    endExtensionDeclaration(extension, null, on, })
+    endExtensionDeclaration(extension, null, on, null, null, })
  endTopLevelDeclaration(void)
  beginMetadataStar(void)
  endMetadataStar(0)
--- a/pkg/front_end/parser_testcases/no-triple-shift/triple_shift_not_triple_shift.dart.intertwined.expect
+++ b/pkg/front_end/parser_testcases/no-triple-shift/triple_shift_not_triple_shift.dart.intertwined.expect
@ -15,6 +15,7 @@ parseUnit(extension)
        listener: handleIdentifier(Symbol, typeReference)
        listener: handleNoTypeArguments({)
        listener: handleType(Symbol, null)
+        listener: handleExtensionShowHide(null, 0, null, 0)
        parseClassOrMixinOrExtensionBody(Symbol, DeclarationKind.Extension, null)
          listener: beginClassOrMixinBody(DeclarationKind.Extension, {)
          notEofOrValue(}, String)
@ -122,7 +123,7 @@ parseUnit(extension)
            listener: endMember()
          notEofOrValue(}, })
          listener: endClassOrMixinBody(DeclarationKind.Extension, 2, {, })
-        listener: endExtensionDeclaration(extension, null, on, })
+        listener: endExtensionDeclaration(extension, null, on, null, null, })
  listener: endTopLevelDeclaration(void)
  parseTopLevelDeclarationImpl(}, Instance of 'DirectiveContext')
    parseMetadataStar(})
--- a/pkg/front_end/test/parser_test_listener.dart
+++ b/pkg/front_end/test/parser_test_listener.dart
@ -222,6 +222,18 @@ class ParserTestListener implements Listener {
        '$interfacesCount)');
  }

+  @override
+  void handleExtensionShowHide(Token? showKeyword, int showElementCount,
+      Token? hideKeyword, int hideElementCount) {
+    seen(showKeyword);
+    seen(hideKeyword);
+    doPrint('handleExtensionShowHide('
+        '$showKeyword, '
+        '$showElementCount, '
+        '$hideKeyword, '
+        '$hideElementCount)');
+  }
+
  @override
  void handleClassHeader(Token begin, Token classKeyword, Token? nativeToken) {
    seen(begin);
@ -300,16 +312,20 @@ class ParserTestListener implements Listener {

  @override
  void endExtensionDeclaration(Token extensionKeyword, Token? typeKeyword,
-      Token onKeyword, Token endToken) {
+      Token onKeyword, Token? showKeyword, Token? hideKeyword, Token endToken) {
    indent--;
    seen(extensionKeyword);
    seen(typeKeyword);
    seen(onKeyword);
+    seen(showKeyword);
+    seen(hideKeyword);
    seen(endToken);
    doPrint('endExtensionDeclaration('
        '$extensionKeyword, '
        '$typeKeyword, '
        '$onKeyword, '
+        '$showKeyword, '
+        '$hideKeyword, '
        '$endToken)');
  }

@ -2083,6 +2099,13 @@ class ParserTestListener implements Listener {
    doPrint('handleIdentifier(' '$token, ' '$context)');
  }

+  @override
+  void handleShowHideIdentifier(Token? modifier, Token identifier) {
+    seen(modifier);
+    seen(identifier);
+    doPrint('handleShowHideIdentifier(' '$modifier, ' '$identifier)');
+  }
+
  @override
  void handleIndexedExpression(
      Token? question, Token openSquareBracket, Token closeSquareBracket) {
--- a/pkg/front_end/test/spell_checking_list_common.txt
+++ b/pkg/front_end/test/spell_checking_list_common.txt
@ -3156,6 +3156,7 @@ unalias
 unaliased
 unaliasing
 unambiguous
+unambiguously
 unary
 unassignable
 unassigned
--- a/pkg/front_end/testcases/extension_types/basic_show.dart
+++ b/pkg/front_end/testcases/extension_types/basic_show.dart
@ -0,0 +1,17 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+extension E1 on int show num {} // Ok.
+
+extension E2 on int {} // Ok.
+
+extension E3 on int show {} // Error.
+
+extension E4 on int show num, Comparable {} // Ok.
+
+extension E5 on int show num, {} // Error.
+
+extension E6 on int show , num {} // Error.
+
+main() {}
--- a/pkg/front_end/testcases/extension_types/basic_show.dart.strong.expect
+++ b/pkg/front_end/testcases/extension_types/basic_show.dart.strong.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:9:26: Error: Expected an identifier, but got '{'.
+// Try inserting an identifier before '{'.
+// extension E3 on int show {} // Error.
+//                          ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:13:31: Error: Expected an identifier, but got '{'.
+// Try inserting an identifier before '{'.
+// extension E5 on int show num, {} // Error.
+//                               ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:26: Error: Expected an identifier, but got ','.
+// Try inserting an identifier before ','.
+// extension E6 on int show , num {} // Error.
+//                          ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:28: Error: A extension declaration must have a body, even if it is empty.
+// Try adding an empty body.
+// extension E6 on int show , num {} // Error.
+//                            ^^^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:28: Error: A function declaration needs an explicit list of parameters.
+// Try adding a parameter list to the function declaration.
+// extension E6 on int show , num {} // Error.
+//                            ^^^
+//
+import self as self;
+import "dart:core" as core;
+
+extension E1 on core::int {
+}
+extension E2 on core::int {
+}
+extension E3 on core::int {
+}
+extension E4 on core::int {
+}
+extension E5 on core::int {
+}
+extension E6 on core::int {
+}
+static method num() → dynamic {}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/basic_show.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/basic_show.dart.strong.transformed.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:9:26: Error: Expected an identifier, but got '{'.
+// Try inserting an identifier before '{'.
+// extension E3 on int show {} // Error.
+//                          ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:13:31: Error: Expected an identifier, but got '{'.
+// Try inserting an identifier before '{'.
+// extension E5 on int show num, {} // Error.
+//                               ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:26: Error: Expected an identifier, but got ','.
+// Try inserting an identifier before ','.
+// extension E6 on int show , num {} // Error.
+//                          ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:28: Error: A extension declaration must have a body, even if it is empty.
+// Try adding an empty body.
+// extension E6 on int show , num {} // Error.
+//                            ^^^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:28: Error: A function declaration needs an explicit list of parameters.
+// Try adding a parameter list to the function declaration.
+// extension E6 on int show , num {} // Error.
+//                            ^^^
+//
+import self as self;
+import "dart:core" as core;
+
+extension E1 on core::int {
+}
+extension E2 on core::int {
+}
+extension E3 on core::int {
+}
+extension E4 on core::int {
+}
+extension E5 on core::int {
+}
+extension E6 on core::int {
+}
+static method num() → dynamic {}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/basic_show.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/extension_types/basic_show.dart.textual_outline.expect
@ -0,0 +1,8 @@
+extension E1 on int show num {}
+extension E2 on int {}
+extension E3 on int show {}
+extension E4 on int show num, Comparable {}
+extension E5 on int show num, {}
+extension E6 on int show , {}
+num (){}
+main() {}
--- a/pkg/front_end/testcases/extension_types/basic_show.dart.weak.expect
+++ b/pkg/front_end/testcases/extension_types/basic_show.dart.weak.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:9:26: Error: Expected an identifier, but got '{'.
+// Try inserting an identifier before '{'.
+// extension E3 on int show {} // Error.
+//                          ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:13:31: Error: Expected an identifier, but got '{'.
+// Try inserting an identifier before '{'.
+// extension E5 on int show num, {} // Error.
+//                               ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:26: Error: Expected an identifier, but got ','.
+// Try inserting an identifier before ','.
+// extension E6 on int show , num {} // Error.
+//                          ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:28: Error: A extension declaration must have a body, even if it is empty.
+// Try adding an empty body.
+// extension E6 on int show , num {} // Error.
+//                            ^^^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:28: Error: A function declaration needs an explicit list of parameters.
+// Try adding a parameter list to the function declaration.
+// extension E6 on int show , num {} // Error.
+//                            ^^^
+//
+import self as self;
+import "dart:core" as core;
+
+extension E1 on core::int {
+}
+extension E2 on core::int {
+}
+extension E3 on core::int {
+}
+extension E4 on core::int {
+}
+extension E5 on core::int {
+}
+extension E6 on core::int {
+}
+static method num() → dynamic {}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/basic_show.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/extension_types/basic_show.dart.weak.outline.expect
@ -0,0 +1,48 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:9:26: Error: Expected an identifier, but got '{'.
+// Try inserting an identifier before '{'.
+// extension E3 on int show {} // Error.
+//                          ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:13:31: Error: Expected an identifier, but got '{'.
+// Try inserting an identifier before '{'.
+// extension E5 on int show num, {} // Error.
+//                               ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:26: Error: Expected an identifier, but got ','.
+// Try inserting an identifier before ','.
+// extension E6 on int show , num {} // Error.
+//                          ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:28: Error: A extension declaration must have a body, even if it is empty.
+// Try adding an empty body.
