[cfe] Report error on bottom type used as representation type

Closes https://github.com/dart-lang/sdk/issues/53824 Part of https://github.com/dart-lang/sdk/issues/49731 Change-Id: I79776eb4f8b736f518898adff1078461895269f0 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/331660 Reviewed-by: Johnni Winther <johnniwinther@google.com> Commit-Queue: Chloe Stefantsova <cstefantsova@google.com>
2024-11-02 08:20:31 +00:00 · 2023-10-24 10:28:09 +00:00 · 2023-10-24 10:28:09 +00:00 · 67d45616c6
commit 67d45616c6
parent 216d98555c
33 changed files with 926 additions and 461 deletions
--- a/pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart
@ -4455,6 +4455,16 @@ const MessageCode messageExtensionTypeMemberOneOfContext = const MessageCode(
    problemMessage:
        r"""This is one of the inherited extension type members.""");

+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Code<Null> codeExtensionTypeRepresentationTypeBottom =
+    messageExtensionTypeRepresentationTypeBottom;
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const MessageCode messageExtensionTypeRepresentationTypeBottom =
+    const MessageCode("ExtensionTypeRepresentationTypeBottom",
+        analyzerCodes: <String>["EXTENSION_TYPE_REPRESENTATION_TYPE_BOTTOM"],
+        problemMessage: r"""The representation type can't be a bottom type.""");
+
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Code<Null> codeExtensionTypeWith = messageExtensionTypeWith;

--- a/pkg/front_end/lib/src/fasta/source/source_extension_type_declaration_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_extension_type_declaration_builder.dart
@ -259,6 +259,14 @@ class SourceExtensionTypeDeclarationBuilder
                  typeBuilder.fileUri);
            }
          }
+          if (isBottom(representationType)) {
+            libraryBuilder.addProblem(
+                messageExtensionTypeRepresentationTypeBottom,
+                representationFieldBuilder!.charOffset,
+                representationFieldBuilder!.name.length,
+                representationFieldBuilder!.fileUri);
+            representationType = const InvalidType();
+          }
        }
      } else {
        representationType = const DynamicType();
--- a/pkg/front_end/messages.yaml
+++ b/pkg/front_end/messages.yaml
@ -7440,3 +7440,10 @@ ExtensionTypeConstructorWithSuperFormalParameter:
  script: |
    extension type A(int foo) { A.named(this.foo, super.bar); }
  analyzerCode: EXTENSION_TYPE_CONSTRUCTOR_WITH_SUPER_FORMAL_PARAMETER
+
+ExtensionTypeRepresentationTypeBottom:
+  problemMessage: "The representation type can't be a bottom type."
+  experiments: inline-class
+  script: |
+    extension type E1(Never foo) {}
+  analyzerCode: EXTENSION_TYPE_REPRESENTATION_TYPE_BOTTOM
--- a/pkg/front_end/testcases/extension_types/implement_all.dart
+++ b/pkg/front_end/testcases/extension_types/implement_all.dart
@ -30,7 +30,7 @@ extension type ET_Null(Null it) implements Null /* Error */ {}

 extension type ET_Dynamic(dynamic it) implements dynamic /* Error */ {}

-extension type ET_Void(Never it) implements void /* Error */ {}
+extension type ET_Void(Null it) implements void /* Error */ {}

 extension type ET_Never(Never it) implements Never /* Error */ {}

@ -54,7 +54,7 @@ extension type ET_NullableExtensionType(int it)
 extension type ET_FutureOr(FutureOr<int> it)
    implements FutureOr<int> /* Error */ {}

-extension type ET_Extension(Never it) implements Extension /* Error */ {}
+extension type ET_Extension(int it) implements Extension /* Error */ {}