+// extension E6 on int show , num {} // Error.
+//                            ^^^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:28: Error: A function declaration needs an explicit list of parameters.
+// Try adding a parameter list to the function declaration.
+// extension E6 on int show , num {} // Error.
+//                            ^^^
+//
+import self as self;
+import "dart:core" as core;
+
+extension E1 on core::int {
+}
+extension E2 on core::int {
+}
+extension E3 on core::int {
+}
+extension E4 on core::int {
+}
+extension E5 on core::int {
+}
+extension E6 on core::int {
+}
+static method num() → dynamic
+  ;
+static method main() → dynamic
+  ;
--- a/pkg/front_end/testcases/extension_types/basic_show.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/basic_show.dart.weak.transformed.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:9:26: Error: Expected an identifier, but got '{'.
+// Try inserting an identifier before '{'.
+// extension E3 on int show {} // Error.
+//                          ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:13:31: Error: Expected an identifier, but got '{'.
+// Try inserting an identifier before '{'.
+// extension E5 on int show num, {} // Error.
+//                               ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:26: Error: Expected an identifier, but got ','.
+// Try inserting an identifier before ','.
+// extension E6 on int show , num {} // Error.
+//                          ^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:28: Error: A extension declaration must have a body, even if it is empty.
+// Try adding an empty body.
+// extension E6 on int show , num {} // Error.
+//                            ^^^
+//
+// pkg/front_end/testcases/extension_types/basic_show.dart:15:28: Error: A function declaration needs an explicit list of parameters.
+// Try adding a parameter list to the function declaration.
+// extension E6 on int show , num {} // Error.
+//                            ^^^
+//
+import self as self;
+import "dart:core" as core;
+
+extension E1 on core::int {
+}
+extension E2 on core::int {
+}
+extension E3 on core::int {
+}
+extension E4 on core::int {
+}
+extension E5 on core::int {
+}
+extension E6 on core::int {
+}
+static method num() → dynamic {}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart
+++ b/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart
@ -0,0 +1,13 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class A {
+  void mixin() {}
+  void as() {}
+}
+
+extension type E1 on A show mixin, as {}
+extension type E2 on A hide mixin, as {}
+
+main() {}
--- a/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart.strong.expect
+++ b/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart.strong.expect
@ -0,0 +1,16 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  method mixin() → void {}
+  method as() → void {}
+}
+extension type E1 on self::A {
+}
+extension type E2 on self::A {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart.strong.transformed.expect
@ -0,0 +1,16 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  method mixin() → void {}
+  method as() → void {}
+}
+extension type E1 on self::A {
+}
+extension type E2 on self::A {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart.textual_outline.expect
@ -0,0 +1,7 @@
+class A {
+  void mixin() {}
+  void as() {}
+}
+extension type E1 on A show mixin, as {}
+extension type E2 on A hide mixin, as {}
+main() {}
--- a/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart.weak.expect
+++ b/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart.weak.expect
@ -0,0 +1,16 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  method mixin() → void {}
+  method as() → void {}
+}
+extension type E1 on self::A {
+}
+extension type E2 on self::A {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart.weak.outline.expect
@ -0,0 +1,18 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    ;
+  method mixin() → void
+    ;
+  method as() → void
+    ;
+}
+extension type E1 on self::A {
+}
+extension type E2 on self::A {
+}
+static method main() → dynamic
+  ;
--- a/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/keyword_in_show_hide_element.dart.weak.transformed.expect
@ -0,0 +1,16 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  method mixin() → void {}
+  method as() → void {}
+}
+extension type E1 on self::A {
+}
+extension type E2 on self::A {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart
+++ b/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart
@ -0,0 +1,33 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class I1<X, Y> {}
+
+class I2<X, Y, Z> {}
+
+class A {}
+
+class B extends A implements I1<int, int> {
+  void methodB() {}
+  void methodB2() {}
+  int get getterB => throw 42;
+  void set setterB(int value) {}
+  B operator *(B other) => throw 42;
+}
+
+class C extends B {}
+
+class D extends C implements I2<int, int, int> {
+  void methodD() {}
+  int get getterD => throw 42;
+  void set setterD(int value) {}
+  D operator +(D other) => throw 42;
+}
+
+extension type E on D
+  show C, I2<int, int, int>, methodD, get getterD, set setterD, operator +
+  hide A, I1<int, int>, methodB, methodB2, get getterB, set setterB, operator *
+  {}
+
+main() {}
--- a/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart.strong.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart.strong.expect
@ -0,0 +1,50 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class I1<X extends core::Object? = dynamic, Y extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::I1<self::I1::X%, self::I1::Y%>
+    : super core::Object::•()
+    ;
+}
+class I2<X extends core::Object? = dynamic, Y extends core::Object? = dynamic, Z extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::I2<self::I2::X%, self::I2::Y%, self::I2::Z%>
+    : super core::Object::•()
+    ;
+}
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+}
+class B extends self::A implements self::I1<core::int, core::int> {
+  synthetic constructor •() → self::B
+    : super self::A::•()
+    ;
+  method methodB() → void {}
+  method methodB2() → void {}
+  get getterB() → core::int
+    return throw 42;
+  set setterB(core::int value) → void {}
+  operator *(self::B other) → self::B
+    return throw 42;
+}
+class C extends self::B {
+  synthetic constructor •() → self::C
+    : super self::B::•()
+    ;
+}
+class D extends self::C implements self::I2<core::int, core::int, core::int> {
+  synthetic constructor •() → self::D
+    : super self::C::•()
+    ;
+  method methodD() → void {}
+  get getterD() → core::int
+    return throw 42;
+  set setterD(core::int value) → void {}
+  operator +(self::D other) → self::D
+    return throw 42;
+}
+extension type E on self::D {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart.strong.transformed.expect
@ -0,0 +1,50 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class I1<X extends core::Object? = dynamic, Y extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::I1<self::I1::X%, self::I1::Y%>
+    : super core::Object::•()
+    ;
+}
+class I2<X extends core::Object? = dynamic, Y extends core::Object? = dynamic, Z extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::I2<self::I2::X%, self::I2::Y%, self::I2::Z%>
+    : super core::Object::•()
+    ;
+}
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+}
+class B extends self::A implements self::I1<core::int, core::int> {
+  synthetic constructor •() → self::B
+    : super self::A::•()
+    ;
+  method methodB() → void {}
+  method methodB2() → void {}
+  get getterB() → core::int
+    return throw 42;
+  set setterB(core::int value) → void {}
+  operator *(self::B other) → self::B
+    return throw 42;
+}
+class C extends self::B {
+  synthetic constructor •() → self::C
+    : super self::B::•()
+    ;
+}
+class D extends self::C implements self::I2<core::int, core::int, core::int> {
+  synthetic constructor •() → self::D
+    : super self::C::•()
+    ;
+  method methodD() → void {}
+  get getterD() → core::int
+    return throw 42;
+  set setterD(core::int value) → void {}
+  operator +(self::D other) → self::D
+    return throw 42;
+}
+extension type E on self::D {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart.textual_outline.expect
@ -0,0 +1,19 @@
+class I1<X, Y> {}
+class I2<X, Y, Z> {}
+class A {}
+class B extends A implements I1<int, int> {
+  void methodB() {}
+  void methodB2() {}
+  int get getterB => throw 42;
+  void set setterB(int value) {}
+  B operator *(B other) => throw 42;
+}
+class C extends B {}
+class D extends C implements I2<int, int, int> {
+  void methodD() {}
+  int get getterD => throw 42;
+  void set setterD(int value) {}
+  D operator +(D other) => throw 42;
+}
+extension type E on D show C, I2<int, int, int>, methodD, get getterD, set setterD, operator + hide A, I1<int, int>, methodB, methodB2, get getterB, set setterB, operator * {}
+main() {}
--- a/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart.weak.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart.weak.expect
@ -0,0 +1,50 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class I1<X extends core::Object? = dynamic, Y extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::I1<self::I1::X%, self::I1::Y%>
+    : super core::Object::•()
+    ;
+}
+class I2<X extends core::Object? = dynamic, Y extends core::Object? = dynamic, Z extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::I2<self::I2::X%, self::I2::Y%, self::I2::Z%>
+    : super core::Object::•()
+    ;
+}
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+}
+class B extends self::A implements self::I1<core::int, core::int> {
+  synthetic constructor •() → self::B
+    : super self::A::•()
+    ;
+  method methodB() → void {}
+  method methodB2() → void {}
+  get getterB() → core::int
+    return throw 42;
+  set setterB(core::int value) → void {}
+  operator *(self::B other) → self::B
+    return throw 42;
+}
+class C extends self::B {
+  synthetic constructor •() → self::C
+    : super self::B::•()
+    ;
+}
+class D extends self::C implements self::I2<core::int, core::int, core::int> {
+  synthetic constructor •() → self::D
+    : super self::C::•()
+    ;
+  method methodD() → void {}
+  get getterD() → core::int
+    return throw 42;
+  set setterD(core::int value) → void {}
+  operator +(self::D other) → self::D
+    return throw 42;
+}
+extension type E on self::D {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart.weak.outline.expect
@ -0,0 +1,50 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class I1<X extends core::Object? = dynamic, Y extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::I1<self::I1::X%, self::I1::Y%>
+    ;
+}