 extension type ET_TypeVariable<T>(T it) implements T /* Error */ {}

--- a/pkg/front_end/testcases/extension_types/implement_all.dart.strong.expect
+++ b/pkg/front_end/testcases/extension_types/implement_all.dart.strong.expect
@ -2,18 +2,18 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: Type 'void' can't be used here.
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: Type 'void' can't be used here.
 // Try removing 'void' keyword or replace it with 'var', 'final', or a type.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^^^^
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^^^^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: Type 'void' not found.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^^^^
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: Type 'void' not found.
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^^^^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:57:50: Error: 'Extension' isn't a type.
-// extension type ET_Extension(Never it) implements Extension /* Error */ {}
-//                                                  ^^^^^^^^^
+// pkg/front_end/testcases/extension_types/implement_all.dart:57:48: Error: 'Extension' isn't a type.
+// extension type ET_Extension(int it) implements Extension /* Error */ {}
+//                                                ^^^^^^^^^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:29:44: Error: The type 'Null' can't be implemented by an extension type.
 // extension type ET_Null(Null it) implements Null /* Error */ {}
@ -23,14 +23,18 @@ library;
 // extension type ET_Dynamic(dynamic it) implements dynamic /* Error */ {}
 //                                                  ^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: The type 'void' can't be implemented by an extension type.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: The type 'void' can't be implemented by an extension type.
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:35:46: Error: The type 'Never' can't be implemented by an extension type.
 // extension type ET_Never(Never it) implements Never /* Error */ {}
 //                                              ^
 //
+// pkg/front_end/testcases/extension_types/implement_all.dart:35:31: Error: The representation type can't be a bottom type.
+// extension type ET_Never(Never it) implements Never /* Error */ {}
+//                               ^^
+//
 // pkg/front_end/testcases/extension_types/implement_all.dart:37:48: Error: The type 'Object' can't be implemented by an extension type.
 // extension type ET_Object(Object it) implements Object /* Error */ {}
 //                                                ^
@ -75,9 +79,9 @@ library;
 //     implements FutureOr<int> /* Error */ {}
 //                ^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:57:50: Error: The type 'Extension' can't be implemented by an extension type.
-// extension type ET_Extension(Never it) implements Extension /* Error */ {}
-//                                                  ^
+// pkg/front_end/testcases/extension_types/implement_all.dart:57:48: Error: The type 'Extension' can't be implemented by an extension type.
+// extension type ET_Extension(int it) implements Extension /* Error */ {}
+//                                                ^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:59:52: Error: The type variable 'T' can't be implemented by an extension type.
 // extension type ET_TypeVariable<T>(T it) implements T /* Error */ {}
@ -126,12 +130,12 @@ extension type ET_Dynamic(dynamic it) {
  constructor • = self::ET_Dynamic|constructor#;
  constructor tearoff • = self::ET_Dynamic|constructor#_#new#tearOff;
 }
-extension type ET_Void(Never it) {
-  abstract inline-class-member representation-field get it() → Never;
+extension type ET_Void(Null it) {
+  abstract inline-class-member representation-field get it() → Null;
  constructor • = self::ET_Void|constructor#;
  constructor tearoff • = self::ET_Void|constructor#_#new#tearOff;
 }
-extension type ET_Never(Never it) {
+extension type ET_Never(invalid-type it) {
  abstract inline-class-member representation-field get it() → Never;
  constructor • = self::ET_Never|constructor#;
  constructor tearoff • = self::ET_Never|constructor#_#new#tearOff;
@ -176,8 +180,8 @@ extension type ET_FutureOr(FutureOr<core::int>it) {
  constructor • = self::ET_FutureOr|constructor#;
  constructor tearoff • = self::ET_FutureOr|constructor#_#new#tearOff;
 }
-extension type ET_Extension(Never it) {
-  abstract inline-class-member representation-field get it() → Never;
+extension type ET_Extension(core::int it) {
+  abstract inline-class-member representation-field get it() → core::int;
  constructor • = self::ET_Extension|constructor#;
  constructor tearoff • = self::ET_Extension|constructor#_#new#tearOff;
 }
@ -240,17 +244,17 @@ static inline-class-member method ET_Dynamic|constructor#(dynamic it) → self::
 }
 static inline-class-member method ET_Dynamic|constructor#_#new#tearOff(dynamic it) → self::ET_Dynamic /* = dynamic */
  return self::ET_Dynamic|constructor#(it);
-static inline-class-member method ET_Void|constructor#(Never it) → self::ET_Void /* = Never */ {
-  lowered final self::ET_Void /* = Never */ #this = it;
+static inline-class-member method ET_Void|constructor#(Null it) → self::ET_Void /* = Null */ {
+  lowered final self::ET_Void /* = Null */ #this = it;
  return #this;
 }
-static inline-class-member method ET_Void|constructor#_#new#tearOff(Never it) → self::ET_Void /* = Never */
+static inline-class-member method ET_Void|constructor#_#new#tearOff(Null it) → self::ET_Void /* = Null */
  return self::ET_Void|constructor#(it);
-static inline-class-member method ET_Never|constructor#(Never it) → self::ET_Never /* = Never */ {
-  lowered final self::ET_Never /* = Never */ #this = it;
+static inline-class-member method ET_Never|constructor#(Never it) → self::ET_Never /* = invalid-type */ {
+  lowered final self::ET_Never /* = invalid-type */ #this = it;
  return #this;
 }
-static inline-class-member method ET_Never|constructor#_#new#tearOff(Never it) → self::ET_Never /* = Never */
+static inline-class-member method ET_Never|constructor#_#new#tearOff(Never it) → self::ET_Never /* = invalid-type */
  return self::ET_Never|constructor#(it);
 static inline-class-member method ET_Object|constructor#(core::Object it) → self::ET_Object /* = core::Object */ {
  lowered final self::ET_Object /* = core::Object */ #this = it;
@ -300,10 +304,10 @@ static inline-class-member method ET_FutureOr|constructor#(FutureOr<core::int>it
 }
 static inline-class-member method ET_FutureOr|constructor#_#new#tearOff(FutureOr<core::int>it) → self::ET_FutureOr /* = FutureOr<core::int>*/
  return self::ET_FutureOr|constructor#(it);
-static inline-class-member method ET_Extension|constructor#_#new#tearOff(Never it) → self::ET_Extension /* = Never */
+static inline-class-member method ET_Extension|constructor#_#new#tearOff(core::int it) → self::ET_Extension /* = core::int */
  return self::ET_Extension|constructor#(it);
-static inline-class-member method ET_Extension|constructor#(Never it) → self::ET_Extension /* = Never */ {
-  lowered final self::ET_Extension /* = Never */ #this = it;
+static inline-class-member method ET_Extension|constructor#(core::int it) → self::ET_Extension /* = core::int */ {
+  lowered final self::ET_Extension /* = core::int */ #this = it;
  return #this;
 }
 static inline-class-member method ET_TypeVariable|constructor#<T extends core::Object? = dynamic>(self::ET_TypeVariable|constructor#::T% it) → self::ET_TypeVariable<self::ET_TypeVariable|constructor#::T%> /* = self::ET_TypeVariable|constructor#::T% */ {
--- a/pkg/front_end/testcases/extension_types/implement_all.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/implement_all.dart.strong.transformed.expect
@ -2,18 +2,18 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: Type 'void' can't be used here.
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: Type 'void' can't be used here.
 // Try removing 'void' keyword or replace it with 'var', 'final', or a type.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^^^^
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^^^^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: Type 'void' not found.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^^^^
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: Type 'void' not found.
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^^^^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:57:50: Error: 'Extension' isn't a type.
-// extension type ET_Extension(Never it) implements Extension /* Error */ {}
-//                                                  ^^^^^^^^^
+// pkg/front_end/testcases/extension_types/implement_all.dart:57:48: Error: 'Extension' isn't a type.
+// extension type ET_Extension(int it) implements Extension /* Error */ {}
+//                                                ^^^^^^^^^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:29:44: Error: The type 'Null' can't be implemented by an extension type.
 // extension type ET_Null(Null it) implements Null /* Error */ {}
@ -23,14 +23,18 @@ library;
 // extension type ET_Dynamic(dynamic it) implements dynamic /* Error */ {}
 //                                                  ^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: The type 'void' can't be implemented by an extension type.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: The type 'void' can't be implemented by an extension type.
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:35:46: Error: The type 'Never' can't be implemented by an extension type.
 // extension type ET_Never(Never it) implements Never /* Error */ {}
 //                                              ^
 //
+// pkg/front_end/testcases/extension_types/implement_all.dart:35:31: Error: The representation type can't be a bottom type.
+// extension type ET_Never(Never it) implements Never /* Error */ {}
+//                               ^^
+//
 // pkg/front_end/testcases/extension_types/implement_all.dart:37:48: Error: The type 'Object' can't be implemented by an extension type.
 // extension type ET_Object(Object it) implements Object /* Error */ {}
 //                                                ^
@ -75,9 +79,9 @@ library;
 //     implements FutureOr<int> /* Error */ {}
 //                ^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:57:50: Error: The type 'Extension' can't be implemented by an extension type.
-// extension type ET_Extension(Never it) implements Extension /* Error */ {}
-//                                                  ^
+// pkg/front_end/testcases/extension_types/implement_all.dart:57:48: Error: The type 'Extension' can't be implemented by an extension type.
+// extension type ET_Extension(int it) implements Extension /* Error */ {}
+//                                                ^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:59:52: Error: The type variable 'T' can't be implemented by an extension type.
 // extension type ET_TypeVariable<T>(T it) implements T /* Error */ {}
@ -126,12 +130,12 @@ extension type ET_Dynamic(dynamic it) {
  constructor • = self::ET_Dynamic|constructor#;
  constructor tearoff • = self::ET_Dynamic|constructor#_#new#tearOff;
 }
-extension type ET_Void(Never it) {
-  abstract inline-class-member representation-field get it() → Never;
+extension type ET_Void(Null it) {
+  abstract inline-class-member representation-field get it() → Null;
  constructor • = self::ET_Void|constructor#;
  constructor tearoff • = self::ET_Void|constructor#_#new#tearOff;
 }
-extension type ET_Never(Never it) {
+extension type ET_Never(invalid-type it) {
  abstract inline-class-member representation-field get it() → Never;
  constructor • = self::ET_Never|constructor#;
  constructor tearoff • = self::ET_Never|constructor#_#new#tearOff;
@ -176,8 +180,8 @@ extension type ET_FutureOr(FutureOr<core::int>it) {
  constructor • = self::ET_FutureOr|constructor#;
  constructor tearoff • = self::ET_FutureOr|constructor#_#new#tearOff;
 }
-extension type ET_Extension(Never it) {
-  abstract inline-class-member representation-field get it() → Never;
+extension type ET_Extension(core::int it) {
+  abstract inline-class-member representation-field get it() → core::int;
  constructor • = self::ET_Extension|constructor#;
  constructor tearoff • = self::ET_Extension|constructor#_#new#tearOff;
 }
@ -240,17 +244,17 @@ static inline-class-member method ET_Dynamic|constructor#(dynamic it) → self::
 }
 static inline-class-member method ET_Dynamic|constructor#_#new#tearOff(dynamic it) → self::ET_Dynamic /* = dynamic */
  return self::ET_Dynamic|constructor#(it);
-static inline-class-member method ET_Void|constructor#(Never it) → self::ET_Void /* = Never */ {
-  lowered final self::ET_Void /* = Never */ #this = it;
+static inline-class-member method ET_Void|constructor#(Null it) → self::ET_Void /* = Null */ {
+  lowered final self::ET_Void /* = Null */ #this = it;
  return #this;
 }
-static inline-class-member method ET_Void|constructor#_#new#tearOff(Never it) → self::ET_Void /* = Never */
+static inline-class-member method ET_Void|constructor#_#new#tearOff(Null it) → self::ET_Void /* = Null */
  return self::ET_Void|constructor#(it);
-static inline-class-member method ET_Never|constructor#(Never it) → self::ET_Never /* = Never */ {
-  lowered final self::ET_Never /* = Never */ #this = it;
+static inline-class-member method ET_Never|constructor#(Never it) → self::ET_Never /* = invalid-type */ {
+  lowered final self::ET_Never /* = invalid-type */ #this = it;
  return #this;
 }
-static inline-class-member method ET_Never|constructor#_#new#tearOff(Never it) → self::ET_Never /* = Never */
+static inline-class-member method ET_Never|constructor#_#new#tearOff(Never it) → self::ET_Never /* = invalid-type */
  return self::ET_Never|constructor#(it);
 static inline-class-member method ET_Object|constructor#(core::Object it) → self::ET_Object /* = core::Object */ {
  lowered final self::ET_Object /* = core::Object */ #this = it;
@ -300,10 +304,10 @@ static inline-class-member method ET_FutureOr|constructor#(FutureOr<core::int>it
 }
 static inline-class-member method ET_FutureOr|constructor#_#new#tearOff(FutureOr<core::int>it) → self::ET_FutureOr /* = FutureOr<core::int>*/
  return self::ET_FutureOr|constructor#(it);
-static inline-class-member method ET_Extension|constructor#_#new#tearOff(Never it) → self::ET_Extension /* = Never */
+static inline-class-member method ET_Extension|constructor#_#new#tearOff(core::int it) → self::ET_Extension /* = core::int */
  return self::ET_Extension|constructor#(it);
-static inline-class-member method ET_Extension|constructor#(Never it) → self::ET_Extension /* = Never */ {
-  lowered final self::ET_Extension /* = Never */ #this = it;
+static inline-class-member method ET_Extension|constructor#(core::int it) → self::ET_Extension /* = core::int */ {
+  lowered final self::ET_Extension /* = core::int */ #this = it;
  return #this;
 }
 static inline-class-member method ET_TypeVariable|constructor#<T extends core::Object? = dynamic>(self::ET_TypeVariable|constructor#::T% it) → self::ET_TypeVariable<self::ET_TypeVariable|constructor#::T%> /* = self::ET_TypeVariable|constructor#::T% */ {
--- a/pkg/front_end/testcases/extension_types/implement_all.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/extension_types/implement_all.dart.textual_outline.expect
@ -12,7 +12,7 @@ extension type ExtensionType(int it) {}
 extension type GenericExtensionType<T>(T it) {}
 extension type ET_Null(Null it) implements Null {}
 extension type ET_Dynamic(dynamic it) implements dynamic {}
-extension type ET_Void(Never it) implements void {}
+extension type ET_Void(Null it) implements void {}
 extension type ET_Never(Never it) implements Never {}
 extension type ET_Object(Object it) implements Object {}
 extension type ET_Record(Record it) implements Record {}
@ -22,7 +22,7 @@ extension type ET_FunctionType(FunctionType it) implements FunctionType {}
 extension type ET_NullableInterfaceType(NullableInterfaceType it) implements NullableInterfaceType {}