+class I2<X extends core::Object? = dynamic, Y extends core::Object? = dynamic, Z extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::I2<self::I2::X%, self::I2::Y%, self::I2::Z%>
+    ;
+}
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    ;
+}
+class B extends self::A implements self::I1<core::int, core::int> {
+  synthetic constructor •() → self::B
+    ;
+  method methodB() → void
+    ;
+  method methodB2() → void
+    ;
+  get getterB() → core::int
+    ;
+  set setterB(core::int value) → void
+    ;
+  operator *(self::B other) → self::B
+    ;
+}
+class C extends self::B {
+  synthetic constructor •() → self::C
+    ;
+}
+class D extends self::C implements self::I2<core::int, core::int, core::int> {
+  synthetic constructor •() → self::D
+    ;
+  method methodD() → void
+    ;
+  get getterD() → core::int
+    ;
+  set setterD(core::int value) → void
+    ;
+  operator +(self::D other) → self::D
+    ;
+}
+extension type E on self::D {
+}
+static method main() → dynamic
+  ;
--- a/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_and_hide.dart.weak.transformed.expect
@ -0,0 +1,50 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class I1<X extends core::Object? = dynamic, Y extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::I1<self::I1::X%, self::I1::Y%>
+    : super core::Object::•()
+    ;
+}
+class I2<X extends core::Object? = dynamic, Y extends core::Object? = dynamic, Z extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::I2<self::I2::X%, self::I2::Y%, self::I2::Z%>
+    : super core::Object::•()
+    ;
+}
+class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+}
+class B extends self::A implements self::I1<core::int, core::int> {
+  synthetic constructor •() → self::B
+    : super self::A::•()
+    ;
+  method methodB() → void {}
+  method methodB2() → void {}
+  get getterB() → core::int
+    return throw 42;
+  set setterB(core::int value) → void {}
+  operator *(self::B other) → self::B
+    return throw 42;
+}
+class C extends self::B {
+  synthetic constructor •() → self::C
+    : super self::B::•()
+    ;
+}
+class D extends self::C implements self::I2<core::int, core::int, core::int> {
+  synthetic constructor •() → self::D
+    : super self::C::•()
+    ;
+  method methodD() → void {}
+  get getterD() → core::int
+    return throw 42;
+  set setterD(core::int value) → void {}
+  operator +(self::D other) → self::D
+    return throw 42;
+}
+extension type E on self::D {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/simple_show_hide.dart.strong.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_hide.dart.strong.expect
@ -2,99 +2,6 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:5:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E1 on int show num {}
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:5:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E1 on int show num {}
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E2 on int show num hide ceil {}
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:26: Error: Expected ';' after this.
-// extension E2 on int show num hide ceil {}
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:26: Error: 'num' is already declared in this scope.
-// extension E2 on int show num hide ceil {}
-//                          ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:5:26: Context: Previous declaration of 'num'.
-// extension E1 on int show num {}
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:35: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E2 on int show num hide ceil {}
-//                                   ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:21:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E3 on int hide isEven {}
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:21:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E3 on int hide isEven {}
-//                          ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:25: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension type MyInt on int show num, isEven hide floor {
-//                         ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:39: Error: Expected ';' after this.
-// extension type MyInt on int show num, isEven hide floor {
-//                                       ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:34: Error: 'num' is already declared in this scope.
-// extension type MyInt on int show num, isEven hide floor {
-//                                  ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:26: Context: Previous declaration of 'num'.
-// extension E2 on int show num hide ceil {}
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:39: Error: 'isEven' is already declared in this scope.
-// extension type MyInt on int show num, isEven hide floor {
-//                                       ^^^^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:21:26: Context: Previous declaration of 'isEven'.
-// extension E3 on int hide isEven {}
-//                          ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:51: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension type MyInt on int show num, isEven hide floor {
-//                                                   ^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:5:21: Error: Type 'show' not found.
-// extension E1 on int show num {}
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:21: Error: Type 'show' not found.
-// extension E2 on int show num hide ceil {}
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:30: Error: Type 'hide' not found.
-// extension E2 on int show num hide ceil {}
-//                              ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:21:21: Error: Type 'hide' not found.
-// extension E3 on int hide isEven {}
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:29: Error: Type 'show' not found.
-// extension type MyInt on int show num, isEven hide floor {
-//                             ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:46: Error: Type 'hide' not found.
-// extension type MyInt on int show num, isEven hide floor {
-//                                              ^^^^
-//
 // pkg/front_end/testcases/extension_types/simple_show_hide.dart:9:3: Error: Undefined name 'e2'.
 //   e2.floor(); // Ok.
 //   ^^
@ -109,10 +16,6 @@ library /*isNonNullableByDefault*/;
 //   e1.isEven; // Error.
 //      ^^^^^^
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:21: Error: 'show' isn't a type.
-// extension E2 on int show num hide ceil {}
-//                     ^^^^
-//
 // pkg/front_end/testcases/extension_types/simple_show_hide.dart:16:6: Error: The method 'ceil' isn't defined for the extension 'E2'.
 // Try correcting the name to the name of an existing method, or defining a method name 'ceil'.
 //   e2.ceil(); // Error.
@ -138,28 +41,6 @@ library /*isNonNullableByDefault*/;
 //   e3.isEven; // Error.
 //      ^^^^^^
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:29: Error: 'show' isn't a type.
-// extension type MyInt on int show num, isEven hide floor {
-//                             ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:29:7: Error: Expected ';' after this.
-//   int get twice => 2 * this;
-//       ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:29:17: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-//   int get twice => 2 * this;
-//                 ^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:29:24: Error: Expected identifier, but got 'this'.
-//   int get twice => 2 * this;
-//                        ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:34:5: Error: The getter 'twice' isn't defined for the extension 'MyInt'.
-// Try correcting the name to the name of an existing getter, or defining a getter or field named 'twice'.
-//   m.twice; // OK, in the extension type.
-//     ^^^^^
-//
 // pkg/front_end/testcases/extension_types/simple_show_hide.dart:35:5: Error: The getter 'isEven' isn't defined for the extension 'MyInt'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'isEven'.
 //   m.isEven; // OK, a shown instance member.
@ -185,8 +66,8 @@ extension E2 on core::int {
 extension E3 on core::int {
 }
 extension type MyInt on core::int {
+  get twice = self::MyInt|get#twice;
 }
-static method num() → invalid-type {}
 static method test1(self::E1 e1) → dynamic {
  invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide.dart:8:6: Error: The method 'ceil' isn't defined for the extension 'E1'.
 Try correcting the name to the name of an existing method, or defining a method name 'ceil'.
@ -200,7 +81,6 @@ Try correcting the name to the name of an existing getter, or defining a getter
  e1.isEven; // Error.
     ^^^^^^" in e1{<unresolved>}.isEven;
 }
-static method ceil() → invalid-type {}
 static method test2(self::E2 e2) → dynamic {
  invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide.dart:16:6: Error: The method 'ceil' isn't defined for the extension 'E2'.
 Try correcting the name to the name of an existing method, or defining a method name 'ceil'.
@ -215,7 +95,6 @@ Try correcting the name to the name of an existing getter, or defining a getter
  e2.isEven; // Error.
     ^^^^^^" in e2{<unresolved>}.isEven;
 }
-static method isEven() → invalid-type {}
 static method test3(self::E3 e3) → dynamic {
  invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide.dart:24:6: Error: The getter 'isOdd' isn't defined for the extension 'E3'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'isOdd'.
@ -226,19 +105,11 @@ Try correcting the name to the name of an existing getter, or defining a getter
  e3.isEven; // Error.
     ^^^^^^" in e3{<unresolved>}.isEven;
 }
-static method floor() → invalid-type {
-  core::int get;
-  function twice() → core::double
-    return 2.{core::num::*}(invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide.dart:29:24: Error: Expected identifier, but got 'this'.
-  int get twice => 2 * this;
-                       ^^^^"){(core::num) → core::double};
-}
+static method MyInt|get#twice(lowered final core::int #this) → core::int
+  return 2.{core::num::*}(#this){(core::num) → core::int};
 static method test() → dynamic {
  self::MyInt m = 42;
-  invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide.dart:34:5: Error: The getter 'twice' isn't defined for the extension 'MyInt'.
-Try correcting the name to the name of an existing getter, or defining a getter or field named 'twice'.
-  m.twice; // OK, in the extension type.
-    ^^^^^" in m{<unresolved>}.twice;
+  self::MyInt|get#twice(m);
  invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide.dart:35:5: Error: The getter 'isEven' isn't defined for the extension 'MyInt'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'isEven'.
  m.isEven; // OK, a shown instance member.