 extension type ET_NullableExtensionType(int it) implements NullableExtensionType {}
 extension type ET_FutureOr(FutureOr<int> it) implements FutureOr<int> {}
-extension type ET_Extension(Never it) implements Extension {}
+extension type ET_Extension(int it) implements Extension {}
 extension type ET_TypeVariable<T>(T it) implements T {}
 extension type ET_Class(Class it) implements Class {}
 extension type ET_GenericClass<T>(GenericClass<T> it) implements GenericClass<T> {}
--- a/pkg/front_end/testcases/extension_types/implement_all.dart.textual_outline_modelled.expect
+++ b/pkg/front_end/testcases/extension_types/implement_all.dart.textual_outline_modelled.expect
@ -5,7 +5,7 @@ extension Extension on int {}
 extension type ET_Alias(Alias it) implements Alias {}
 extension type ET_Class(Class it) implements Class {}
 extension type ET_Dynamic(dynamic it) implements dynamic {}
-extension type ET_Extension(Never it) implements Extension {}
+extension type ET_Extension(int it) implements Extension {}
 extension type ET_ExtensionType(int it) implements ExtensionType {}
 extension type ET_Function(Function it) implements Function {}
 extension type ET_FunctionType(FunctionType it) implements FunctionType {}
@ -21,7 +21,7 @@ extension type ET_Object(Object it) implements Object {}
 extension type ET_Record(Record it) implements Record {}
 extension type ET_RecordType(RecordType it) implements RecordType {}
 extension type ET_TypeVariable<T>(T it) implements T {}
-extension type ET_Void(Never it) implements void {}
+extension type ET_Void(Null it) implements void {}
 extension type ExtensionType(int it) {}
 extension type GenericExtensionType<T>(T it) {}
 typedef Alias = Class;
--- a/pkg/front_end/testcases/extension_types/implement_all.dart.weak.expect
+++ b/pkg/front_end/testcases/extension_types/implement_all.dart.weak.expect
@ -2,18 +2,18 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: Type 'void' can't be used here.
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: Type 'void' can't be used here.
 // Try removing 'void' keyword or replace it with 'var', 'final', or a type.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^^^^
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^^^^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: Type 'void' not found.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^^^^
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: Type 'void' not found.
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^^^^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:57:50: Error: 'Extension' isn't a type.
-// extension type ET_Extension(Never it) implements Extension /* Error */ {}
-//                                                  ^^^^^^^^^
+// pkg/front_end/testcases/extension_types/implement_all.dart:57:48: Error: 'Extension' isn't a type.
+// extension type ET_Extension(int it) implements Extension /* Error */ {}
+//                                                ^^^^^^^^^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:29:44: Error: The type 'Null' can't be implemented by an extension type.
 // extension type ET_Null(Null it) implements Null /* Error */ {}
@ -23,14 +23,18 @@ library;
 // extension type ET_Dynamic(dynamic it) implements dynamic /* Error */ {}
 //                                                  ^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: The type 'void' can't be implemented by an extension type.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: The type 'void' can't be implemented by an extension type.
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:35:46: Error: The type 'Never' can't be implemented by an extension type.
 // extension type ET_Never(Never it) implements Never /* Error */ {}
 //                                              ^
 //
+// pkg/front_end/testcases/extension_types/implement_all.dart:35:31: Error: The representation type can't be a bottom type.
+// extension type ET_Never(Never it) implements Never /* Error */ {}
+//                               ^^
+//
 // pkg/front_end/testcases/extension_types/implement_all.dart:37:48: Error: The type 'Object' can't be implemented by an extension type.
 // extension type ET_Object(Object it) implements Object /* Error */ {}
 //                                                ^
@ -75,9 +79,9 @@ library;
 //     implements FutureOr<int> /* Error */ {}
 //                ^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:57:50: Error: The type 'Extension' can't be implemented by an extension type.
-// extension type ET_Extension(Never it) implements Extension /* Error */ {}
-//                                                  ^
+// pkg/front_end/testcases/extension_types/implement_all.dart:57:48: Error: The type 'Extension' can't be implemented by an extension type.
+// extension type ET_Extension(int it) implements Extension /* Error */ {}
+//                                                ^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:59:52: Error: The type variable 'T' can't be implemented by an extension type.
 // extension type ET_TypeVariable<T>(T it) implements T /* Error */ {}
@ -126,12 +130,12 @@ extension type ET_Dynamic(dynamic it) {
  constructor • = self::ET_Dynamic|constructor#;
  constructor tearoff • = self::ET_Dynamic|constructor#_#new#tearOff;
 }
-extension type ET_Void(Never it) {
-  abstract inline-class-member representation-field get it() → Never;
+extension type ET_Void(Null it) {
+  abstract inline-class-member representation-field get it() → Null;
  constructor • = self::ET_Void|constructor#;
  constructor tearoff • = self::ET_Void|constructor#_#new#tearOff;
 }
-extension type ET_Never(Never it) {
+extension type ET_Never(invalid-type it) {
  abstract inline-class-member representation-field get it() → Never;
  constructor • = self::ET_Never|constructor#;
  constructor tearoff • = self::ET_Never|constructor#_#new#tearOff;
@ -176,8 +180,8 @@ extension type ET_FutureOr(FutureOr<core::int>it) {
  constructor • = self::ET_FutureOr|constructor#;
  constructor tearoff • = self::ET_FutureOr|constructor#_#new#tearOff;
 }
-extension type ET_Extension(Never it) {
-  abstract inline-class-member representation-field get it() → Never;
+extension type ET_Extension(core::int it) {
+  abstract inline-class-member representation-field get it() → core::int;
  constructor • = self::ET_Extension|constructor#;
  constructor tearoff • = self::ET_Extension|constructor#_#new#tearOff;
 }
@ -240,17 +244,17 @@ static inline-class-member method ET_Dynamic|constructor#(dynamic it) → self::
 }
 static inline-class-member method ET_Dynamic|constructor#_#new#tearOff(dynamic it) → self::ET_Dynamic /* = dynamic */
  return self::ET_Dynamic|constructor#(it);
-static inline-class-member method ET_Void|constructor#(Never it) → self::ET_Void /* = Never */ {
-  lowered final self::ET_Void /* = Never */ #this = it;
+static inline-class-member method ET_Void|constructor#(Null it) → self::ET_Void /* = Null */ {
+  lowered final self::ET_Void /* = Null */ #this = it;
  return #this;
 }
-static inline-class-member method ET_Void|constructor#_#new#tearOff(Never it) → self::ET_Void /* = Never */
+static inline-class-member method ET_Void|constructor#_#new#tearOff(Null it) → self::ET_Void /* = Null */
  return self::ET_Void|constructor#(it);
-static inline-class-member method ET_Never|constructor#(Never it) → self::ET_Never /* = Never */ {
-  lowered final self::ET_Never /* = Never */ #this = it;
+static inline-class-member method ET_Never|constructor#(Never it) → self::ET_Never /* = invalid-type */ {
+  lowered final self::ET_Never /* = invalid-type */ #this = it;
  return #this;
 }
-static inline-class-member method ET_Never|constructor#_#new#tearOff(Never it) → self::ET_Never /* = Never */
+static inline-class-member method ET_Never|constructor#_#new#tearOff(Never it) → self::ET_Never /* = invalid-type */
  return self::ET_Never|constructor#(it);
 static inline-class-member method ET_Object|constructor#(core::Object it) → self::ET_Object /* = core::Object */ {
  lowered final self::ET_Object /* = core::Object */ #this = it;
@ -300,10 +304,10 @@ static inline-class-member method ET_FutureOr|constructor#(FutureOr<core::int>it
 }
 static inline-class-member method ET_FutureOr|constructor#_#new#tearOff(FutureOr<core::int>it) → self::ET_FutureOr /* = FutureOr<core::int>*/
  return self::ET_FutureOr|constructor#(it);
-static inline-class-member method ET_Extension|constructor#_#new#tearOff(Never it) → self::ET_Extension /* = Never */
+static inline-class-member method ET_Extension|constructor#_#new#tearOff(core::int it) → self::ET_Extension /* = core::int */
  return self::ET_Extension|constructor#(it);
-static inline-class-member method ET_Extension|constructor#(Never it) → self::ET_Extension /* = Never */ {
-  lowered final self::ET_Extension /* = Never */ #this = it;
+static inline-class-member method ET_Extension|constructor#(core::int it) → self::ET_Extension /* = core::int */ {
+  lowered final self::ET_Extension /* = core::int */ #this = it;
  return #this;
 }
 static inline-class-member method ET_TypeVariable|constructor#<T extends core::Object? = dynamic>(self::ET_TypeVariable|constructor#::T% it) → self::ET_TypeVariable<self::ET_TypeVariable|constructor#::T%> /* = self::ET_TypeVariable|constructor#::T% */ {
--- a/pkg/front_end/testcases/extension_types/implement_all.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/extension_types/implement_all.dart.weak.modular.expect
@ -2,18 +2,18 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: Type 'void' can't be used here.
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: Type 'void' can't be used here.
 // Try removing 'void' keyword or replace it with 'var', 'final', or a type.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^^^^
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^^^^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: Type 'void' not found.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^^^^
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: Type 'void' not found.
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^^^^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:57:50: Error: 'Extension' isn't a type.
-// extension type ET_Extension(Never it) implements Extension /* Error */ {}
-//                                                  ^^^^^^^^^
+// pkg/front_end/testcases/extension_types/implement_all.dart:57:48: Error: 'Extension' isn't a type.
+// extension type ET_Extension(int it) implements Extension /* Error */ {}
+//                                                ^^^^^^^^^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:29:44: Error: The type 'Null' can't be implemented by an extension type.
 // extension type ET_Null(Null it) implements Null /* Error */ {}
@ -23,14 +23,18 @@ library;
 // extension type ET_Dynamic(dynamic it) implements dynamic /* Error */ {}
 //                                                  ^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: The type 'void' can't be implemented by an extension type.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: The type 'void' can't be implemented by an extension type.
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:35:46: Error: The type 'Never' can't be implemented by an extension type.
 // extension type ET_Never(Never it) implements Never /* Error */ {}
 //                                              ^
 //
+// pkg/front_end/testcases/extension_types/implement_all.dart:35:31: Error: The representation type can't be a bottom type.
+// extension type ET_Never(Never it) implements Never /* Error */ {}
+//                               ^^
+//
 // pkg/front_end/testcases/extension_types/implement_all.dart:37:48: Error: The type 'Object' can't be implemented by an extension type.
 // extension type ET_Object(Object it) implements Object /* Error */ {}
 //                                                ^
@ -75,9 +79,9 @@ library;
 //     implements FutureOr<int> /* Error */ {}
 //                ^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:57:50: Error: The type 'Extension' can't be implemented by an extension type.
-// extension type ET_Extension(Never it) implements Extension /* Error */ {}
-//                                                  ^
+// pkg/front_end/testcases/extension_types/implement_all.dart:57:48: Error: The type 'Extension' can't be implemented by an extension type.
+// extension type ET_Extension(int it) implements Extension /* Error */ {}
+//                                                ^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:59:52: Error: The type variable 'T' can't be implemented by an extension type.
 // extension type ET_TypeVariable<T>(T it) implements T /* Error */ {}
@ -126,12 +130,12 @@ extension type ET_Dynamic(dynamic it) {
  constructor • = self::ET_Dynamic|constructor#;
  constructor tearoff • = self::ET_Dynamic|constructor#_#new#tearOff;
 }
-extension type ET_Void(Never it) {
-  abstract inline-class-member representation-field get it() → Never;
+extension type ET_Void(Null it) {
+  abstract inline-class-member representation-field get it() → Null;
  constructor • = self::ET_Void|constructor#;
  constructor tearoff • = self::ET_Void|constructor#_#new#tearOff;
 }
-extension type ET_Never(Never it) {
+extension type ET_Never(invalid-type it) {
  abstract inline-class-member representation-field get it() → Never;
  constructor • = self::ET_Never|constructor#;
  constructor tearoff • = self::ET_Never|constructor#_#new#tearOff;
@ -176,8 +180,8 @@ extension type ET_FutureOr(FutureOr<core::int>it) {
  constructor • = self::ET_FutureOr|constructor#;
  constructor tearoff • = self::ET_FutureOr|constructor#_#new#tearOff;
 }
-extension type ET_Extension(Never it) {
-  abstract inline-class-member representation-field get it() → Never;
+extension type ET_Extension(core::int it) {
+  abstract inline-class-member representation-field get it() → core::int;
  constructor • = self::ET_Extension|constructor#;
  constructor tearoff • = self::ET_Extension|constructor#_#new#tearOff;
 }
@ -240,17 +244,17 @@ static inline-class-member method ET_Dynamic|constructor#(dynamic it) → self::
 }
 static inline-class-member method ET_Dynamic|constructor#_#new#tearOff(dynamic it) → self::ET_Dynamic /* = dynamic */
  return self::ET_Dynamic|constructor#(it);
-static inline-class-member method ET_Void|constructor#(Never it) → self::ET_Void /* = Never */ {
-  lowered final self::ET_Void /* = Never */ #this = it;
+static inline-class-member method ET_Void|constructor#(Null it) → self::ET_Void /* = Null */ {
+  lowered final self::ET_Void /* = Null */ #this = it;
  return #this;
 }
-static inline-class-member method ET_Void|constructor#_#new#tearOff(Never it) → self::ET_Void /* = Never */
+static inline-class-member method ET_Void|constructor#_#new#tearOff(Null it) → self::ET_Void /* = Null */
  return self::ET_Void|constructor#(it);
-static inline-class-member method ET_Never|constructor#(Never it) → self::ET_Never /* = Never */ {
-  lowered final self::ET_Never /* = Never */ #this = it;
+static inline-class-member method ET_Never|constructor#(Never it) → self::ET_Never /* = invalid-type */ {
+  lowered final self::ET_Never /* = invalid-type */ #this = it;
  return #this;
 }
-static inline-class-member method ET_Never|constructor#_#new#tearOff(Never it) → self::ET_Never /* = Never */
+static inline-class-member method ET_Never|constructor#_#new#tearOff(Never it) → self::ET_Never /* = invalid-type */
  return self::ET_Never|constructor#(it);
 static inline-class-member method ET_Object|constructor#(core::Object it) → self::ET_Object /* = core::Object */ {
  lowered final self::ET_Object /* = core::Object */ #this = it;
@ -300,10 +304,10 @@ static inline-class-member method ET_FutureOr|constructor#(FutureOr<core::int>it
 }
 static inline-class-member method ET_FutureOr|constructor#_#new#tearOff(FutureOr<core::int>it) → self::ET_FutureOr /* = FutureOr<core::int>*/
  return self::ET_FutureOr|constructor#(it);
-static inline-class-member method ET_Extension|constructor#_#new#tearOff(Never it) → self::ET_Extension /* = Never */
+static inline-class-member method ET_Extension|constructor#_#new#tearOff(core::int it) → self::ET_Extension /* = core::int */
  return self::ET_Extension|constructor#(it);
-static inline-class-member method ET_Extension|constructor#(Never it) → self::ET_Extension /* = Never */ {
-  lowered final self::ET_Extension /* = Never */ #this = it;
+static inline-class-member method ET_Extension|constructor#(core::int it) → self::ET_Extension /* = core::int */ {
+  lowered final self::ET_Extension /* = core::int */ #this = it;
  return #this;
 }
 static inline-class-member method ET_TypeVariable|constructor#<T extends core::Object? = dynamic>(self::ET_TypeVariable|constructor#::T% it) → self::ET_TypeVariable<self::ET_TypeVariable|constructor#::T%> /* = self::ET_TypeVariable|constructor#::T% */ {
--- a/pkg/front_end/testcases/extension_types/implement_all.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/extension_types/implement_all.dart.weak.outline.expect
@ -2,18 +2,18 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: Type 'void' can't be used here.
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: Type 'void' can't be used here.
 // Try removing 'void' keyword or replace it with 'var', 'final', or a type.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^^^^
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^^^^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: Type 'void' not found.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^^^^
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: Type 'void' not found.
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^^^^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:57:50: Error: 'Extension' isn't a type.
-// extension type ET_Extension(Never it) implements Extension /* Error */ {}
-//                                                  ^^^^^^^^^
+// pkg/front_end/testcases/extension_types/implement_all.dart:57:48: Error: 'Extension' isn't a type.
+// extension type ET_Extension(int it) implements Extension /* Error */ {}
+//                                                ^^^^^^^^^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:29:44: Error: The type 'Null' can't be implemented by an extension type.
 // extension type ET_Null(Null it) implements Null /* Error */ {}
@ -23,14 +23,18 @@ library;
 // extension type ET_Dynamic(dynamic it) implements dynamic /* Error */ {}
 //                                                  ^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: The type 'void' can't be implemented by an extension type.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: The type 'void' can't be implemented by an extension type.
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:35:46: Error: The type 'Never' can't be implemented by an extension type.
 // extension type ET_Never(Never it) implements Never /* Error */ {}
 //                                              ^
 //
+// pkg/front_end/testcases/extension_types/implement_all.dart:35:31: Error: The representation type can't be a bottom type.
+// extension type ET_Never(Never it) implements Never /* Error */ {}
+//                               ^^
+//
 // pkg/front_end/testcases/extension_types/implement_all.dart:37:48: Error: The type 'Object' can't be implemented by an extension type.
 // extension type ET_Object(Object it) implements Object /* Error */ {}
 //                                                ^
@ -75,9 +79,9 @@ library;
 //     implements FutureOr<int> /* Error */ {}
 //                ^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:57:50: Error: The type 'Extension' can't be implemented by an extension type.
-// extension type ET_Extension(Never it) implements Extension /* Error */ {}
-//                                                  ^
+// pkg/front_end/testcases/extension_types/implement_all.dart:57:48: Error: The type 'Extension' can't be implemented by an extension type.
+// extension type ET_Extension(int it) implements Extension /* Error */ {}
+//                                                ^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:59:52: Error: The type variable 'T' can't be implemented by an extension type.
 // extension type ET_TypeVariable<T>(T it) implements T /* Error */ {}
@ -124,12 +128,12 @@ extension type ET_Dynamic(dynamic it) {
  constructor • = self::ET_Dynamic|constructor#;
  constructor tearoff • = self::ET_Dynamic|constructor#_#new#tearOff;
 }
-extension type ET_Void(Never it) {
-  abstract inline-class-member representation-field get it() → Never;
+extension type ET_Void(Null it) {
+  abstract inline-class-member representation-field get it() → Null;
  constructor • = self::ET_Void|constructor#;
  constructor tearoff • = self::ET_Void|constructor#_#new#tearOff;
 }
-extension type ET_Never(Never it) {
+extension type ET_Never(invalid-type it) {
  abstract inline-class-member representation-field get it() → Never;
  constructor • = self::ET_Never|constructor#;
  constructor tearoff • = self::ET_Never|constructor#_#new#tearOff;
@ -174,8 +178,8 @@ extension type ET_FutureOr(FutureOr<core::int>it) {
  constructor • = self::ET_FutureOr|constructor#;
  constructor tearoff • = self::ET_FutureOr|constructor#_#new#tearOff;
 }
-extension type ET_Extension(Never it) {
-  abstract inline-class-member representation-field get it() → Never;
+extension type ET_Extension(core::int it) {
+  abstract inline-class-member representation-field get it() → core::int;
  constructor • = self::ET_Extension|constructor#;
  constructor tearoff • = self::ET_Extension|constructor#_#new#tearOff;
 }
@ -230,13 +234,13 @@ static inline-class-member method ET_Dynamic|constructor#(dynamic it) → self::
  ;
 static inline-class-member method ET_Dynamic|constructor#_#new#tearOff(dynamic it) → self::ET_Dynamic /* = dynamic */
  return self::ET_Dynamic|constructor#(it);
-static inline-class-member method ET_Void|constructor#(Never it) → self::ET_Void /* = Never */
+static inline-class-member method ET_Void|constructor#(Null it) → self::ET_Void /* = Null */
  ;
-static inline-class-member method ET_Void|constructor#_#new#tearOff(Never it) → self::ET_Void /* = Never */
+static inline-class-member method ET_Void|constructor#_#new#tearOff(Null it) → self::ET_Void /* = Null */
  return self::ET_Void|constructor#(it);
-static inline-class-member method ET_Never|constructor#(Never it) → self::ET_Never /* = Never */
+static inline-class-member method ET_Never|constructor#(Never it) → self::ET_Never /* = invalid-type */
  ;
-static inline-class-member method ET_Never|constructor#_#new#tearOff(Never it) → self::ET_Never /* = Never */
+static inline-class-member method ET_Never|constructor#_#new#tearOff(Never it) → self::ET_Never /* = invalid-type */
  return self::ET_Never|constructor#(it);
 static inline-class-member method ET_Object|constructor#(core::Object it) → self::ET_Object /* = core::Object */
  ;
@ -270,9 +274,9 @@ static inline-class-member method ET_FutureOr|constructor#(FutureOr<core::int>it
  ;
 static inline-class-member method ET_FutureOr|constructor#_#new#tearOff(FutureOr<core::int>it) → self::ET_FutureOr /* = FutureOr<core::int>*/
  return self::ET_FutureOr|constructor#(it);
-static inline-class-member method ET_Extension|constructor#_#new#tearOff(Never it) → self::ET_Extension /* = Never */
+static inline-class-member method ET_Extension|constructor#_#new#tearOff(core::int it) → self::ET_Extension /* = core::int */
  return self::ET_Extension|constructor#(it);
-static inline-class-member method ET_Extension|constructor#(Never it) → self::ET_Extension /* = Never */
+static inline-class-member method ET_Extension|constructor#(core::int it) → self::ET_Extension /* = core::int */
  ;
 static inline-class-member method ET_TypeVariable|constructor#<T extends core::Object? = dynamic>(self::ET_TypeVariable|constructor#::T% it) → self::ET_TypeVariable<self::ET_TypeVariable|constructor#::T%> /* = self::ET_TypeVariable|constructor#::T% */
  ;
--- a/pkg/front_end/testcases/extension_types/implement_all.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/implement_all.dart.weak.transformed.expect
@ -2,18 +2,18 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: Type 'void' can't be used here.
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: Type 'void' can't be used here.
 // Try removing 'void' keyword or replace it with 'var', 'final', or a type.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^^^^
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^^^^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: Type 'void' not found.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^^^^
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: Type 'void' not found.
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^^^^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:57:50: Error: 'Extension' isn't a type.
-// extension type ET_Extension(Never it) implements Extension /* Error */ {}
-//                                                  ^^^^^^^^^
+// pkg/front_end/testcases/extension_types/implement_all.dart:57:48: Error: 'Extension' isn't a type.
+// extension type ET_Extension(int it) implements Extension /* Error */ {}
+//                                                ^^^^^^^^^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:29:44: Error: The type 'Null' can't be implemented by an extension type.
 // extension type ET_Null(Null it) implements Null /* Error */ {}
@ -23,14 +23,18 @@ library;
 // extension type ET_Dynamic(dynamic it) implements dynamic /* Error */ {}
 //                                                  ^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:33:45: Error: The type 'void' can't be implemented by an extension type.
-// extension type ET_Void(Never it) implements void /* Error */ {}
-//                                             ^
+// pkg/front_end/testcases/extension_types/implement_all.dart:33:44: Error: The type 'void' can't be implemented by an extension type.
+// extension type ET_Void(Null it) implements void /* Error */ {}
+//                                            ^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:35:46: Error: The type 'Never' can't be implemented by an extension type.
 // extension type ET_Never(Never it) implements Never /* Error */ {}
 //                                              ^
 //
+// pkg/front_end/testcases/extension_types/implement_all.dart:35:31: Error: The representation type can't be a bottom type.
+// extension type ET_Never(Never it) implements Never /* Error */ {}
+//                               ^^
+//
 // pkg/front_end/testcases/extension_types/implement_all.dart:37:48: Error: The type 'Object' can't be implemented by an extension type.
 // extension type ET_Object(Object it) implements Object /* Error */ {}
 //                                                ^
@ -75,9 +79,9 @@ library;
 //     implements FutureOr<int> /* Error */ {}
 //                ^
 //
-// pkg/front_end/testcases/extension_types/implement_all.dart:57:50: Error: The type 'Extension' can't be implemented by an extension type.
-// extension type ET_Extension(Never it) implements Extension /* Error */ {}
-//                                                  ^
+// pkg/front_end/testcases/extension_types/implement_all.dart:57:48: Error: The type 'Extension' can't be implemented by an extension type.
+// extension type ET_Extension(int it) implements Extension /* Error */ {}
+//                                                ^
 //
 // pkg/front_end/testcases/extension_types/implement_all.dart:59:52: Error: The type variable 'T' can't be implemented by an extension type.
 // extension type ET_TypeVariable<T>(T it) implements T /* Error */ {}
@ -126,12 +130,12 @@ extension type ET_Dynamic(dynamic it) {
  constructor • = self::ET_Dynamic|constructor#;
  constructor tearoff • = self::ET_Dynamic|constructor#_#new#tearOff;
 }
-extension type ET_Void(Never it) {
-  abstract inline-class-member representation-field get it() → Never;
+extension type ET_Void(Null it) {
+  abstract inline-class-member representation-field get it() → Null;
  constructor • = self::ET_Void|constructor#;
  constructor tearoff • = self::ET_Void|constructor#_#new#tearOff;
 }
-extension type ET_Never(Never it) {
+extension type ET_Never(invalid-type it) {
  abstract inline-class-member representation-field get it() → Never;
  constructor • = self::ET_Never|constructor#;
  constructor tearoff • = self::ET_Never|constructor#_#new#tearOff;
@ -176,8 +180,8 @@ extension type ET_FutureOr(FutureOr<core::int>it) {
  constructor • = self::ET_FutureOr|constructor#;
  constructor tearoff • = self::ET_FutureOr|constructor#_#new#tearOff;
 }
-extension type ET_Extension(Never it) {
-  abstract inline-class-member representation-field get it() → Never;
+extension type ET_Extension(core::int it) {
+  abstract inline-class-member representation-field get it() → core::int;
  constructor • = self::ET_Extension|constructor#;
  constructor tearoff • = self::ET_Extension|constructor#_#new#tearOff;
 }
@ -240,17 +244,17 @@ static inline-class-member method ET_Dynamic|constructor#(dynamic it) → self::
 }
 static inline-class-member method ET_Dynamic|constructor#_#new#tearOff(dynamic it) → self::ET_Dynamic /* = dynamic */
  return self::ET_Dynamic|constructor#(it);
-static inline-class-member method ET_Void|constructor#(Never it) → self::ET_Void /* = Never */ {
-  lowered final self::ET_Void /* = Never */ #this = it;
+static inline-class-member method ET_Void|constructor#(Null it) → self::ET_Void /* = Null */ {
+  lowered final self::ET_Void /* = Null */ #this = it;
  return #this;
 }
-static inline-class-member method ET_Void|constructor#_#new#tearOff(Never it) → self::ET_Void /* = Never */
+static inline-class-member method ET_Void|constructor#_#new#tearOff(Null it) → self::ET_Void /* = Null */
  return self::ET_Void|constructor#(it);
-static inline-class-member method ET_Never|constructor#(Never it) → self::ET_Never /* = Never */ {
-  lowered final self::ET_Never /* = Never */ #this = it;
+static inline-class-member method ET_Never|constructor#(Never it) → self::ET_Never /* = invalid-type */ {
+  lowered final self::ET_Never /* = invalid-type */ #this = it;
  return #this;
 }
-static inline-class-member method ET_Never|constructor#_#new#tearOff(Never it) → self::ET_Never /* = Never */
+static inline-class-member method ET_Never|constructor#_#new#tearOff(Never it) → self::ET_Never /* = invalid-type */
  return self::ET_Never|constructor#(it);
 static inline-class-member method ET_Object|constructor#(core::Object it) → self::ET_Object /* = core::Object */ {
  lowered final self::ET_Object /* = core::Object */ #this = it;
@ -300,10 +304,10 @@ static inline-class-member method ET_FutureOr|constructor#(FutureOr<core::int>it
 }
 static inline-class-member method ET_FutureOr|constructor#_#new#tearOff(FutureOr<core::int>it) → self::ET_FutureOr /* = FutureOr<core::int>*/
  return self::ET_FutureOr|constructor#(it);
-static inline-class-member method ET_Extension|constructor#_#new#tearOff(Never it) → self::ET_Extension /* = Never */
+static inline-class-member method ET_Extension|constructor#_#new#tearOff(core::int it) → self::ET_Extension /* = core::int */
  return self::ET_Extension|constructor#(it);
-static inline-class-member method ET_Extension|constructor#(Never it) → self::ET_Extension /* = Never */ {
-  lowered final self::ET_Extension /* = Never */ #this = it;
+static inline-class-member method ET_Extension|constructor#(core::int it) → self::ET_Extension /* = core::int */ {
+  lowered final self::ET_Extension /* = core::int */ #this = it;
  return #this;
 }
 static inline-class-member method ET_TypeVariable|constructor#<T extends core::Object? = dynamic>(self::ET_TypeVariable|constructor#::T% it) → self::ET_TypeVariable<self::ET_TypeVariable|constructor#::T%> /* = self::ET_TypeVariable|constructor#::T% */ {
--- a/pkg/front_end/testcases/extension_types/implements_conflicts.dart
+++ b/pkg/front_end/testcases/extension_types/implements_conflicts.dart
@ -70,7 +70,7 @@ extension type ExtensionType9(int i) {
 extension type ExtensionType9a(Never n) /* Error */
  implements ClassJ, ClassK {}