--- a/pkg/front_end/testcases/extension_types/simple_show_hide.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_hide.dart.textual_outline.expect
@ -1,15 +1,11 @@
-extension E1 on int {}
-show num (){}
+extension E1 on int show num {}
 test1(E1 e1) {}
-extension E2 on int {}
-show num ;
-hide ceil (){}
+extension E2 on int show num hide ceil {}
 test2(E2 e2) {}
-extension E3 on int {}
-hide isEven (){}
+extension E3 on int hide isEven {}
 test3(E3 e3) {}
-extension type MyInt on int {}
-show num, isEven ;
-hide floor (){}
+extension type MyInt on int show num, isEven hide floor {
+  int get twice => 2 * this;
+}
 test() {}
 main() {}
--- a/pkg/front_end/testcases/extension_types/simple_show_hide.dart.weak.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_hide.dart.weak.expect
@ -2,99 +2,6 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:5:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E1 on int show num {}
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:5:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E1 on int show num {}
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E2 on int show num hide ceil {}
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:26: Error: Expected ';' after this.
-// extension E2 on int show num hide ceil {}
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:26: Error: 'num' is already declared in this scope.
-// extension E2 on int show num hide ceil {}
-//                          ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:5:26: Context: Previous declaration of 'num'.
-// extension E1 on int show num {}
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:35: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E2 on int show num hide ceil {}
-//                                   ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:21:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E3 on int hide isEven {}
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:21:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E3 on int hide isEven {}
-//                          ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:25: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension type MyInt on int show num, isEven hide floor {
-//                         ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:39: Error: Expected ';' after this.
-// extension type MyInt on int show num, isEven hide floor {
-//                                       ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:34: Error: 'num' is already declared in this scope.
-// extension type MyInt on int show num, isEven hide floor {
-//                                  ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:26: Context: Previous declaration of 'num'.
-// extension E2 on int show num hide ceil {}
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:39: Error: 'isEven' is already declared in this scope.
-// extension type MyInt on int show num, isEven hide floor {
-//                                       ^^^^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:21:26: Context: Previous declaration of 'isEven'.
-// extension E3 on int hide isEven {}
-//                          ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:51: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension type MyInt on int show num, isEven hide floor {
-//                                                   ^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:5:21: Error: Type 'show' not found.
-// extension E1 on int show num {}
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:21: Error: Type 'show' not found.
-// extension E2 on int show num hide ceil {}
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:30: Error: Type 'hide' not found.
-// extension E2 on int show num hide ceil {}
-//                              ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:21:21: Error: Type 'hide' not found.
-// extension E3 on int hide isEven {}
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:29: Error: Type 'show' not found.
-// extension type MyInt on int show num, isEven hide floor {
-//                             ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:46: Error: Type 'hide' not found.
-// extension type MyInt on int show num, isEven hide floor {
-//                                              ^^^^
-//
 // pkg/front_end/testcases/extension_types/simple_show_hide.dart:9:3: Error: Undefined name 'e2'.
 //   e2.floor(); // Ok.
 //   ^^
@ -109,10 +16,6 @@ library /*isNonNullableByDefault*/;
 //   e1.isEven; // Error.
 //      ^^^^^^
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:21: Error: 'show' isn't a type.
-// extension E2 on int show num hide ceil {}
-//                     ^^^^
-//
 // pkg/front_end/testcases/extension_types/simple_show_hide.dart:16:6: Error: The method 'ceil' isn't defined for the extension 'E2'.
 // Try correcting the name to the name of an existing method, or defining a method name 'ceil'.
 //   e2.ceil(); // Error.
@ -138,28 +41,6 @@ library /*isNonNullableByDefault*/;
 //   e3.isEven; // Error.
 //      ^^^^^^
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:29: Error: 'show' isn't a type.
-// extension type MyInt on int show num, isEven hide floor {
-//                             ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:29:7: Error: Expected ';' after this.
-//   int get twice => 2 * this;
-//       ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:29:17: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-//   int get twice => 2 * this;
-//                 ^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:29:24: Error: Expected identifier, but got 'this'.
-//   int get twice => 2 * this;
-//                        ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:34:5: Error: The getter 'twice' isn't defined for the extension 'MyInt'.
-// Try correcting the name to the name of an existing getter, or defining a getter or field named 'twice'.
-//   m.twice; // OK, in the extension type.
-//     ^^^^^
-//
 // pkg/front_end/testcases/extension_types/simple_show_hide.dart:35:5: Error: The getter 'isEven' isn't defined for the extension 'MyInt'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'isEven'.
 //   m.isEven; // OK, a shown instance member.
@ -185,8 +66,8 @@ extension E2 on core::int {
 extension E3 on core::int {
 }
 extension type MyInt on core::int {
+  get twice = self::MyInt|get#twice;
 }
-static method num() → invalid-type {}
 static method test1(self::E1 e1) → dynamic {
  invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide.dart:8:6: Error: The method 'ceil' isn't defined for the extension 'E1'.
 Try correcting the name to the name of an existing method, or defining a method name 'ceil'.
@ -200,7 +81,6 @@ Try correcting the name to the name of an existing getter, or defining a getter
  e1.isEven; // Error.
     ^^^^^^" in e1{<unresolved>}.isEven;
 }
-static method ceil() → invalid-type {}
 static method test2(self::E2 e2) → dynamic {
  invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide.dart:16:6: Error: The method 'ceil' isn't defined for the extension 'E2'.
 Try correcting the name to the name of an existing method, or defining a method name 'ceil'.
@ -215,7 +95,6 @@ Try correcting the name to the name of an existing getter, or defining a getter
  e2.isEven; // Error.
     ^^^^^^" in e2{<unresolved>}.isEven;
 }
-static method isEven() → invalid-type {}
 static method test3(self::E3 e3) → dynamic {
  invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide.dart:24:6: Error: The getter 'isOdd' isn't defined for the extension 'E3'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'isOdd'.
@ -226,19 +105,11 @@ Try correcting the name to the name of an existing getter, or defining a getter
  e3.isEven; // Error.
     ^^^^^^" in e3{<unresolved>}.isEven;
 }
-static method floor() → invalid-type {
-  core::int get;
-  function twice() → core::double
-    return 2.{core::num::*}(invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide.dart:29:24: Error: Expected identifier, but got 'this'.
-  int get twice => 2 * this;
-                       ^^^^"){(core::num) → core::double};
-}
+static method MyInt|get#twice(lowered final core::int #this) → core::int
+  return 2.{core::num::*}(#this){(core::num) → core::int};
 static method test() → dynamic {
  self::MyInt m = 42;
-  invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide.dart:34:5: Error: The getter 'twice' isn't defined for the extension 'MyInt'.
-Try correcting the name to the name of an existing getter, or defining a getter or field named 'twice'.
-  m.twice; // OK, in the extension type.
-    ^^^^^" in m{<unresolved>}.twice;
+  self::MyInt|get#twice(m);
  invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide.dart:35:5: Error: The getter 'isEven' isn't defined for the extension 'MyInt'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'isEven'.
  m.isEven; // OK, a shown instance member.
--- a/pkg/front_end/testcases/extension_types/simple_show_hide.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_hide.dart.weak.outline.expect
@ -1,100 +1,4 @@
 library /*isNonNullableByDefault*/;
-//
-// Problems in library:
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:5:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E1 on int show num {}
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:5:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E1 on int show num {}
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E2 on int show num hide ceil {}
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:26: Error: Expected ';' after this.
-// extension E2 on int show num hide ceil {}
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:26: Error: 'num' is already declared in this scope.
-// extension E2 on int show num hide ceil {}
-//                          ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:5:26: Context: Previous declaration of 'num'.
-// extension E1 on int show num {}
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:35: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E2 on int show num hide ceil {}
-//                                   ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:21:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E3 on int hide isEven {}
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:21:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E3 on int hide isEven {}
-//                          ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:25: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension type MyInt on int show num, isEven hide floor {
-//                         ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:39: Error: Expected ';' after this.
-// extension type MyInt on int show num, isEven hide floor {
-//                                       ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:34: Error: 'num' is already declared in this scope.
-// extension type MyInt on int show num, isEven hide floor {
-//                                  ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:26: Context: Previous declaration of 'num'.
-// extension E2 on int show num hide ceil {}
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:39: Error: 'isEven' is already declared in this scope.
-// extension type MyInt on int show num, isEven hide floor {
-//                                       ^^^^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:21:26: Context: Previous declaration of 'isEven'.
-// extension E3 on int hide isEven {}
-//                          ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:51: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension type MyInt on int show num, isEven hide floor {
-//                                                   ^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:5:21: Error: Type 'show' not found.
-// extension E1 on int show num {}
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:21: Error: Type 'show' not found.
-// extension E2 on int show num hide ceil {}
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:13:30: Error: Type 'hide' not found.
-// extension E2 on int show num hide ceil {}
-//                              ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:21:21: Error: Type 'hide' not found.
-// extension E3 on int hide isEven {}
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:29: Error: Type 'show' not found.
-// extension type MyInt on int show num, isEven hide floor {
-//                             ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide.dart:28:46: Error: Type 'hide' not found.
-// extension type MyInt on int show num, isEven hide floor {
-//                                              ^^^^
-//
 import self as self;
 import "dart:core" as core;

@ -105,20 +9,15 @@ extension E2 on core::int {
 extension E3 on core::int {
 }
 extension type MyInt on core::int {
+  get twice = self::MyInt|get#twice;
 }
-static method num() → invalid-type
-  ;
 static method test1(self::E1 e1) → dynamic
  ;
-static method ceil() → invalid-type
-  ;
 static method test2(self::E2 e2) → dynamic
  ;
-static method isEven() → invalid-type
-  ;
 static method test3(self::E3 e3) → dynamic
  ;
-static method floor() → invalid-type
+static method MyInt|get#twice(lowered final core::int #this) → core::int
  ;
 static method test() → dynamic
  ;
--- a/pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart.strong.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart.strong.expect
@ -2,140 +2,58 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E1 on int show num {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E1 on int show num {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E2 on int show num hide ceil {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:26: Error: Expected ';' after this.
-// extension E2 on int show num hide ceil {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:26: Error: 'num' is already declared in this scope.
-// extension E2 on int show num hide ceil {
-//                          ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:26: Context: Previous declaration of 'num'.
-// extension E1 on int show num {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:35: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E2 on int show num hide ceil {
-//                                   ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E3 on int hide isEven {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E3 on int hide isEven {
-//                          ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:21: Error: Type 'show' not found.
-// extension E1 on int show num {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:21: Error: Type 'show' not found.
-// extension E2 on int show num hide ceil {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:30: Error: Type 'hide' not found.
-// extension E2 on int show num hide ceil {
-//                              ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:21: Error: Type 'hide' not found.
-// extension E3 on int hide isEven {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
 //   int ceil() {} // Error.
-//   ^
+//       ^
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:21: Error: 'show' isn't a type.
-// extension E2 on int show num hide ceil {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
 //   int ceil() {} // Ok.
-//   ^
+//       ^
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
 //   int floor() {} // Error.
-//   ^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:16:8: Error: Expected ';' after this.
-//   bool get isOdd => throw 42; // Error.
-//        ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:16:18: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-//   bool get isOdd => throw 42; // Error.
-//                  ^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:8: Error: 'get' is already declared in this scope.
-//   bool get isEven => throw 42; // Ok.
-//        ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:16:8: Context: Previous declaration of 'get'.
-//   bool get isOdd => throw 42; // Error.
-//        ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:8: Error: Expected ';' after this.
-//   bool get isEven => throw 42; // Ok.
-//        ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:19: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-//   bool get isEven => throw 42; // Ok.
-//                   ^^
+//       ^
 //
 import self as self;
 import "dart:core" as core;

 extension E1 on core::int {
+  method ceil = self::E1|ceil;
+  tearoff ceil = self::E1|get#ceil;
 }
 extension E2 on core::int {
+  method ceil = self::E2|ceil;
+  tearoff ceil = self::E2|get#ceil;
+  method floor = self::E2|floor;
+  tearoff floor = self::E2|get#floor;
 }
 extension E3 on core::int {
+  get isOdd = self::E3|get#isOdd;
+  get isEven = self::E3|get#isEven;
 }
-static method num() → invalid-type {
-  function ceil() → core::int {
-    return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+static method E1|ceil(lowered final core::int #this) → core::int {
+  return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
  int ceil() {} // Error.
-  ^" in null;
-  }
+      ^" in null;
 }
-static method ceil() → invalid-type {
-  function ceil() → core::int {
-    return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+static method E1|get#ceil(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E1|ceil(#this);
+static method E2|ceil(lowered final core::int #this) → core::int {
+  return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
  int ceil() {} // Ok.
-  ^" in null;
-  }
-  function floor() → core::int {
-    return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+      ^" in null;
+}
+static method E2|get#ceil(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E2|ceil(#this);
+static method E2|floor(lowered final core::int #this) → core::int {
+  return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
  int floor() {} // Error.
-  ^" in null;
-  }
-}
-static method isEven() → invalid-type {
-  core::bool get;
-  function isOdd() → Never
-    return throw 42;
-  core::bool get = invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:8: Error: 'get' is already declared in this scope.
-  bool get isEven => throw 42; // Ok.
-       ^^^";
-  function isEven() → Never
-    return throw 42;
+      ^" in null;
 }
+static method E2|get#floor(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E2|floor(#this);
+static method E3|get#isOdd(lowered final core::int #this) → core::bool
+  return throw 42;
+static method E3|get#isEven(lowered final core::int #this) → core::bool
+  return throw 42;
 static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart.strong.transformed.expect
@ -2,140 +2,58 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E1 on int show num {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E1 on int show num {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E2 on int show num hide ceil {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:26: Error: Expected ';' after this.
-// extension E2 on int show num hide ceil {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:26: Error: 'num' is already declared in this scope.
-// extension E2 on int show num hide ceil {
-//                          ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:26: Context: Previous declaration of 'num'.
-// extension E1 on int show num {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:35: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E2 on int show num hide ceil {
-//                                   ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E3 on int hide isEven {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E3 on int hide isEven {
-//                          ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:21: Error: Type 'show' not found.
-// extension E1 on int show num {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:21: Error: Type 'show' not found.
-// extension E2 on int show num hide ceil {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:30: Error: Type 'hide' not found.
-// extension E2 on int show num hide ceil {
-//                              ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:21: Error: Type 'hide' not found.
-// extension E3 on int hide isEven {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
 //   int ceil() {} // Error.
-//   ^
+//       ^
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:21: Error: 'show' isn't a type.
-// extension E2 on int show num hide ceil {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
 //   int ceil() {} // Ok.
-//   ^
+//       ^
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
 //   int floor() {} // Error.
-//   ^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:16:8: Error: Expected ';' after this.
-//   bool get isOdd => throw 42; // Error.
-//        ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:16:18: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-//   bool get isOdd => throw 42; // Error.
-//                  ^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:8: Error: 'get' is already declared in this scope.
-//   bool get isEven => throw 42; // Ok.
-//        ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:16:8: Context: Previous declaration of 'get'.
-//   bool get isOdd => throw 42; // Error.
-//        ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:8: Error: Expected ';' after this.
-//   bool get isEven => throw 42; // Ok.
-//        ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:19: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-//   bool get isEven => throw 42; // Ok.
-//                   ^^
+//       ^
 //
 import self as self;
 import "dart:core" as core;

 extension E1 on core::int {
+  method ceil = self::E1|ceil;
+  tearoff ceil = self::E1|get#ceil;
 }
 extension E2 on core::int {
+  method ceil = self::E2|ceil;
+  tearoff ceil = self::E2|get#ceil;
+  method floor = self::E2|floor;
+  tearoff floor = self::E2|get#floor;
 }
 extension E3 on core::int {
+  get isOdd = self::E3|get#isOdd;
+  get isEven = self::E3|get#isEven;
 }
-static method num() → invalid-type {
-  function ceil() → core::int {
-    return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+static method E1|ceil(lowered final core::int #this) → core::int {
+  return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
  int ceil() {} // Error.
-  ^" in null;
-  }
+      ^" in null;
 }
-static method ceil() → invalid-type {
-  function ceil() → core::int {
-    return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+static method E1|get#ceil(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E1|ceil(#this);
+static method E2|ceil(lowered final core::int #this) → core::int {
+  return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
  int ceil() {} // Ok.
-  ^" in null;
-  }
-  function floor() → core::int {
-    return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+      ^" in null;
+}
+static method E2|get#ceil(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E2|ceil(#this);
+static method E2|floor(lowered final core::int #this) → core::int {
+  return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
  int floor() {} // Error.
-  ^" in null;
-  }
-}
-static method isEven() → invalid-type {
-  core::bool get;
-  function isOdd() → Never
-    return throw 42;
-  core::bool get = invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:8: Error: 'get' is already declared in this scope.
-  bool get isEven => throw 42; // Ok.
-       ^^^";
-  function isEven() → Never
-    return throw 42;
+      ^" in null;
 }
+static method E2|get#floor(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E2|floor(#this);
+static method E3|get#isOdd(lowered final core::int #this) → core::bool
+  return throw 42;
+static method E3|get#isEven(lowered final core::int #this) → core::bool
+  return throw 42;
 static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart.textual_outline.expect
@ -1,13 +1,12 @@
-extension E1 on int {}
-
-show num() {}
-
-extension E2 on int {}
-
-show num;
-hide ceil() {}
-
-extension E3 on int {}
-
-hide isEven() {}
+extension E1 on int show num {
+  int ceil() {}
+}
+extension E2 on int show num hide ceil {
+  int ceil() {}
+  int floor() {}
+}
+extension E3 on int hide isEven {
+  bool get isOdd => throw 42;
+  bool get isEven => throw 42;
+}
 main() {}
--- a/pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart.weak.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart.weak.expect
@ -2,140 +2,58 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E1 on int show num {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E1 on int show num {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E2 on int show num hide ceil {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:26: Error: Expected ';' after this.