-extension type ExtensionType9b(Never n) /* Ok */
+extension type ExtensionType9b(Never n) /* Error */
    implements ClassJ, ClassK {
  int get method4 => 42; /* Ok */
  void set method4(int value) {} /* Ok */
@ -85,7 +85,7 @@ extension type ExtensionType10b(int i) /* Ok */
  void set method4(int value) {} /* Ok */
 }

-extension type ExtensionType11(Never n) {
+extension type ExtensionType11(Null n) {
  int get property => 42; /* Error */
  void set property(String value) {} /* Error */
 }
@ -94,17 +94,17 @@ extension type ExtensionType11(Never n) {
 extension type ExtensionType12a(Never n) /* Error */
    implements ClassL, ClassM {}

-extension type ExtensionType12b(Never n) /* Error */
+extension type ExtensionType12b(ClassL n) /* Error */
    implements ClassL {
  void set property(bool value) {} /* Error */
 }

-extension type ExtensionType12c(Never n) /* Error */
+extension type ExtensionType12c(ClassM n) /* Error */
    implements ClassM {
  bool get property => true; /* Error */
 }

-extension type ExtensionType12d(Never n) /* Ok */
+extension type ExtensionType12d(Never n) /* Error */
    implements ClassL, ClassM {
  bool get property => true; /* Ok */
  void set property(bool value) {} /* Ok */
--- a/pkg/front_end/testcases/extension_types/implements_conflicts.dart.strong.expect
+++ b/pkg/front_end/testcases/extension_types/implements_conflicts.dart.strong.expect
@ -10,6 +10,22 @@ library;
 //   void method4() {} /* Error */
 //        ^
 //
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:70:38: Error: The representation type can't be a bottom type.
+// extension type ExtensionType9a(Never n) /* Error */
+//                                      ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:73:38: Error: The representation type can't be a bottom type.
+// extension type ExtensionType9b(Never n) /* Error */
+//                                      ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:94:39: Error: The representation type can't be a bottom type.
+// extension type ExtensionType12a(Never n) /* Error */
+//                                       ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:107:39: Error: The representation type can't be a bottom type.
+// extension type ExtensionType12d(Never n) /* Error */
+//                                       ^
+//
 // pkg/front_end/testcases/extension_types/implements_conflicts.dart:7:16: Error: The extension type 'ExtensionType1a' can't inherit the member 'method1' as both an extension type member and a non-extension type member.
 // Try declaring a member 'method1' in 'ExtensionType1a' to resolve the conflict.
 // extension type ExtensionType1a(ClassA c) /* Error */
@ -217,12 +233,12 @@ extension type ExtensionType9(core::int i) {
  constructor • = self::ExtensionType9|constructor#;
  constructor tearoff • = self::ExtensionType9|constructor#_#new#tearOff;
 }
-extension type ExtensionType9a(Never n) implements imp::ClassJ, imp::ClassK {
+extension type ExtensionType9a(invalid-type n) implements imp::ClassJ, imp::ClassK {
  abstract inline-class-member representation-field get n() → Never;
  constructor • = self::ExtensionType9a|constructor#;
  constructor tearoff • = self::ExtensionType9a|constructor#_#new#tearOff;
 }
-extension type ExtensionType9b(Never n) implements imp::ClassJ, imp::ClassK {
+extension type ExtensionType9b(invalid-type n) implements imp::ClassJ, imp::ClassK {
  abstract inline-class-member representation-field get n() → Never;
  get method4 = self::ExtensionType9b|get#method4;
  set method4 = self::ExtensionType9b|set#method4;
@ -241,31 +257,31 @@ extension type ExtensionType10b(core::int i) implements imp::ExtensionTypeJ /* =
  constructor • = self::ExtensionType10b|constructor#;
  constructor tearoff • = self::ExtensionType10b|constructor#_#new#tearOff;
 }
-extension type ExtensionType11(Never n) {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType11(Null n) {
+  abstract inline-class-member representation-field get n() → Null;
  get property = self::ExtensionType11|get#property;
  set property = self::ExtensionType11|set#property;
  constructor • = self::ExtensionType11|constructor#;
  constructor tearoff • = self::ExtensionType11|constructor#_#new#tearOff;
 }
-extension type ExtensionType12a(Never n) implements imp::ClassL, imp::ClassM {
+extension type ExtensionType12a(invalid-type n) implements imp::ClassL, imp::ClassM {
  abstract inline-class-member representation-field get n() → Never;
  constructor • = self::ExtensionType12a|constructor#;
  constructor tearoff • = self::ExtensionType12a|constructor#_#new#tearOff;
 }
-extension type ExtensionType12b(Never n) implements imp::ClassL {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType12b(imp::ClassL n) implements imp::ClassL {
+  abstract inline-class-member representation-field get n() → imp::ClassL;
  set property = self::ExtensionType12b|set#property;
  constructor • = self::ExtensionType12b|constructor#;
  constructor tearoff • = self::ExtensionType12b|constructor#_#new#tearOff;
 }
-extension type ExtensionType12c(Never n) implements imp::ClassM {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType12c(imp::ClassM n) implements imp::ClassM {
+  abstract inline-class-member representation-field get n() → imp::ClassM;
  get property = self::ExtensionType12c|get#property;
  constructor • = self::ExtensionType12c|constructor#;
  constructor tearoff • = self::ExtensionType12c|constructor#_#new#tearOff;
 }
-extension type ExtensionType12d(Never n) implements imp::ClassL, imp::ClassM {
+extension type ExtensionType12d(invalid-type n) implements imp::ClassL, imp::ClassM {
  abstract inline-class-member representation-field get n() → Never;
  get property = self::ExtensionType12d|get#property;
  set property = self::ExtensionType12d|set#property;
@ -414,21 +430,21 @@ static inline-class-member method ExtensionType9|method4(lowered final self::Ext
 static inline-class-member method ExtensionType9|get#method4(lowered final self::ExtensionType9 /* = core::int */ #this) → () → void
  return () → void => self::ExtensionType9|method4(#this);
 static inline-class-member method ExtensionType9|set#method4(lowered final self::ExtensionType9 /* = core::int */ #this, core::int value) → void {}
-static inline-class-member method ExtensionType9a|constructor#(Never n) → self::ExtensionType9a /* = Never */ {
-  lowered final self::ExtensionType9a /* = Never */ #this = n;
+static inline-class-member method ExtensionType9a|constructor#(Never n) → self::ExtensionType9a /* = invalid-type */ {
+  lowered final self::ExtensionType9a /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType9a|constructor#_#new#tearOff(Never n) → self::ExtensionType9a /* = Never */
+static inline-class-member method ExtensionType9a|constructor#_#new#tearOff(Never n) → self::ExtensionType9a /* = invalid-type */
  return self::ExtensionType9a|constructor#(n);
-static inline-class-member method ExtensionType9b|constructor#(Never n) → self::ExtensionType9b /* = Never */ {
-  lowered final self::ExtensionType9b /* = Never */ #this = n;
+static inline-class-member method ExtensionType9b|constructor#(Never n) → self::ExtensionType9b /* = invalid-type */ {
+  lowered final self::ExtensionType9b /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType9b|constructor#_#new#tearOff(Never n) → self::ExtensionType9b /* = Never */
+static inline-class-member method ExtensionType9b|constructor#_#new#tearOff(Never n) → self::ExtensionType9b /* = invalid-type */
  return self::ExtensionType9b|constructor#(n);
-static inline-class-member method ExtensionType9b|get#method4(lowered final self::ExtensionType9b /* = Never */ #this) → core::int
+static inline-class-member method ExtensionType9b|get#method4(lowered final self::ExtensionType9b /* = invalid-type */ #this) → core::int
  return 42;
-static inline-class-member method ExtensionType9b|set#method4(lowered final self::ExtensionType9b /* = Never */ #this, core::int value) → void {}
+static inline-class-member method ExtensionType9b|set#method4(lowered final self::ExtensionType9b /* = invalid-type */ #this, core::int value) → void {}
 static inline-class-member method ExtensionType10a|constructor#(core::int i) → self::ExtensionType10a /* = core::int */ {
  lowered final self::ExtensionType10a /* = core::int */ #this = i;
  return #this;
@ -444,45 +460,45 @@ static inline-class-member method ExtensionType10b|constructor#(core::int i) →
 static inline-class-member method ExtensionType10b|get#method4(lowered final self::ExtensionType10b /* = core::int */ #this) → core::int
  return 42;
 static inline-class-member method ExtensionType10b|set#method4(lowered final self::ExtensionType10b /* = core::int */ #this, core::int value) → void {}
-static inline-class-member method ExtensionType11|constructor#(Never n) → self::ExtensionType11 /* = Never */ {
-  lowered final self::ExtensionType11 /* = Never */ #this = n;
+static inline-class-member method ExtensionType11|constructor#(Null n) → self::ExtensionType11 /* = Null */ {
+  lowered final self::ExtensionType11 /* = Null */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType11|constructor#_#new#tearOff(Never n) → self::ExtensionType11 /* = Never */
+static inline-class-member method ExtensionType11|constructor#_#new#tearOff(Null n) → self::ExtensionType11 /* = Null */
  return self::ExtensionType11|constructor#(n);
-static inline-class-member method ExtensionType11|get#property(lowered final self::ExtensionType11 /* = Never */ #this) → core::int
+static inline-class-member method ExtensionType11|get#property(lowered final self::ExtensionType11 /* = Null */ #this) → core::int
  return 42;
-static inline-class-member method ExtensionType11|set#property(lowered final self::ExtensionType11 /* = Never */ #this, core::String value) → void {}
-static inline-class-member method ExtensionType12a|constructor#(Never n) → self::ExtensionType12a /* = Never */ {
-  lowered final self::ExtensionType12a /* = Never */ #this = n;
+static inline-class-member method ExtensionType11|set#property(lowered final self::ExtensionType11 /* = Null */ #this, core::String value) → void {}
+static inline-class-member method ExtensionType12a|constructor#(Never n) → self::ExtensionType12a /* = invalid-type */ {
+  lowered final self::ExtensionType12a /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12a|constructor#_#new#tearOff(Never n) → self::ExtensionType12a /* = Never */
+static inline-class-member method ExtensionType12a|constructor#_#new#tearOff(Never n) → self::ExtensionType12a /* = invalid-type */
  return self::ExtensionType12a|constructor#(n);
-static inline-class-member method ExtensionType12b|constructor#(Never n) → self::ExtensionType12b /* = Never */ {
-  lowered final self::ExtensionType12b /* = Never */ #this = n;
+static inline-class-member method ExtensionType12b|constructor#(imp::ClassL n) → self::ExtensionType12b /* = imp::ClassL */ {
+  lowered final self::ExtensionType12b /* = imp::ClassL */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12b|constructor#_#new#tearOff(Never n) → self::ExtensionType12b /* = Never */
+static inline-class-member method ExtensionType12b|constructor#_#new#tearOff(imp::ClassL n) → self::ExtensionType12b /* = imp::ClassL */
  return self::ExtensionType12b|constructor#(n);
-static inline-class-member method ExtensionType12b|set#property(lowered final self::ExtensionType12b /* = Never */ #this, core::bool value) → void {}
-static inline-class-member method ExtensionType12c|constructor#(Never n) → self::ExtensionType12c /* = Never */ {
-  lowered final self::ExtensionType12c /* = Never */ #this = n;
+static inline-class-member method ExtensionType12b|set#property(lowered final self::ExtensionType12b /* = imp::ClassL */ #this, core::bool value) → void {}
+static inline-class-member method ExtensionType12c|constructor#(imp::ClassM n) → self::ExtensionType12c /* = imp::ClassM */ {
+  lowered final self::ExtensionType12c /* = imp::ClassM */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12c|constructor#_#new#tearOff(Never n) → self::ExtensionType12c /* = Never */
+static inline-class-member method ExtensionType12c|constructor#_#new#tearOff(imp::ClassM n) → self::ExtensionType12c /* = imp::ClassM */
  return self::ExtensionType12c|constructor#(n);
-static inline-class-member method ExtensionType12c|get#property(lowered final self::ExtensionType12c /* = Never */ #this) → core::bool
+static inline-class-member method ExtensionType12c|get#property(lowered final self::ExtensionType12c /* = imp::ClassM */ #this) → core::bool
  return true;
-static inline-class-member method ExtensionType12d|constructor#(Never n) → self::ExtensionType12d /* = Never */ {
-  lowered final self::ExtensionType12d /* = Never */ #this = n;
+static inline-class-member method ExtensionType12d|constructor#(Never n) → self::ExtensionType12d /* = invalid-type */ {
+  lowered final self::ExtensionType12d /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12d|constructor#_#new#tearOff(Never n) → self::ExtensionType12d /* = Never */
+static inline-class-member method ExtensionType12d|constructor#_#new#tearOff(Never n) → self::ExtensionType12d /* = invalid-type */
  return self::ExtensionType12d|constructor#(n);
-static inline-class-member method ExtensionType12d|get#property(lowered final self::ExtensionType12d /* = Never */ #this) → core::bool
+static inline-class-member method ExtensionType12d|get#property(lowered final self::ExtensionType12d /* = invalid-type */ #this) → core::bool
  return true;
-static inline-class-member method ExtensionType12d|set#property(lowered final self::ExtensionType12d /* = Never */ #this, core::bool value) → void {}
+static inline-class-member method ExtensionType12d|set#property(lowered final self::ExtensionType12d /* = invalid-type */ #this, core::bool value) → void {}
 static inline-class-member method ExtensionType13a|constructor#(core::int i) → self::ExtensionType13a /* = core::int */ {
  lowered final self::ExtensionType13a /* = core::int */ #this = i;
  return #this;
--- a/pkg/front_end/testcases/extension_types/implements_conflicts.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/implements_conflicts.dart.strong.transformed.expect
@ -10,6 +10,22 @@ library;
 //   void method4() {} /* Error */
 //        ^
 //
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:70:38: Error: The representation type can't be a bottom type.
+// extension type ExtensionType9a(Never n) /* Error */
+//                                      ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:73:38: Error: The representation type can't be a bottom type.
+// extension type ExtensionType9b(Never n) /* Error */
+//                                      ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:94:39: Error: The representation type can't be a bottom type.
+// extension type ExtensionType12a(Never n) /* Error */
+//                                       ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:107:39: Error: The representation type can't be a bottom type.
+// extension type ExtensionType12d(Never n) /* Error */
+//                                       ^
+//
 // pkg/front_end/testcases/extension_types/implements_conflicts.dart:7:16: Error: The extension type 'ExtensionType1a' can't inherit the member 'method1' as both an extension type member and a non-extension type member.
 // Try declaring a member 'method1' in 'ExtensionType1a' to resolve the conflict.
 // extension type ExtensionType1a(ClassA c) /* Error */
@ -217,12 +233,12 @@ extension type ExtensionType9(core::int i) {
  constructor • = self::ExtensionType9|constructor#;
  constructor tearoff • = self::ExtensionType9|constructor#_#new#tearOff;
 }
-extension type ExtensionType9a(Never n) implements imp::ClassJ, imp::ClassK {
+extension type ExtensionType9a(invalid-type n) implements imp::ClassJ, imp::ClassK {
  abstract inline-class-member representation-field get n() → Never;
  constructor • = self::ExtensionType9a|constructor#;
  constructor tearoff • = self::ExtensionType9a|constructor#_#new#tearOff;
 }
-extension type ExtensionType9b(Never n) implements imp::ClassJ, imp::ClassK {
+extension type ExtensionType9b(invalid-type n) implements imp::ClassJ, imp::ClassK {
  abstract inline-class-member representation-field get n() → Never;
  get method4 = self::ExtensionType9b|get#method4;
  set method4 = self::ExtensionType9b|set#method4;
@ -241,31 +257,31 @@ extension type ExtensionType10b(core::int i) implements imp::ExtensionTypeJ /* =
  constructor • = self::ExtensionType10b|constructor#;
  constructor tearoff • = self::ExtensionType10b|constructor#_#new#tearOff;
 }
-extension type ExtensionType11(Never n) {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType11(Null n) {
+  abstract inline-class-member representation-field get n() → Null;
  get property = self::ExtensionType11|get#property;
  set property = self::ExtensionType11|set#property;
  constructor • = self::ExtensionType11|constructor#;
  constructor tearoff • = self::ExtensionType11|constructor#_#new#tearOff;
 }
-extension type ExtensionType12a(Never n) implements imp::ClassL, imp::ClassM {
+extension type ExtensionType12a(invalid-type n) implements imp::ClassL, imp::ClassM {
  abstract inline-class-member representation-field get n() → Never;
  constructor • = self::ExtensionType12a|constructor#;
  constructor tearoff • = self::ExtensionType12a|constructor#_#new#tearOff;
 }
-extension type ExtensionType12b(Never n) implements imp::ClassL {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType12b(imp::ClassL n) implements imp::ClassL {
+  abstract inline-class-member representation-field get n() → imp::ClassL;
  set property = self::ExtensionType12b|set#property;
  constructor • = self::ExtensionType12b|constructor#;
  constructor tearoff • = self::ExtensionType12b|constructor#_#new#tearOff;
 }
-extension type ExtensionType12c(Never n) implements imp::ClassM {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType12c(imp::ClassM n) implements imp::ClassM {
+  abstract inline-class-member representation-field get n() → imp::ClassM;
  get property = self::ExtensionType12c|get#property;
  constructor • = self::ExtensionType12c|constructor#;
  constructor tearoff • = self::ExtensionType12c|constructor#_#new#tearOff;
 }
-extension type ExtensionType12d(Never n) implements imp::ClassL, imp::ClassM {
+extension type ExtensionType12d(invalid-type n) implements imp::ClassL, imp::ClassM {
  abstract inline-class-member representation-field get n() → Never;
  get property = self::ExtensionType12d|get#property;
  set property = self::ExtensionType12d|set#property;
@ -414,21 +430,21 @@ static inline-class-member method ExtensionType9|method4(lowered final self::Ext
 static inline-class-member method ExtensionType9|get#method4(lowered final self::ExtensionType9 /* = core::int */ #this) → () → void
  return () → void => self::ExtensionType9|method4(#this);
 static inline-class-member method ExtensionType9|set#method4(lowered final self::ExtensionType9 /* = core::int */ #this, core::int value) → void {}
-static inline-class-member method ExtensionType9a|constructor#(Never n) → self::ExtensionType9a /* = Never */ {
-  lowered final self::ExtensionType9a /* = Never */ #this = n;
+static inline-class-member method ExtensionType9a|constructor#(Never n) → self::ExtensionType9a /* = invalid-type */ {
+  lowered final self::ExtensionType9a /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType9a|constructor#_#new#tearOff(Never n) → self::ExtensionType9a /* = Never */
+static inline-class-member method ExtensionType9a|constructor#_#new#tearOff(Never n) → self::ExtensionType9a /* = invalid-type */
  return self::ExtensionType9a|constructor#(n);
-static inline-class-member method ExtensionType9b|constructor#(Never n) → self::ExtensionType9b /* = Never */ {
-  lowered final self::ExtensionType9b /* = Never */ #this = n;
+static inline-class-member method ExtensionType9b|constructor#(Never n) → self::ExtensionType9b /* = invalid-type */ {
+  lowered final self::ExtensionType9b /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType9b|constructor#_#new#tearOff(Never n) → self::ExtensionType9b /* = Never */
+static inline-class-member method ExtensionType9b|constructor#_#new#tearOff(Never n) → self::ExtensionType9b /* = invalid-type */
  return self::ExtensionType9b|constructor#(n);
-static inline-class-member method ExtensionType9b|get#method4(lowered final self::ExtensionType9b /* = Never */ #this) → core::int
+static inline-class-member method ExtensionType9b|get#method4(lowered final self::ExtensionType9b /* = invalid-type */ #this) → core::int
  return 42;
-static inline-class-member method ExtensionType9b|set#method4(lowered final self::ExtensionType9b /* = Never */ #this, core::int value) → void {}
+static inline-class-member method ExtensionType9b|set#method4(lowered final self::ExtensionType9b /* = invalid-type */ #this, core::int value) → void {}
 static inline-class-member method ExtensionType10a|constructor#(core::int i) → self::ExtensionType10a /* = core::int */ {
  lowered final self::ExtensionType10a /* = core::int */ #this = i;
  return #this;
@ -444,45 +460,45 @@ static inline-class-member method ExtensionType10b|constructor#(core::int i) →
 static inline-class-member method ExtensionType10b|get#method4(lowered final self::ExtensionType10b /* = core::int */ #this) → core::int
  return 42;
 static inline-class-member method ExtensionType10b|set#method4(lowered final self::ExtensionType10b /* = core::int */ #this, core::int value) → void {}
-static inline-class-member method ExtensionType11|constructor#(Never n) → self::ExtensionType11 /* = Never */ {
-  lowered final self::ExtensionType11 /* = Never */ #this = n;
+static inline-class-member method ExtensionType11|constructor#(Null n) → self::ExtensionType11 /* = Null */ {
+  lowered final self::ExtensionType11 /* = Null */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType11|constructor#_#new#tearOff(Never n) → self::ExtensionType11 /* = Never */
+static inline-class-member method ExtensionType11|constructor#_#new#tearOff(Null n) → self::ExtensionType11 /* = Null */
  return self::ExtensionType11|constructor#(n);
-static inline-class-member method ExtensionType11|get#property(lowered final self::ExtensionType11 /* = Never */ #this) → core::int
+static inline-class-member method ExtensionType11|get#property(lowered final self::ExtensionType11 /* = Null */ #this) → core::int
  return 42;
-static inline-class-member method ExtensionType11|set#property(lowered final self::ExtensionType11 /* = Never */ #this, core::String value) → void {}
-static inline-class-member method ExtensionType12a|constructor#(Never n) → self::ExtensionType12a /* = Never */ {
-  lowered final self::ExtensionType12a /* = Never */ #this = n;
+static inline-class-member method ExtensionType11|set#property(lowered final self::ExtensionType11 /* = Null */ #this, core::String value) → void {}
+static inline-class-member method ExtensionType12a|constructor#(Never n) → self::ExtensionType12a /* = invalid-type */ {
+  lowered final self::ExtensionType12a /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12a|constructor#_#new#tearOff(Never n) → self::ExtensionType12a /* = Never */
+static inline-class-member method ExtensionType12a|constructor#_#new#tearOff(Never n) → self::ExtensionType12a /* = invalid-type */
  return self::ExtensionType12a|constructor#(n);
-static inline-class-member method ExtensionType12b|constructor#(Never n) → self::ExtensionType12b /* = Never */ {
-  lowered final self::ExtensionType12b /* = Never */ #this = n;
+static inline-class-member method ExtensionType12b|constructor#(imp::ClassL n) → self::ExtensionType12b /* = imp::ClassL */ {
+  lowered final self::ExtensionType12b /* = imp::ClassL */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12b|constructor#_#new#tearOff(Never n) → self::ExtensionType12b /* = Never */
+static inline-class-member method ExtensionType12b|constructor#_#new#tearOff(imp::ClassL n) → self::ExtensionType12b /* = imp::ClassL */
  return self::ExtensionType12b|constructor#(n);
-static inline-class-member method ExtensionType12b|set#property(lowered final self::ExtensionType12b /* = Never */ #this, core::bool value) → void {}
-static inline-class-member method ExtensionType12c|constructor#(Never n) → self::ExtensionType12c /* = Never */ {
-  lowered final self::ExtensionType12c /* = Never */ #this = n;
+static inline-class-member method ExtensionType12b|set#property(lowered final self::ExtensionType12b /* = imp::ClassL */ #this, core::bool value) → void {}
+static inline-class-member method ExtensionType12c|constructor#(imp::ClassM n) → self::ExtensionType12c /* = imp::ClassM */ {
+  lowered final self::ExtensionType12c /* = imp::ClassM */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12c|constructor#_#new#tearOff(Never n) → self::ExtensionType12c /* = Never */
+static inline-class-member method ExtensionType12c|constructor#_#new#tearOff(imp::ClassM n) → self::ExtensionType12c /* = imp::ClassM */
  return self::ExtensionType12c|constructor#(n);
-static inline-class-member method ExtensionType12c|get#property(lowered final self::ExtensionType12c /* = Never */ #this) → core::bool
+static inline-class-member method ExtensionType12c|get#property(lowered final self::ExtensionType12c /* = imp::ClassM */ #this) → core::bool
  return true;
-static inline-class-member method ExtensionType12d|constructor#(Never n) → self::ExtensionType12d /* = Never */ {
-  lowered final self::ExtensionType12d /* = Never */ #this = n;
+static inline-class-member method ExtensionType12d|constructor#(Never n) → self::ExtensionType12d /* = invalid-type */ {
+  lowered final self::ExtensionType12d /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12d|constructor#_#new#tearOff(Never n) → self::ExtensionType12d /* = Never */
+static inline-class-member method ExtensionType12d|constructor#_#new#tearOff(Never n) → self::ExtensionType12d /* = invalid-type */
  return self::ExtensionType12d|constructor#(n);
-static inline-class-member method ExtensionType12d|get#property(lowered final self::ExtensionType12d /* = Never */ #this) → core::bool
+static inline-class-member method ExtensionType12d|get#property(lowered final self::ExtensionType12d /* = invalid-type */ #this) → core::bool
  return true;
-static inline-class-member method ExtensionType12d|set#property(lowered final self::ExtensionType12d /* = Never */ #this, core::bool value) → void {}
+static inline-class-member method ExtensionType12d|set#property(lowered final self::ExtensionType12d /* = invalid-type */ #this, core::bool value) → void {}
 static inline-class-member method ExtensionType13a|constructor#(core::int i) → self::ExtensionType13a /* = core::int */ {
  lowered final self::ExtensionType13a /* = core::int */ #this = i;
  return #this;
--- a/pkg/front_end/testcases/extension_types/implements_conflicts.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/extension_types/implements_conflicts.dart.textual_outline.expect
@ -40,15 +40,15 @@ extension type ExtensionType10b(int i) implements ExtensionTypeJ, ExtensionTypeK
  int get method4 => 42;
  void set method4(int value) {}
 }
-extension type ExtensionType11(Never n) {
+extension type ExtensionType11(Null n) {
  int get property => 42;
  void set property(String value) {}
 }
 extension type ExtensionType12a(Never n) implements ClassL, ClassM {}
-extension type ExtensionType12b(Never n) implements ClassL {
+extension type ExtensionType12b(ClassL n) implements ClassL {
  void set property(bool value) {}
 }
-extension type ExtensionType12c(Never n) implements ClassM {
+extension type ExtensionType12c(ClassM n) implements ClassM {
  bool get property => true;
 }
 extension type ExtensionType12d(Never n) implements ClassL, ClassM {
--- a/pkg/front_end/testcases/extension_types/implements_conflicts.dart.textual_outline_modelled.expect
+++ b/pkg/front_end/testcases/extension_types/implements_conflicts.dart.textual_outline_modelled.expect
@ -4,15 +4,15 @@ extension type ExtensionType10b(int i) implements ExtensionTypeJ, ExtensionTypeK
  int get method4 => 42;
  void set method4(int value) {}
 }
-extension type ExtensionType11(Never n) {
+extension type ExtensionType11(Null n) {
  int get property => 42;
  void set property(String value) {}
 }
 extension type ExtensionType12a(Never n) implements ClassL, ClassM {}
-extension type ExtensionType12b(Never n) implements ClassL {
+extension type ExtensionType12b(ClassL n) implements ClassL {
  void set property(bool value) {}
 }
-extension type ExtensionType12c(Never n) implements ClassM {
+extension type ExtensionType12c(ClassM n) implements ClassM {
  bool get property => true;
 }
 extension type ExtensionType12d(Never n) implements ClassL, ClassM {
--- a/pkg/front_end/testcases/extension_types/implements_conflicts.dart.weak.expect
+++ b/pkg/front_end/testcases/extension_types/implements_conflicts.dart.weak.expect
@ -10,6 +10,22 @@ library;
 //   void method4() {} /* Error */
 //        ^
 //
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:70:38: Error: The representation type can't be a bottom type.
+// extension type ExtensionType9a(Never n) /* Error */
+//                                      ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:73:38: Error: The representation type can't be a bottom type.
+// extension type ExtensionType9b(Never n) /* Error */
+//                                      ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:94:39: Error: The representation type can't be a bottom type.
+// extension type ExtensionType12a(Never n) /* Error */
+//                                       ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:107:39: Error: The representation type can't be a bottom type.
+// extension type ExtensionType12d(Never n) /* Error */
+//                                       ^
+//
 // pkg/front_end/testcases/extension_types/implements_conflicts.dart:7:16: Error: The extension type 'ExtensionType1a' can't inherit the member 'method1' as both an extension type member and a non-extension type member.
 // Try declaring a member 'method1' in 'ExtensionType1a' to resolve the conflict.
 // extension type ExtensionType1a(ClassA c) /* Error */
@ -217,12 +233,12 @@ extension type ExtensionType9(core::int i) {
  constructor • = self::ExtensionType9|constructor#;
  constructor tearoff • = self::ExtensionType9|constructor#_#new#tearOff;
 }
-extension type ExtensionType9a(Never n) implements imp::ClassJ, imp::ClassK {
+extension type ExtensionType9a(invalid-type n) implements imp::ClassJ, imp::ClassK {
  abstract inline-class-member representation-field get n() → Never;
  constructor • = self::ExtensionType9a|constructor#;
  constructor tearoff • = self::ExtensionType9a|constructor#_#new#tearOff;
 }
-extension type ExtensionType9b(Never n) implements imp::ClassJ, imp::ClassK {
+extension type ExtensionType9b(invalid-type n) implements imp::ClassJ, imp::ClassK {
  abstract inline-class-member representation-field get n() → Never;
  get method4 = self::ExtensionType9b|get#method4;
  set method4 = self::ExtensionType9b|set#method4;
@ -241,31 +257,31 @@ extension type ExtensionType10b(core::int i) implements imp::ExtensionTypeJ /* =
  constructor • = self::ExtensionType10b|constructor#;
  constructor tearoff • = self::ExtensionType10b|constructor#_#new#tearOff;
 }
-extension type ExtensionType11(Never n) {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType11(Null n) {
+  abstract inline-class-member representation-field get n() → Null;
  get property = self::ExtensionType11|get#property;
  set property = self::ExtensionType11|set#property;
  constructor • = self::ExtensionType11|constructor#;
  constructor tearoff • = self::ExtensionType11|constructor#_#new#tearOff;
 }
-extension type ExtensionType12a(Never n) implements imp::ClassL, imp::ClassM {
+extension type ExtensionType12a(invalid-type n) implements imp::ClassL, imp::ClassM {
  abstract inline-class-member representation-field get n() → Never;
  constructor • = self::ExtensionType12a|constructor#;
  constructor tearoff • = self::ExtensionType12a|constructor#_#new#tearOff;
 }
-extension type ExtensionType12b(Never n) implements imp::ClassL {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType12b(imp::ClassL n) implements imp::ClassL {
+  abstract inline-class-member representation-field get n() → imp::ClassL;
  set property = self::ExtensionType12b|set#property;
  constructor • = self::ExtensionType12b|constructor#;
  constructor tearoff • = self::ExtensionType12b|constructor#_#new#tearOff;
 }
-extension type ExtensionType12c(Never n) implements imp::ClassM {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType12c(imp::ClassM n) implements imp::ClassM {
+  abstract inline-class-member representation-field get n() → imp::ClassM;
  get property = self::ExtensionType12c|get#property;
  constructor • = self::ExtensionType12c|constructor#;
  constructor tearoff • = self::ExtensionType12c|constructor#_#new#tearOff;
 }
-extension type ExtensionType12d(Never n) implements imp::ClassL, imp::ClassM {
+extension type ExtensionType12d(invalid-type n) implements imp::ClassL, imp::ClassM {
  abstract inline-class-member representation-field get n() → Never;
  get property = self::ExtensionType12d|get#property;
  set property = self::ExtensionType12d|set#property;
@ -414,21 +430,21 @@ static inline-class-member method ExtensionType9|method4(lowered final self::Ext
 static inline-class-member method ExtensionType9|get#method4(lowered final self::ExtensionType9 /* = core::int */ #this) → () → void
  return () → void => self::ExtensionType9|method4(#this);
 static inline-class-member method ExtensionType9|set#method4(lowered final self::ExtensionType9 /* = core::int */ #this, core::int value) → void {}
-static inline-class-member method ExtensionType9a|constructor#(Never n) → self::ExtensionType9a /* = Never */ {
-  lowered final self::ExtensionType9a /* = Never */ #this = n;
+static inline-class-member method ExtensionType9a|constructor#(Never n) → self::ExtensionType9a /* = invalid-type */ {
+  lowered final self::ExtensionType9a /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType9a|constructor#_#new#tearOff(Never n) → self::ExtensionType9a /* = Never */
+static inline-class-member method ExtensionType9a|constructor#_#new#tearOff(Never n) → self::ExtensionType9a /* = invalid-type */
  return self::ExtensionType9a|constructor#(n);
-static inline-class-member method ExtensionType9b|constructor#(Never n) → self::ExtensionType9b /* = Never */ {
-  lowered final self::ExtensionType9b /* = Never */ #this = n;
+static inline-class-member method ExtensionType9b|constructor#(Never n) → self::ExtensionType9b /* = invalid-type */ {
+  lowered final self::ExtensionType9b /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType9b|constructor#_#new#tearOff(Never n) → self::ExtensionType9b /* = Never */
+static inline-class-member method ExtensionType9b|constructor#_#new#tearOff(Never n) → self::ExtensionType9b /* = invalid-type */
  return self::ExtensionType9b|constructor#(n);
-static inline-class-member method ExtensionType9b|get#method4(lowered final self::ExtensionType9b /* = Never */ #this) → core::int
+static inline-class-member method ExtensionType9b|get#method4(lowered final self::ExtensionType9b /* = invalid-type */ #this) → core::int
  return 42;
-static inline-class-member method ExtensionType9b|set#method4(lowered final self::ExtensionType9b /* = Never */ #this, core::int value) → void {}
+static inline-class-member method ExtensionType9b|set#method4(lowered final self::ExtensionType9b /* = invalid-type */ #this, core::int value) → void {}
 static inline-class-member method ExtensionType10a|constructor#(core::int i) → self::ExtensionType10a /* = core::int */ {
  lowered final self::ExtensionType10a /* = core::int */ #this = i;
  return #this;
@ -444,45 +460,45 @@ static inline-class-member method ExtensionType10b|constructor#(core::int i) →
 static inline-class-member method ExtensionType10b|get#method4(lowered final self::ExtensionType10b /* = core::int */ #this) → core::int
  return 42;
 static inline-class-member method ExtensionType10b|set#method4(lowered final self::ExtensionType10b /* = core::int */ #this, core::int value) → void {}
-static inline-class-member method ExtensionType11|constructor#(Never n) → self::ExtensionType11 /* = Never */ {
-  lowered final self::ExtensionType11 /* = Never */ #this = n;
+static inline-class-member method ExtensionType11|constructor#(Null n) → self::ExtensionType11 /* = Null */ {
+  lowered final self::ExtensionType11 /* = Null */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType11|constructor#_#new#tearOff(Never n) → self::ExtensionType11 /* = Never */
+static inline-class-member method ExtensionType11|constructor#_#new#tearOff(Null n) → self::ExtensionType11 /* = Null */
  return self::ExtensionType11|constructor#(n);
-static inline-class-member method ExtensionType11|get#property(lowered final self::ExtensionType11 /* = Never */ #this) → core::int
+static inline-class-member method ExtensionType11|get#property(lowered final self::ExtensionType11 /* = Null */ #this) → core::int
  return 42;
-static inline-class-member method ExtensionType11|set#property(lowered final self::ExtensionType11 /* = Never */ #this, core::String value) → void {}