-// extension E2 on int show num hide ceil {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:26: Error: 'num' is already declared in this scope.
-// extension E2 on int show num hide ceil {
-//                          ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:26: Context: Previous declaration of 'num'.
-// extension E1 on int show num {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:35: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E2 on int show num hide ceil {
-//                                   ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E3 on int hide isEven {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E3 on int hide isEven {
-//                          ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:21: Error: Type 'show' not found.
-// extension E1 on int show num {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:21: Error: Type 'show' not found.
-// extension E2 on int show num hide ceil {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:30: Error: Type 'hide' not found.
-// extension E2 on int show num hide ceil {
-//                              ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:21: Error: Type 'hide' not found.
-// extension E3 on int hide isEven {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
 //   int ceil() {} // Error.
-//   ^
+//       ^
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:21: Error: 'show' isn't a type.
-// extension E2 on int show num hide ceil {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
 //   int ceil() {} // Ok.
-//   ^
+//       ^
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
 //   int floor() {} // Error.
-//   ^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:16:8: Error: Expected ';' after this.
-//   bool get isOdd => throw 42; // Error.
-//        ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:16:18: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-//   bool get isOdd => throw 42; // Error.
-//                  ^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:8: Error: 'get' is already declared in this scope.
-//   bool get isEven => throw 42; // Ok.
-//        ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:16:8: Context: Previous declaration of 'get'.
-//   bool get isOdd => throw 42; // Error.
-//        ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:8: Error: Expected ';' after this.
-//   bool get isEven => throw 42; // Ok.
-//        ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:19: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-//   bool get isEven => throw 42; // Ok.
-//                   ^^
+//       ^
 //
 import self as self;
 import "dart:core" as core;

 extension E1 on core::int {
+  method ceil = self::E1|ceil;
+  tearoff ceil = self::E1|get#ceil;
 }
 extension E2 on core::int {
+  method ceil = self::E2|ceil;
+  tearoff ceil = self::E2|get#ceil;
+  method floor = self::E2|floor;
+  tearoff floor = self::E2|get#floor;
 }
 extension E3 on core::int {
+  get isOdd = self::E3|get#isOdd;
+  get isEven = self::E3|get#isEven;
 }
-static method num() → invalid-type {
-  function ceil() → core::int {
-    return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+static method E1|ceil(lowered final core::int #this) → core::int {
+  return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
  int ceil() {} // Error.
-  ^" in null;
-  }
+      ^" in null;
 }
-static method ceil() → invalid-type {
-  function ceil() → core::int {
-    return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+static method E1|get#ceil(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E1|ceil(#this);
+static method E2|ceil(lowered final core::int #this) → core::int {
+  return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
  int ceil() {} // Ok.
-  ^" in null;
-  }
-  function floor() → core::int {
-    return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+      ^" in null;
+}
+static method E2|get#ceil(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E2|ceil(#this);
+static method E2|floor(lowered final core::int #this) → core::int {
+  return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
  int floor() {} // Error.
-  ^" in null;
-  }
-}
-static method isEven() → invalid-type {
-  core::bool get;
-  function isOdd() → Never
-    return throw 42;
-  core::bool get = invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:8: Error: 'get' is already declared in this scope.
-  bool get isEven => throw 42; // Ok.
-       ^^^";
-  function isEven() → Never
-    return throw 42;
+      ^" in null;
 }
+static method E2|get#floor(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E2|floor(#this);
+static method E3|get#isOdd(lowered final core::int #this) → core::bool
+  return throw 42;
+static method E3|get#isEven(lowered final core::int #this) → core::bool
+  return throw 42;
 static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart.weak.outline.expect
@ -1,78 +1,36 @@
 library /*isNonNullableByDefault*/;
-//
-// Problems in library:
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E1 on int show num {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E1 on int show num {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E2 on int show num hide ceil {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:26: Error: Expected ';' after this.
-// extension E2 on int show num hide ceil {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:26: Error: 'num' is already declared in this scope.
-// extension E2 on int show num hide ceil {
-//                          ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:26: Context: Previous declaration of 'num'.
-// extension E1 on int show num {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:35: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E2 on int show num hide ceil {
-//                                   ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E3 on int hide isEven {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E3 on int hide isEven {
-//                          ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:21: Error: Type 'show' not found.
-// extension E1 on int show num {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:21: Error: Type 'show' not found.
-// extension E2 on int show num hide ceil {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:30: Error: Type 'hide' not found.
-// extension E2 on int show num hide ceil {
-//                              ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:21: Error: Type 'hide' not found.
-// extension E3 on int hide isEven {
-//                     ^^^^
-//
 import self as self;
 import "dart:core" as core;

 extension E1 on core::int {
+  method ceil = self::E1|ceil;
+  tearoff ceil = self::E1|get#ceil;
 }
 extension E2 on core::int {
+  method ceil = self::E2|ceil;
+  tearoff ceil = self::E2|get#ceil;
+  method floor = self::E2|floor;
+  tearoff floor = self::E2|get#floor;
 }
 extension E3 on core::int {
+  get isOdd = self::E3|get#isOdd;
+  get isEven = self::E3|get#isEven;
 }
-static method num() → invalid-type
+static method E1|ceil(lowered final core::int #this) → core::int
  ;
-static method ceil() → invalid-type
+static method E1|get#ceil(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E1|ceil(#this);
+static method E2|ceil(lowered final core::int #this) → core::int
  ;
-static method isEven() → invalid-type
+static method E2|get#ceil(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E2|ceil(#this);
+static method E2|floor(lowered final core::int #this) → core::int
+  ;
+static method E2|get#floor(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E2|floor(#this);
+static method E3|get#isOdd(lowered final core::int #this) → core::bool
+  ;
+static method E3|get#isEven(lowered final core::int #this) → core::bool
  ;
 static method main() → dynamic
  ;
--- a/pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart.weak.transformed.expect
@ -2,140 +2,58 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E1 on int show num {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E1 on int show num {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E2 on int show num hide ceil {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:26: Error: Expected ';' after this.
-// extension E2 on int show num hide ceil {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:26: Error: 'num' is already declared in this scope.
-// extension E2 on int show num hide ceil {
-//                          ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:26: Context: Previous declaration of 'num'.
-// extension E1 on int show num {
-//                          ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:35: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E2 on int show num hide ceil {
-//                                   ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:17: Error: A extension declaration must have a body, even if it is empty.
-// Try adding an empty body.
-// extension E3 on int hide isEven {
-//                 ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:26: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-// extension E3 on int hide isEven {
-//                          ^^^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:5:21: Error: Type 'show' not found.
-// extension E1 on int show num {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:21: Error: Type 'show' not found.
-// extension E2 on int show num hide ceil {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:30: Error: Type 'hide' not found.
-// extension E2 on int show num hide ceil {
-//                              ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:14:21: Error: Type 'hide' not found.
-// extension E3 on int hide isEven {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
 //   int ceil() {} // Error.
-//   ^
+//       ^
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:9:21: Error: 'show' isn't a type.
-// extension E2 on int show num hide ceil {
-//                     ^^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
 //   int ceil() {} // Ok.
-//   ^
+//       ^
 //
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
 //   int floor() {} // Error.
-//   ^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:16:8: Error: Expected ';' after this.
-//   bool get isOdd => throw 42; // Error.
-//        ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:16:18: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-//   bool get isOdd => throw 42; // Error.
-//                  ^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:8: Error: 'get' is already declared in this scope.
-//   bool get isEven => throw 42; // Ok.
-//        ^^^
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:16:8: Context: Previous declaration of 'get'.
-//   bool get isOdd => throw 42; // Error.
-//        ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:8: Error: Expected ';' after this.
-//   bool get isEven => throw 42; // Ok.
-//        ^^^
-//
-// pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:19: Error: A function declaration needs an explicit list of parameters.
-// Try adding a parameter list to the function declaration.
-//   bool get isEven => throw 42; // Ok.
-//                   ^^
+//       ^
 //
 import self as self;
 import "dart:core" as core;

 extension E1 on core::int {
+  method ceil = self::E1|ceil;
+  tearoff ceil = self::E1|get#ceil;
 }
 extension E2 on core::int {
+  method ceil = self::E2|ceil;
+  tearoff ceil = self::E2|get#ceil;
+  method floor = self::E2|floor;
+  tearoff floor = self::E2|get#floor;
 }
 extension E3 on core::int {
+  get isOdd = self::E3|get#isOdd;
+  get isEven = self::E3|get#isEven;
 }
-static method num() → invalid-type {
-  function ceil() → core::int {
-    return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+static method E1|ceil(lowered final core::int #this) → core::int {
+  return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:6:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
  int ceil() {} // Error.
-  ^" in null;
-  }
+      ^" in null;
 }
-static method ceil() → invalid-type {
-  function ceil() → core::int {
-    return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+static method E1|get#ceil(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E1|ceil(#this);
+static method E2|ceil(lowered final core::int #this) → core::int {
+  return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:10:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
  int ceil() {} // Ok.
-  ^" in null;
-  }
-  function floor() → core::int {
-    return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:3: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
+      ^" in null;
+}
+static method E2|get#ceil(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E2|ceil(#this);
+static method E2|floor(lowered final core::int #this) → core::int {
+  return invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:11:7: Error: A non-null value must be returned since the return type 'int' doesn't allow null.
  int floor() {} // Error.
-  ^" in null;
-  }
-}
-static method isEven() → invalid-type {
-  core::bool get;
-  function isOdd() → Never
-    return throw 42;
-  core::bool get = invalid-expression "pkg/front_end/testcases/extension_types/simple_show_hide_conflicts.dart:17:8: Error: 'get' is already declared in this scope.
-  bool get isEven => throw 42; // Ok.
-       ^^^";
-  function isEven() → Never
-    return throw 42;
+      ^" in null;
 }
+static method E2|get#floor(lowered final core::int #this) → () → core::int
+  return () → core::int => self::E2|floor(#this);
+static method E3|get#isOdd(lowered final core::int #this) → core::bool
+  return throw 42;
+static method E3|get#isEven(lowered final core::int #this) → core::bool
+  return throw 42;
 static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/various_hide_elements.dart
+++ b/pkg/front_end/testcases/extension_types/various_hide_elements.dart
@ -0,0 +1,32 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:core' as prefixedCore;
+import 'dart:core';
+
+extension type E0 on int hide operator * {}
+extension type E1 on int hide get isEven {}
+extension type E2<T> on List<T> hide set length {}
+extension type E3 on int hide num {}
+extension type E4 on List<int> hide prefixedCore.Iterable<int> {} // Error?
+extension type E5 on List hide prefixedCore.Iterable {} // Error?
+extension type E6 on List<int> hide Iterable<int> {}
+
+abstract class A {
+  A operator *(A other);
+}
+
+class B<X> implements A {
+  bool get foo => throw 42;
+  A operator *(A other) => throw 42;
+}
+
+class C extends B<int> {
+  void set bar(int value) {}
+  void baz() {}
+}
+
+extension type E on C hide A, B<int>, operator *, get foo, set bar, baz {}
+
+main() {}
--- a/pkg/front_end/testcases/extension_types/various_hide_elements.dart.strong.expect
+++ b/pkg/front_end/testcases/extension_types/various_hide_elements.dart.strong.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+import "dart:core" as prefixedCore;
+import "dart:core";
+
+abstract class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  abstract operator *(self::A other) → self::A;
+}
+class B<X extends core::Object? = dynamic> extends core::Object implements self::A {
+  synthetic constructor •() → self::B<self::B::X%>
+    : super core::Object::•()
+    ;
+  get foo() → core::bool
+    return throw 42;
+  operator *(self::A other) → self::A
+    return throw 42;
+}
+class C extends self::B<core::int> {
+  synthetic constructor •() → self::C
+    : super self::B::•()
+    ;
+  set bar(core::int value) → void {}
+  method baz() → void {}
+}
+extension type E0 on core::int {
+}
+extension type E1 on core::int {
+}
+extension type E2<T extends core::Object? = dynamic> on core::List<T%> {
+}
+extension type E3 on core::int {
+}
+extension type E4 on core::List<core::int> {
+}
+extension type E5 on core::List<dynamic> {
+}
+extension type E6 on core::List<core::int> {
+}
+extension type E on self::C {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/various_hide_elements.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/various_hide_elements.dart.strong.transformed.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+import "dart:core" as prefixedCore;
+import "dart:core";
+
+abstract class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  abstract operator *(self::A other) → self::A;
+}
+class B<X extends core::Object? = dynamic> extends core::Object implements self::A {
+  synthetic constructor •() → self::B<self::B::X%>
+    : super core::Object::•()
+    ;
+  get foo() → core::bool
+    return throw 42;
+  operator *(self::A other) → self::A
+    return throw 42;
+}
+class C extends self::B<core::int> {
+  synthetic constructor •() → self::C
+    : super self::B::•()
+    ;
+  set bar(core::int value) → void {}
+  method baz() → void {}
+}
+extension type E0 on core::int {
+}
+extension type E1 on core::int {
+}
+extension type E2<T extends core::Object? = dynamic> on core::List<T%> {
+}
+extension type E3 on core::int {
+}
+extension type E4 on core::List<core::int> {
+}
+extension type E5 on core::List<dynamic> {
+}
+extension type E6 on core::List<core::int> {
+}
+extension type E on self::C {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/various_hide_elements.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/extension_types/various_hide_elements.dart.textual_outline.expect
@ -0,0 +1,22 @@
+import 'dart:core' as prefixedCore;
+import 'dart:core';
+extension type E0 on int hide operator * {}
+extension type E1 on int hide get isEven {}
+extension type E2<T> on List<T> hide set length {}
+extension type E3 on int hide num {}
+extension type E4 on List<int> hide prefixedCore.Iterable<int> {}
+extension type E5 on List hide prefixedCore.Iterable {}
+extension type E6 on List<int> hide Iterable<int> {}
+abstract class A {
+  A operator *(A other);
+}
+class B<X> implements A {
+  bool get foo => throw 42;
+  A operator *(A other) => throw 42;
+}
+class C extends B<int> {
+  void set bar(int value) {}
+  void baz() {}
+}
+extension type E on C hide A, B<int>, operator *, get foo, set bar, baz {}
+main() {}
--- a/pkg/front_end/testcases/extension_types/various_hide_elements.dart.weak.expect
+++ b/pkg/front_end/testcases/extension_types/various_hide_elements.dart.weak.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+import "dart:core" as prefixedCore;
+import "dart:core";
+
+abstract class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  abstract operator *(self::A other) → self::A;
+}
+class B<X extends core::Object? = dynamic> extends core::Object implements self::A {
+  synthetic constructor •() → self::B<self::B::X%>
+    : super core::Object::•()
+    ;
+  get foo() → core::bool
+    return throw 42;
+  operator *(self::A other) → self::A
+    return throw 42;
+}
+class C extends self::B<core::int> {
+  synthetic constructor •() → self::C
+    : super self::B::•()
+    ;
+  set bar(core::int value) → void {}
+  method baz() → void {}
+}
+extension type E0 on core::int {
+}
+extension type E1 on core::int {
+}
+extension type E2<T extends core::Object? = dynamic> on core::List<T%> {
+}
+extension type E3 on core::int {
+}
+extension type E4 on core::List<core::int> {
+}
+extension type E5 on core::List<dynamic> {
+}
+extension type E6 on core::List<core::int> {
+}
+extension type E on self::C {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/various_hide_elements.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/extension_types/various_hide_elements.dart.weak.outline.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+import "dart:core" as prefixedCore;
+import "dart:core";
+
+abstract class A extends core::Object {
+  synthetic constructor •() → self::A
+    ;
+  abstract operator *(self::A other) → self::A;
+}
+class B<X extends core::Object? = dynamic> extends core::Object implements self::A {
+  synthetic constructor •() → self::B<self::B::X%>
+    ;
+  get foo() → core::bool
+    ;
+  operator *(self::A other) → self::A
+    ;
+}
+class C extends self::B<core::int> {
+  synthetic constructor •() → self::C
+    ;
+  set bar(core::int value) → void
+    ;
+  method baz() → void
+    ;
+}
+extension type E0 on core::int {
+}
+extension type E1 on core::int {
+}
+extension type E2<T extends core::Object? = dynamic> on core::List<T%> {
+}
+extension type E3 on core::int {
+}
+extension type E4 on core::List<core::int> {
+}
+extension type E5 on core::List<dynamic> {
+}
+extension type E6 on core::List<core::int> {
+}
+extension type E on self::C {
+}
+static method main() → dynamic
+  ;
--- a/pkg/front_end/testcases/extension_types/various_hide_elements.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/various_hide_elements.dart.weak.transformed.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+import "dart:core" as prefixedCore;
+import "dart:core";
+
+abstract class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  abstract operator *(self::A other) → self::A;
+}
+class B<X extends core::Object? = dynamic> extends core::Object implements self::A {
+  synthetic constructor •() → self::B<self::B::X%>
+    : super core::Object::•()
+    ;
+  get foo() → core::bool
+    return throw 42;
+  operator *(self::A other) → self::A
+    return throw 42;
+}
+class C extends self::B<core::int> {
+  synthetic constructor •() → self::C
+    : super self::B::•()
+    ;
+  set bar(core::int value) → void {}
+  method baz() → void {}
+}
+extension type E0 on core::int {
+}
+extension type E1 on core::int {
+}
+extension type E2<T extends core::Object? = dynamic> on core::List<T%> {
+}
+extension type E3 on core::int {
+}
+extension type E4 on core::List<core::int> {
+}
+extension type E5 on core::List<dynamic> {
+}
+extension type E6 on core::List<core::int> {
+}
+extension type E on self::C {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/various_show_elements.dart
+++ b/pkg/front_end/testcases/extension_types/various_show_elements.dart
@ -0,0 +1,32 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:core' as prefixedCore;
+import 'dart:core';
+
+extension type E0 on int show operator * {}
+extension type E1 on int show get isEven {}
+extension type E2<T> on List<T> show set length {}
+extension type E3 on int show num {}
+extension type E4 on List<int> show prefixedCore.Iterable<int> {} // Error?