-static inline-class-member method ExtensionType12a|constructor#(Never n) → self::ExtensionType12a /* = Never */ {
-  lowered final self::ExtensionType12a /* = Never */ #this = n;
+static inline-class-member method ExtensionType11|set#property(lowered final self::ExtensionType11 /* = Null */ #this, core::String value) → void {}
+static inline-class-member method ExtensionType12a|constructor#(Never n) → self::ExtensionType12a /* = invalid-type */ {
+  lowered final self::ExtensionType12a /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12a|constructor#_#new#tearOff(Never n) → self::ExtensionType12a /* = Never */
+static inline-class-member method ExtensionType12a|constructor#_#new#tearOff(Never n) → self::ExtensionType12a /* = invalid-type */
  return self::ExtensionType12a|constructor#(n);
-static inline-class-member method ExtensionType12b|constructor#(Never n) → self::ExtensionType12b /* = Never */ {
-  lowered final self::ExtensionType12b /* = Never */ #this = n;
+static inline-class-member method ExtensionType12b|constructor#(imp::ClassL n) → self::ExtensionType12b /* = imp::ClassL */ {
+  lowered final self::ExtensionType12b /* = imp::ClassL */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12b|constructor#_#new#tearOff(Never n) → self::ExtensionType12b /* = Never */
+static inline-class-member method ExtensionType12b|constructor#_#new#tearOff(imp::ClassL n) → self::ExtensionType12b /* = imp::ClassL */
  return self::ExtensionType12b|constructor#(n);
-static inline-class-member method ExtensionType12b|set#property(lowered final self::ExtensionType12b /* = Never */ #this, core::bool value) → void {}
-static inline-class-member method ExtensionType12c|constructor#(Never n) → self::ExtensionType12c /* = Never */ {
-  lowered final self::ExtensionType12c /* = Never */ #this = n;
+static inline-class-member method ExtensionType12b|set#property(lowered final self::ExtensionType12b /* = imp::ClassL */ #this, core::bool value) → void {}
+static inline-class-member method ExtensionType12c|constructor#(imp::ClassM n) → self::ExtensionType12c /* = imp::ClassM */ {
+  lowered final self::ExtensionType12c /* = imp::ClassM */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12c|constructor#_#new#tearOff(Never n) → self::ExtensionType12c /* = Never */
+static inline-class-member method ExtensionType12c|constructor#_#new#tearOff(imp::ClassM n) → self::ExtensionType12c /* = imp::ClassM */
  return self::ExtensionType12c|constructor#(n);
-static inline-class-member method ExtensionType12c|get#property(lowered final self::ExtensionType12c /* = Never */ #this) → core::bool
+static inline-class-member method ExtensionType12c|get#property(lowered final self::ExtensionType12c /* = imp::ClassM */ #this) → core::bool
  return true;
-static inline-class-member method ExtensionType12d|constructor#(Never n) → self::ExtensionType12d /* = Never */ {
-  lowered final self::ExtensionType12d /* = Never */ #this = n;
+static inline-class-member method ExtensionType12d|constructor#(Never n) → self::ExtensionType12d /* = invalid-type */ {
+  lowered final self::ExtensionType12d /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12d|constructor#_#new#tearOff(Never n) → self::ExtensionType12d /* = Never */
+static inline-class-member method ExtensionType12d|constructor#_#new#tearOff(Never n) → self::ExtensionType12d /* = invalid-type */
  return self::ExtensionType12d|constructor#(n);
-static inline-class-member method ExtensionType12d|get#property(lowered final self::ExtensionType12d /* = Never */ #this) → core::bool
+static inline-class-member method ExtensionType12d|get#property(lowered final self::ExtensionType12d /* = invalid-type */ #this) → core::bool
  return true;
-static inline-class-member method ExtensionType12d|set#property(lowered final self::ExtensionType12d /* = Never */ #this, core::bool value) → void {}
+static inline-class-member method ExtensionType12d|set#property(lowered final self::ExtensionType12d /* = invalid-type */ #this, core::bool value) → void {}
 static inline-class-member method ExtensionType13a|constructor#(core::int i) → self::ExtensionType13a /* = core::int */ {
  lowered final self::ExtensionType13a /* = core::int */ #this = i;
  return #this;
--- a/pkg/front_end/testcases/extension_types/implements_conflicts.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/extension_types/implements_conflicts.dart.weak.modular.expect
@ -10,6 +10,22 @@ library;
 //   void method4() {} /* Error */
 //        ^
 //
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:70:38: Error: The representation type can't be a bottom type.
+// extension type ExtensionType9a(Never n) /* Error */
+//                                      ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:73:38: Error: The representation type can't be a bottom type.
+// extension type ExtensionType9b(Never n) /* Error */
+//                                      ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:94:39: Error: The representation type can't be a bottom type.
+// extension type ExtensionType12a(Never n) /* Error */
+//                                       ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:107:39: Error: The representation type can't be a bottom type.
+// extension type ExtensionType12d(Never n) /* Error */
+//                                       ^
+//
 // pkg/front_end/testcases/extension_types/implements_conflicts.dart:7:16: Error: The extension type 'ExtensionType1a' can't inherit the member 'method1' as both an extension type member and a non-extension type member.
 // Try declaring a member 'method1' in 'ExtensionType1a' to resolve the conflict.
 // extension type ExtensionType1a(ClassA c) /* Error */
@ -217,12 +233,12 @@ extension type ExtensionType9(core::int i) {
  constructor • = self::ExtensionType9|constructor#;
  constructor tearoff • = self::ExtensionType9|constructor#_#new#tearOff;
 }
-extension type ExtensionType9a(Never n) implements imp::ClassJ, imp::ClassK {
+extension type ExtensionType9a(invalid-type n) implements imp::ClassJ, imp::ClassK {
  abstract inline-class-member representation-field get n() → Never;
  constructor • = self::ExtensionType9a|constructor#;
  constructor tearoff • = self::ExtensionType9a|constructor#_#new#tearOff;
 }
-extension type ExtensionType9b(Never n) implements imp::ClassJ, imp::ClassK {
+extension type ExtensionType9b(invalid-type n) implements imp::ClassJ, imp::ClassK {
  abstract inline-class-member representation-field get n() → Never;
  get method4 = self::ExtensionType9b|get#method4;
  set method4 = self::ExtensionType9b|set#method4;
@ -241,31 +257,31 @@ extension type ExtensionType10b(core::int i) implements imp::ExtensionTypeJ /* =
  constructor • = self::ExtensionType10b|constructor#;
  constructor tearoff • = self::ExtensionType10b|constructor#_#new#tearOff;
 }
-extension type ExtensionType11(Never n) {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType11(Null n) {
+  abstract inline-class-member representation-field get n() → Null;
  get property = self::ExtensionType11|get#property;
  set property = self::ExtensionType11|set#property;
  constructor • = self::ExtensionType11|constructor#;
  constructor tearoff • = self::ExtensionType11|constructor#_#new#tearOff;
 }
-extension type ExtensionType12a(Never n) implements imp::ClassL, imp::ClassM {
+extension type ExtensionType12a(invalid-type n) implements imp::ClassL, imp::ClassM {
  abstract inline-class-member representation-field get n() → Never;
  constructor • = self::ExtensionType12a|constructor#;
  constructor tearoff • = self::ExtensionType12a|constructor#_#new#tearOff;
 }
-extension type ExtensionType12b(Never n) implements imp::ClassL {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType12b(imp::ClassL n) implements imp::ClassL {
+  abstract inline-class-member representation-field get n() → imp::ClassL;
  set property = self::ExtensionType12b|set#property;
  constructor • = self::ExtensionType12b|constructor#;
  constructor tearoff • = self::ExtensionType12b|constructor#_#new#tearOff;
 }
-extension type ExtensionType12c(Never n) implements imp::ClassM {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType12c(imp::ClassM n) implements imp::ClassM {
+  abstract inline-class-member representation-field get n() → imp::ClassM;
  get property = self::ExtensionType12c|get#property;
  constructor • = self::ExtensionType12c|constructor#;
  constructor tearoff • = self::ExtensionType12c|constructor#_#new#tearOff;
 }
-extension type ExtensionType12d(Never n) implements imp::ClassL, imp::ClassM {
+extension type ExtensionType12d(invalid-type n) implements imp::ClassL, imp::ClassM {
  abstract inline-class-member representation-field get n() → Never;
  get property = self::ExtensionType12d|get#property;
  set property = self::ExtensionType12d|set#property;
@ -414,21 +430,21 @@ static inline-class-member method ExtensionType9|method4(lowered final self::Ext
 static inline-class-member method ExtensionType9|get#method4(lowered final self::ExtensionType9 /* = core::int */ #this) → () → void
  return () → void => self::ExtensionType9|method4(#this);
 static inline-class-member method ExtensionType9|set#method4(lowered final self::ExtensionType9 /* = core::int */ #this, core::int value) → void {}
-static inline-class-member method ExtensionType9a|constructor#(Never n) → self::ExtensionType9a /* = Never */ {
-  lowered final self::ExtensionType9a /* = Never */ #this = n;
+static inline-class-member method ExtensionType9a|constructor#(Never n) → self::ExtensionType9a /* = invalid-type */ {
+  lowered final self::ExtensionType9a /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType9a|constructor#_#new#tearOff(Never n) → self::ExtensionType9a /* = Never */
+static inline-class-member method ExtensionType9a|constructor#_#new#tearOff(Never n) → self::ExtensionType9a /* = invalid-type */
  return self::ExtensionType9a|constructor#(n);
-static inline-class-member method ExtensionType9b|constructor#(Never n) → self::ExtensionType9b /* = Never */ {
-  lowered final self::ExtensionType9b /* = Never */ #this = n;
+static inline-class-member method ExtensionType9b|constructor#(Never n) → self::ExtensionType9b /* = invalid-type */ {
+  lowered final self::ExtensionType9b /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType9b|constructor#_#new#tearOff(Never n) → self::ExtensionType9b /* = Never */
+static inline-class-member method ExtensionType9b|constructor#_#new#tearOff(Never n) → self::ExtensionType9b /* = invalid-type */
  return self::ExtensionType9b|constructor#(n);
-static inline-class-member method ExtensionType9b|get#method4(lowered final self::ExtensionType9b /* = Never */ #this) → core::int
+static inline-class-member method ExtensionType9b|get#method4(lowered final self::ExtensionType9b /* = invalid-type */ #this) → core::int
  return 42;
-static inline-class-member method ExtensionType9b|set#method4(lowered final self::ExtensionType9b /* = Never */ #this, core::int value) → void {}
+static inline-class-member method ExtensionType9b|set#method4(lowered final self::ExtensionType9b /* = invalid-type */ #this, core::int value) → void {}
 static inline-class-member method ExtensionType10a|constructor#(core::int i) → self::ExtensionType10a /* = core::int */ {
  lowered final self::ExtensionType10a /* = core::int */ #this = i;
  return #this;
@ -444,45 +460,45 @@ static inline-class-member method ExtensionType10b|constructor#(core::int i) →
 static inline-class-member method ExtensionType10b|get#method4(lowered final self::ExtensionType10b /* = core::int */ #this) → core::int
  return 42;
 static inline-class-member method ExtensionType10b|set#method4(lowered final self::ExtensionType10b /* = core::int */ #this, core::int value) → void {}
-static inline-class-member method ExtensionType11|constructor#(Never n) → self::ExtensionType11 /* = Never */ {
-  lowered final self::ExtensionType11 /* = Never */ #this = n;
+static inline-class-member method ExtensionType11|constructor#(Null n) → self::ExtensionType11 /* = Null */ {
+  lowered final self::ExtensionType11 /* = Null */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType11|constructor#_#new#tearOff(Never n) → self::ExtensionType11 /* = Never */
+static inline-class-member method ExtensionType11|constructor#_#new#tearOff(Null n) → self::ExtensionType11 /* = Null */
  return self::ExtensionType11|constructor#(n);
-static inline-class-member method ExtensionType11|get#property(lowered final self::ExtensionType11 /* = Never */ #this) → core::int
+static inline-class-member method ExtensionType11|get#property(lowered final self::ExtensionType11 /* = Null */ #this) → core::int
  return 42;
-static inline-class-member method ExtensionType11|set#property(lowered final self::ExtensionType11 /* = Never */ #this, core::String value) → void {}
-static inline-class-member method ExtensionType12a|constructor#(Never n) → self::ExtensionType12a /* = Never */ {
-  lowered final self::ExtensionType12a /* = Never */ #this = n;
+static inline-class-member method ExtensionType11|set#property(lowered final self::ExtensionType11 /* = Null */ #this, core::String value) → void {}
+static inline-class-member method ExtensionType12a|constructor#(Never n) → self::ExtensionType12a /* = invalid-type */ {
+  lowered final self::ExtensionType12a /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12a|constructor#_#new#tearOff(Never n) → self::ExtensionType12a /* = Never */
+static inline-class-member method ExtensionType12a|constructor#_#new#tearOff(Never n) → self::ExtensionType12a /* = invalid-type */
  return self::ExtensionType12a|constructor#(n);
-static inline-class-member method ExtensionType12b|constructor#(Never n) → self::ExtensionType12b /* = Never */ {
-  lowered final self::ExtensionType12b /* = Never */ #this = n;
+static inline-class-member method ExtensionType12b|constructor#(imp::ClassL n) → self::ExtensionType12b /* = imp::ClassL */ {
+  lowered final self::ExtensionType12b /* = imp::ClassL */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12b|constructor#_#new#tearOff(Never n) → self::ExtensionType12b /* = Never */
+static inline-class-member method ExtensionType12b|constructor#_#new#tearOff(imp::ClassL n) → self::ExtensionType12b /* = imp::ClassL */
  return self::ExtensionType12b|constructor#(n);
-static inline-class-member method ExtensionType12b|set#property(lowered final self::ExtensionType12b /* = Never */ #this, core::bool value) → void {}
-static inline-class-member method ExtensionType12c|constructor#(Never n) → self::ExtensionType12c /* = Never */ {
-  lowered final self::ExtensionType12c /* = Never */ #this = n;
+static inline-class-member method ExtensionType12b|set#property(lowered final self::ExtensionType12b /* = imp::ClassL */ #this, core::bool value) → void {}
+static inline-class-member method ExtensionType12c|constructor#(imp::ClassM n) → self::ExtensionType12c /* = imp::ClassM */ {
+  lowered final self::ExtensionType12c /* = imp::ClassM */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12c|constructor#_#new#tearOff(Never n) → self::ExtensionType12c /* = Never */
+static inline-class-member method ExtensionType12c|constructor#_#new#tearOff(imp::ClassM n) → self::ExtensionType12c /* = imp::ClassM */
  return self::ExtensionType12c|constructor#(n);
-static inline-class-member method ExtensionType12c|get#property(lowered final self::ExtensionType12c /* = Never */ #this) → core::bool
+static inline-class-member method ExtensionType12c|get#property(lowered final self::ExtensionType12c /* = imp::ClassM */ #this) → core::bool
  return true;
-static inline-class-member method ExtensionType12d|constructor#(Never n) → self::ExtensionType12d /* = Never */ {
-  lowered final self::ExtensionType12d /* = Never */ #this = n;
+static inline-class-member method ExtensionType12d|constructor#(Never n) → self::ExtensionType12d /* = invalid-type */ {
+  lowered final self::ExtensionType12d /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12d|constructor#_#new#tearOff(Never n) → self::ExtensionType12d /* = Never */
+static inline-class-member method ExtensionType12d|constructor#_#new#tearOff(Never n) → self::ExtensionType12d /* = invalid-type */
  return self::ExtensionType12d|constructor#(n);
-static inline-class-member method ExtensionType12d|get#property(lowered final self::ExtensionType12d /* = Never */ #this) → core::bool
+static inline-class-member method ExtensionType12d|get#property(lowered final self::ExtensionType12d /* = invalid-type */ #this) → core::bool
  return true;
-static inline-class-member method ExtensionType12d|set#property(lowered final self::ExtensionType12d /* = Never */ #this, core::bool value) → void {}
+static inline-class-member method ExtensionType12d|set#property(lowered final self::ExtensionType12d /* = invalid-type */ #this, core::bool value) → void {}
 static inline-class-member method ExtensionType13a|constructor#(core::int i) → self::ExtensionType13a /* = core::int */ {
  lowered final self::ExtensionType13a /* = core::int */ #this = i;
  return #this;
--- a/pkg/front_end/testcases/extension_types/implements_conflicts.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/extension_types/implements_conflicts.dart.weak.outline.expect
@ -10,6 +10,22 @@ library;
 //   void method4() {} /* Error */
 //        ^
 //
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:70:38: Error: The representation type can't be a bottom type.
+// extension type ExtensionType9a(Never n) /* Error */
+//                                      ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:73:38: Error: The representation type can't be a bottom type.
+// extension type ExtensionType9b(Never n) /* Error */
+//                                      ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:94:39: Error: The representation type can't be a bottom type.
+// extension type ExtensionType12a(Never n) /* Error */
+//                                       ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:107:39: Error: The representation type can't be a bottom type.
+// extension type ExtensionType12d(Never n) /* Error */
+//                                       ^
+//
 // pkg/front_end/testcases/extension_types/implements_conflicts.dart:7:16: Error: The extension type 'ExtensionType1a' can't inherit the member 'method1' as both an extension type member and a non-extension type member.
 // Try declaring a member 'method1' in 'ExtensionType1a' to resolve the conflict.
 // extension type ExtensionType1a(ClassA c) /* Error */
@ -217,12 +233,12 @@ extension type ExtensionType9(core::int i) {
  constructor • = self::ExtensionType9|constructor#;