+extension type E5 on List show prefixedCore.Iterable {} // Error?
+extension type E6 on List<int> show Iterable<int> {}
+
+abstract class A {
+  A operator *(A other);
+}
+
+class B<X> implements A {
+  bool get foo => throw 42;
+  A operator *(A other) => throw 42;
+}
+
+class C extends B<int> {
+  void set bar(int value) {}
+  void baz() {}
+}
+
+extension type E on C show A, B<int>, operator *, get foo, set bar, baz {}
+
+main() {}
--- a/pkg/front_end/testcases/extension_types/various_show_elements.dart.strong.expect
+++ b/pkg/front_end/testcases/extension_types/various_show_elements.dart.strong.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+import "dart:core" as prefixedCore;
+import "dart:core";
+
+abstract class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  abstract operator *(self::A other) → self::A;
+}
+class B<X extends core::Object? = dynamic> extends core::Object implements self::A {
+  synthetic constructor •() → self::B<self::B::X%>
+    : super core::Object::•()
+    ;
+  get foo() → core::bool
+    return throw 42;
+  operator *(self::A other) → self::A
+    return throw 42;
+}
+class C extends self::B<core::int> {
+  synthetic constructor •() → self::C
+    : super self::B::•()
+    ;
+  set bar(core::int value) → void {}
+  method baz() → void {}
+}
+extension type E0 on core::int {
+}
+extension type E1 on core::int {
+}
+extension type E2<T extends core::Object? = dynamic> on core::List<T%> {
+}
+extension type E3 on core::int {
+}
+extension type E4 on core::List<core::int> {
+}
+extension type E5 on core::List<dynamic> {
+}
+extension type E6 on core::List<core::int> {
+}
+extension type E on self::C {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/various_show_elements.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/various_show_elements.dart.strong.transformed.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+import "dart:core" as prefixedCore;
+import "dart:core";
+
+abstract class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  abstract operator *(self::A other) → self::A;
+}
+class B<X extends core::Object? = dynamic> extends core::Object implements self::A {
+  synthetic constructor •() → self::B<self::B::X%>
+    : super core::Object::•()
+    ;
+  get foo() → core::bool
+    return throw 42;
+  operator *(self::A other) → self::A
+    return throw 42;
+}
+class C extends self::B<core::int> {
+  synthetic constructor •() → self::C
+    : super self::B::•()
+    ;
+  set bar(core::int value) → void {}
+  method baz() → void {}
+}
+extension type E0 on core::int {
+}
+extension type E1 on core::int {
+}
+extension type E2<T extends core::Object? = dynamic> on core::List<T%> {
+}
+extension type E3 on core::int {
+}
+extension type E4 on core::List<core::int> {
+}
+extension type E5 on core::List<dynamic> {
+}
+extension type E6 on core::List<core::int> {
+}
+extension type E on self::C {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/various_show_elements.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/extension_types/various_show_elements.dart.textual_outline.expect
@ -0,0 +1,22 @@
+import 'dart:core' as prefixedCore;
+import 'dart:core';
+extension type E0 on int show operator * {}
+extension type E1 on int show get isEven {}
+extension type E2<T> on List<T> show set length {}
+extension type E3 on int show num {}
+extension type E4 on List<int> show prefixedCore.Iterable<int> {}
+extension type E5 on List show prefixedCore.Iterable {}
+extension type E6 on List<int> show Iterable<int> {}
+abstract class A {
+  A operator *(A other);
+}
+class B<X> implements A {
+  bool get foo => throw 42;
+  A operator *(A other) => throw 42;
+}
+class C extends B<int> {
+  void set bar(int value) {}
+  void baz() {}
+}
+extension type E on C show A, B<int>, operator *, get foo, set bar, baz {}
+main() {}
--- a/pkg/front_end/testcases/extension_types/various_show_elements.dart.weak.expect
+++ b/pkg/front_end/testcases/extension_types/various_show_elements.dart.weak.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+import "dart:core" as prefixedCore;
+import "dart:core";
+
+abstract class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  abstract operator *(self::A other) → self::A;
+}
+class B<X extends core::Object? = dynamic> extends core::Object implements self::A {
+  synthetic constructor •() → self::B<self::B::X%>
+    : super core::Object::•()
+    ;
+  get foo() → core::bool
+    return throw 42;
+  operator *(self::A other) → self::A
+    return throw 42;
+}
+class C extends self::B<core::int> {
+  synthetic constructor •() → self::C
+    : super self::B::•()
+    ;
+  set bar(core::int value) → void {}
+  method baz() → void {}
+}
+extension type E0 on core::int {
+}
+extension type E1 on core::int {
+}
+extension type E2<T extends core::Object? = dynamic> on core::List<T%> {
+}
+extension type E3 on core::int {
+}
+extension type E4 on core::List<core::int> {
+}
+extension type E5 on core::List<dynamic> {
+}
+extension type E6 on core::List<core::int> {
+}
+extension type E on self::C {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/extension_types/various_show_elements.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/extension_types/various_show_elements.dart.weak.outline.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+import "dart:core" as prefixedCore;
+import "dart:core";
+
+abstract class A extends core::Object {
+  synthetic constructor •() → self::A
+    ;
+  abstract operator *(self::A other) → self::A;
+}
+class B<X extends core::Object? = dynamic> extends core::Object implements self::A {
+  synthetic constructor •() → self::B<self::B::X%>
+    ;
+  get foo() → core::bool
+    ;
+  operator *(self::A other) → self::A
+    ;
+}
+class C extends self::B<core::int> {
+  synthetic constructor •() → self::C
+    ;
+  set bar(core::int value) → void
+    ;
+  method baz() → void
+    ;
+}
+extension type E0 on core::int {
+}
+extension type E1 on core::int {
+}
+extension type E2<T extends core::Object? = dynamic> on core::List<T%> {
+}
+extension type E3 on core::int {
+}
+extension type E4 on core::List<core::int> {
+}
+extension type E5 on core::List<dynamic> {
+}
+extension type E6 on core::List<core::int> {
+}
+extension type E on self::C {
+}
+static method main() → dynamic
+  ;
--- a/pkg/front_end/testcases/extension_types/various_show_elements.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/various_show_elements.dart.weak.transformed.expect
@ -0,0 +1,46 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+import "dart:core" as prefixedCore;
+import "dart:core";
+
+abstract class A extends core::Object {
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  abstract operator *(self::A other) → self::A;
+}
+class B<X extends core::Object? = dynamic> extends core::Object implements self::A {
+  synthetic constructor •() → self::B<self::B::X%>
+    : super core::Object::•()
+    ;
+  get foo() → core::bool
+    return throw 42;
+  operator *(self::A other) → self::A
+    return throw 42;
+}
+class C extends self::B<core::int> {
+  synthetic constructor •() → self::C
+    : super self::B::•()
+    ;
+  set bar(core::int value) → void {}
+  method baz() → void {}
+}
+extension type E0 on core::int {
+}
+extension type E1 on core::int {
+}
+extension type E2<T extends core::Object? = dynamic> on core::List<T%> {
+}
+extension type E3 on core::int {
+}
+extension type E4 on core::List<core::int> {
+}
+extension type E5 on core::List<dynamic> {
+}
+extension type E6 on core::List<core::int> {
+}
+extension type E on self::C {
+}
+static method main() → dynamic {}
--- a/pkg/front_end/testcases/textual_outline.status
+++ b/pkg/front_end/testcases/textual_outline.status
@ -27,11 +27,17 @@ constructor_tearoffs/issue46133: FormatterCrash
 constructor_tearoffs/issue47075: FormatterCrash
 dart2js/late_fields: FormatterCrash
 dart2js/late_statics: FormatterCrash
+extension_types/basic_show: FormatterCrash
+extension_types/keyword_in_show_hide_element: FormatterCrash
 extension_types/simple_getter_resolution: FormatterCrash
 extension_types/simple_method_resolution: FormatterCrash
 extension_types/simple_operator_resolution: FormatterCrash
 extension_types/simple_setter_resolution: FormatterCrash
+extension_types/simple_show_and_hide: FormatterCrash
 extension_types/simple_show_hide: FormatterCrash
+extension_types/simple_show_hide_conflicts: FormatterCrash
+extension_types/various_hide_elements: FormatterCrash
+extension_types/various_show_elements: FormatterCrash
 extensions/extension_constructor: FormatterCrash
 extensions/extension_field_with_type_parameter_usage: FormatterCrash
 extensions/issue38600: FormatterCrash