  constructor tearoff • = self::ExtensionType9|constructor#_#new#tearOff;
 }
-extension type ExtensionType9a(Never n) implements imp::ClassJ, imp::ClassK {
+extension type ExtensionType9a(invalid-type n) implements imp::ClassJ, imp::ClassK {
  abstract inline-class-member representation-field get n() → Never;
  constructor • = self::ExtensionType9a|constructor#;
  constructor tearoff • = self::ExtensionType9a|constructor#_#new#tearOff;
 }
-extension type ExtensionType9b(Never n) implements imp::ClassJ, imp::ClassK {
+extension type ExtensionType9b(invalid-type n) implements imp::ClassJ, imp::ClassK {
  abstract inline-class-member representation-field get n() → Never;
  get method4 = self::ExtensionType9b|get#method4;
  set method4 = self::ExtensionType9b|set#method4;
@ -241,31 +257,31 @@ extension type ExtensionType10b(core::int i) implements imp::ExtensionTypeJ /* =
  constructor • = self::ExtensionType10b|constructor#;
  constructor tearoff • = self::ExtensionType10b|constructor#_#new#tearOff;
 }
-extension type ExtensionType11(Never n) {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType11(Null n) {
+  abstract inline-class-member representation-field get n() → Null;
  get property = self::ExtensionType11|get#property;
  set property = self::ExtensionType11|set#property;
  constructor • = self::ExtensionType11|constructor#;
  constructor tearoff • = self::ExtensionType11|constructor#_#new#tearOff;
 }
-extension type ExtensionType12a(Never n) implements imp::ClassL, imp::ClassM {
+extension type ExtensionType12a(invalid-type n) implements imp::ClassL, imp::ClassM {
  abstract inline-class-member representation-field get n() → Never;
  constructor • = self::ExtensionType12a|constructor#;
  constructor tearoff • = self::ExtensionType12a|constructor#_#new#tearOff;
 }
-extension type ExtensionType12b(Never n) implements imp::ClassL {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType12b(imp::ClassL n) implements imp::ClassL {
+  abstract inline-class-member representation-field get n() → imp::ClassL;
  set property = self::ExtensionType12b|set#property;
  constructor • = self::ExtensionType12b|constructor#;
  constructor tearoff • = self::ExtensionType12b|constructor#_#new#tearOff;
 }
-extension type ExtensionType12c(Never n) implements imp::ClassM {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType12c(imp::ClassM n) implements imp::ClassM {
+  abstract inline-class-member representation-field get n() → imp::ClassM;
  get property = self::ExtensionType12c|get#property;
  constructor • = self::ExtensionType12c|constructor#;
  constructor tearoff • = self::ExtensionType12c|constructor#_#new#tearOff;
 }
-extension type ExtensionType12d(Never n) implements imp::ClassL, imp::ClassM {
+extension type ExtensionType12d(invalid-type n) implements imp::ClassL, imp::ClassM {
  abstract inline-class-member representation-field get n() → Never;
  get property = self::ExtensionType12d|get#property;
  set property = self::ExtensionType12d|set#property;
@ -390,17 +406,17 @@ static inline-class-member method ExtensionType9|get#method4(lowered final self:
  return () → void => self::ExtensionType9|method4(#this);
 static inline-class-member method ExtensionType9|set#method4(lowered final self::ExtensionType9 /* = core::int */ #this, core::int value) → void
  ;
-static inline-class-member method ExtensionType9a|constructor#(Never n) → self::ExtensionType9a /* = Never */
+static inline-class-member method ExtensionType9a|constructor#(Never n) → self::ExtensionType9a /* = invalid-type */
  ;
-static inline-class-member method ExtensionType9a|constructor#_#new#tearOff(Never n) → self::ExtensionType9a /* = Never */
+static inline-class-member method ExtensionType9a|constructor#_#new#tearOff(Never n) → self::ExtensionType9a /* = invalid-type */
  return self::ExtensionType9a|constructor#(n);
-static inline-class-member method ExtensionType9b|constructor#(Never n) → self::ExtensionType9b /* = Never */
+static inline-class-member method ExtensionType9b|constructor#(Never n) → self::ExtensionType9b /* = invalid-type */
  ;
-static inline-class-member method ExtensionType9b|constructor#_#new#tearOff(Never n) → self::ExtensionType9b /* = Never */
+static inline-class-member method ExtensionType9b|constructor#_#new#tearOff(Never n) → self::ExtensionType9b /* = invalid-type */
  return self::ExtensionType9b|constructor#(n);
-static inline-class-member method ExtensionType9b|get#method4(lowered final self::ExtensionType9b /* = Never */ #this) → core::int
+static inline-class-member method ExtensionType9b|get#method4(lowered final self::ExtensionType9b /* = invalid-type */ #this) → core::int
  ;
-static inline-class-member method ExtensionType9b|set#method4(lowered final self::ExtensionType9b /* = Never */ #this, core::int value) → void
+static inline-class-member method ExtensionType9b|set#method4(lowered final self::ExtensionType9b /* = invalid-type */ #this, core::int value) → void
  ;
 static inline-class-member method ExtensionType10a|constructor#(core::int i) → self::ExtensionType10a /* = core::int */
  ;
@ -414,37 +430,37 @@ static inline-class-member method ExtensionType10b|get#method4(lowered final sel
  ;
 static inline-class-member method ExtensionType10b|set#method4(lowered final self::ExtensionType10b /* = core::int */ #this, core::int value) → void
  ;
-static inline-class-member method ExtensionType11|constructor#(Never n) → self::ExtensionType11 /* = Never */
+static inline-class-member method ExtensionType11|constructor#(Null n) → self::ExtensionType11 /* = Null */
  ;
-static inline-class-member method ExtensionType11|constructor#_#new#tearOff(Never n) → self::ExtensionType11 /* = Never */
+static inline-class-member method ExtensionType11|constructor#_#new#tearOff(Null n) → self::ExtensionType11 /* = Null */
  return self::ExtensionType11|constructor#(n);
-static inline-class-member method ExtensionType11|get#property(lowered final self::ExtensionType11 /* = Never */ #this) → core::int
+static inline-class-member method ExtensionType11|get#property(lowered final self::ExtensionType11 /* = Null */ #this) → core::int
  ;
-static inline-class-member method ExtensionType11|set#property(lowered final self::ExtensionType11 /* = Never */ #this, core::String value) → void
+static inline-class-member method ExtensionType11|set#property(lowered final self::ExtensionType11 /* = Null */ #this, core::String value) → void
  ;
-static inline-class-member method ExtensionType12a|constructor#(Never n) → self::ExtensionType12a /* = Never */
+static inline-class-member method ExtensionType12a|constructor#(Never n) → self::ExtensionType12a /* = invalid-type */
  ;
-static inline-class-member method ExtensionType12a|constructor#_#new#tearOff(Never n) → self::ExtensionType12a /* = Never */
+static inline-class-member method ExtensionType12a|constructor#_#new#tearOff(Never n) → self::ExtensionType12a /* = invalid-type */
  return self::ExtensionType12a|constructor#(n);
-static inline-class-member method ExtensionType12b|constructor#(Never n) → self::ExtensionType12b /* = Never */
+static inline-class-member method ExtensionType12b|constructor#(imp::ClassL n) → self::ExtensionType12b /* = imp::ClassL */
  ;
-static inline-class-member method ExtensionType12b|constructor#_#new#tearOff(Never n) → self::ExtensionType12b /* = Never */
+static inline-class-member method ExtensionType12b|constructor#_#new#tearOff(imp::ClassL n) → self::ExtensionType12b /* = imp::ClassL */
  return self::ExtensionType12b|constructor#(n);
-static inline-class-member method ExtensionType12b|set#property(lowered final self::ExtensionType12b /* = Never */ #this, core::bool value) → void
+static inline-class-member method ExtensionType12b|set#property(lowered final self::ExtensionType12b /* = imp::ClassL */ #this, core::bool value) → void
  ;
-static inline-class-member method ExtensionType12c|constructor#(Never n) → self::ExtensionType12c /* = Never */
+static inline-class-member method ExtensionType12c|constructor#(imp::ClassM n) → self::ExtensionType12c /* = imp::ClassM */
  ;
-static inline-class-member method ExtensionType12c|constructor#_#new#tearOff(Never n) → self::ExtensionType12c /* = Never */
+static inline-class-member method ExtensionType12c|constructor#_#new#tearOff(imp::ClassM n) → self::ExtensionType12c /* = imp::ClassM */
  return self::ExtensionType12c|constructor#(n);
-static inline-class-member method ExtensionType12c|get#property(lowered final self::ExtensionType12c /* = Never */ #this) → core::bool
+static inline-class-member method ExtensionType12c|get#property(lowered final self::ExtensionType12c /* = imp::ClassM */ #this) → core::bool
  ;
-static inline-class-member method ExtensionType12d|constructor#(Never n) → self::ExtensionType12d /* = Never */
+static inline-class-member method ExtensionType12d|constructor#(Never n) → self::ExtensionType12d /* = invalid-type */
  ;
-static inline-class-member method ExtensionType12d|constructor#_#new#tearOff(Never n) → self::ExtensionType12d /* = Never */
+static inline-class-member method ExtensionType12d|constructor#_#new#tearOff(Never n) → self::ExtensionType12d /* = invalid-type */
  return self::ExtensionType12d|constructor#(n);
-static inline-class-member method ExtensionType12d|get#property(lowered final self::ExtensionType12d /* = Never */ #this) → core::bool
+static inline-class-member method ExtensionType12d|get#property(lowered final self::ExtensionType12d /* = invalid-type */ #this) → core::bool
  ;
-static inline-class-member method ExtensionType12d|set#property(lowered final self::ExtensionType12d /* = Never */ #this, core::bool value) → void
+static inline-class-member method ExtensionType12d|set#property(lowered final self::ExtensionType12d /* = invalid-type */ #this, core::bool value) → void
  ;
 static inline-class-member method ExtensionType13a|constructor#(core::int i) → self::ExtensionType13a /* = core::int */
  ;
--- a/pkg/front_end/testcases/extension_types/implements_conflicts.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/implements_conflicts.dart.weak.transformed.expect
@ -10,6 +10,22 @@ library;
 //   void method4() {} /* Error */
 //        ^
 //
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:70:38: Error: The representation type can't be a bottom type.
+// extension type ExtensionType9a(Never n) /* Error */
+//                                      ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:73:38: Error: The representation type can't be a bottom type.
+// extension type ExtensionType9b(Never n) /* Error */
+//                                      ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:94:39: Error: The representation type can't be a bottom type.
+// extension type ExtensionType12a(Never n) /* Error */
+//                                       ^
+//
+// pkg/front_end/testcases/extension_types/implements_conflicts.dart:107:39: Error: The representation type can't be a bottom type.
+// extension type ExtensionType12d(Never n) /* Error */
+//                                       ^
+//
 // pkg/front_end/testcases/extension_types/implements_conflicts.dart:7:16: Error: The extension type 'ExtensionType1a' can't inherit the member 'method1' as both an extension type member and a non-extension type member.
 // Try declaring a member 'method1' in 'ExtensionType1a' to resolve the conflict.
 // extension type ExtensionType1a(ClassA c) /* Error */
@ -217,12 +233,12 @@ extension type ExtensionType9(core::int i) {
  constructor • = self::ExtensionType9|constructor#;
  constructor tearoff • = self::ExtensionType9|constructor#_#new#tearOff;
 }
-extension type ExtensionType9a(Never n) implements imp::ClassJ, imp::ClassK {
+extension type ExtensionType9a(invalid-type n) implements imp::ClassJ, imp::ClassK {
  abstract inline-class-member representation-field get n() → Never;
  constructor • = self::ExtensionType9a|constructor#;
  constructor tearoff • = self::ExtensionType9a|constructor#_#new#tearOff;
 }
-extension type ExtensionType9b(Never n) implements imp::ClassJ, imp::ClassK {
+extension type ExtensionType9b(invalid-type n) implements imp::ClassJ, imp::ClassK {
  abstract inline-class-member representation-field get n() → Never;
  get method4 = self::ExtensionType9b|get#method4;
  set method4 = self::ExtensionType9b|set#method4;
@ -241,31 +257,31 @@ extension type ExtensionType10b(core::int i) implements imp::ExtensionTypeJ /* =
  constructor • = self::ExtensionType10b|constructor#;
  constructor tearoff • = self::ExtensionType10b|constructor#_#new#tearOff;
 }
-extension type ExtensionType11(Never n) {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType11(Null n) {
+  abstract inline-class-member representation-field get n() → Null;
  get property = self::ExtensionType11|get#property;
  set property = self::ExtensionType11|set#property;
  constructor • = self::ExtensionType11|constructor#;
  constructor tearoff • = self::ExtensionType11|constructor#_#new#tearOff;
 }
-extension type ExtensionType12a(Never n) implements imp::ClassL, imp::ClassM {
+extension type ExtensionType12a(invalid-type n) implements imp::ClassL, imp::ClassM {
  abstract inline-class-member representation-field get n() → Never;
  constructor • = self::ExtensionType12a|constructor#;
  constructor tearoff • = self::ExtensionType12a|constructor#_#new#tearOff;
 }
-extension type ExtensionType12b(Never n) implements imp::ClassL {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType12b(imp::ClassL n) implements imp::ClassL {
+  abstract inline-class-member representation-field get n() → imp::ClassL;
  set property = self::ExtensionType12b|set#property;
  constructor • = self::ExtensionType12b|constructor#;
  constructor tearoff • = self::ExtensionType12b|constructor#_#new#tearOff;
 }
-extension type ExtensionType12c(Never n) implements imp::ClassM {
-  abstract inline-class-member representation-field get n() → Never;
+extension type ExtensionType12c(imp::ClassM n) implements imp::ClassM {
+  abstract inline-class-member representation-field get n() → imp::ClassM;
  get property = self::ExtensionType12c|get#property;
  constructor • = self::ExtensionType12c|constructor#;
  constructor tearoff • = self::ExtensionType12c|constructor#_#new#tearOff;
 }
-extension type ExtensionType12d(Never n) implements imp::ClassL, imp::ClassM {
+extension type ExtensionType12d(invalid-type n) implements imp::ClassL, imp::ClassM {
  abstract inline-class-member representation-field get n() → Never;
  get property = self::ExtensionType12d|get#property;
  set property = self::ExtensionType12d|set#property;
@ -414,21 +430,21 @@ static inline-class-member method ExtensionType9|method4(lowered final self::Ext
 static inline-class-member method ExtensionType9|get#method4(lowered final self::ExtensionType9 /* = core::int */ #this) → () → void
  return () → void => self::ExtensionType9|method4(#this);
 static inline-class-member method ExtensionType9|set#method4(lowered final self::ExtensionType9 /* = core::int */ #this, core::int value) → void {}
-static inline-class-member method ExtensionType9a|constructor#(Never n) → self::ExtensionType9a /* = Never */ {
-  lowered final self::ExtensionType9a /* = Never */ #this = n;
+static inline-class-member method ExtensionType9a|constructor#(Never n) → self::ExtensionType9a /* = invalid-type */ {
+  lowered final self::ExtensionType9a /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType9a|constructor#_#new#tearOff(Never n) → self::ExtensionType9a /* = Never */
+static inline-class-member method ExtensionType9a|constructor#_#new#tearOff(Never n) → self::ExtensionType9a /* = invalid-type */
  return self::ExtensionType9a|constructor#(n);
-static inline-class-member method ExtensionType9b|constructor#(Never n) → self::ExtensionType9b /* = Never */ {
-  lowered final self::ExtensionType9b /* = Never */ #this = n;
+static inline-class-member method ExtensionType9b|constructor#(Never n) → self::ExtensionType9b /* = invalid-type */ {
+  lowered final self::ExtensionType9b /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType9b|constructor#_#new#tearOff(Never n) → self::ExtensionType9b /* = Never */
+static inline-class-member method ExtensionType9b|constructor#_#new#tearOff(Never n) → self::ExtensionType9b /* = invalid-type */
  return self::ExtensionType9b|constructor#(n);
-static inline-class-member method ExtensionType9b|get#method4(lowered final self::ExtensionType9b /* = Never */ #this) → core::int
+static inline-class-member method ExtensionType9b|get#method4(lowered final self::ExtensionType9b /* = invalid-type */ #this) → core::int
  return 42;
-static inline-class-member method ExtensionType9b|set#method4(lowered final self::ExtensionType9b /* = Never */ #this, core::int value) → void {}
+static inline-class-member method ExtensionType9b|set#method4(lowered final self::ExtensionType9b /* = invalid-type */ #this, core::int value) → void {}
 static inline-class-member method ExtensionType10a|constructor#(core::int i) → self::ExtensionType10a /* = core::int */ {
  lowered final self::ExtensionType10a /* = core::int */ #this = i;
  return #this;
@ -444,45 +460,45 @@ static inline-class-member method ExtensionType10b|constructor#(core::int i) →
 static inline-class-member method ExtensionType10b|get#method4(lowered final self::ExtensionType10b /* = core::int */ #this) → core::int
  return 42;
 static inline-class-member method ExtensionType10b|set#method4(lowered final self::ExtensionType10b /* = core::int */ #this, core::int value) → void {}
-static inline-class-member method ExtensionType11|constructor#(Never n) → self::ExtensionType11 /* = Never */ {
-  lowered final self::ExtensionType11 /* = Never */ #this = n;
+static inline-class-member method ExtensionType11|constructor#(Null n) → self::ExtensionType11 /* = Null */ {
+  lowered final self::ExtensionType11 /* = Null */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType11|constructor#_#new#tearOff(Never n) → self::ExtensionType11 /* = Never */
+static inline-class-member method ExtensionType11|constructor#_#new#tearOff(Null n) → self::ExtensionType11 /* = Null */
  return self::ExtensionType11|constructor#(n);
-static inline-class-member method ExtensionType11|get#property(lowered final self::ExtensionType11 /* = Never */ #this) → core::int
+static inline-class-member method ExtensionType11|get#property(lowered final self::ExtensionType11 /* = Null */ #this) → core::int
  return 42;
-static inline-class-member method ExtensionType11|set#property(lowered final self::ExtensionType11 /* = Never */ #this, core::String value) → void {}
-static inline-class-member method ExtensionType12a|constructor#(Never n) → self::ExtensionType12a /* = Never */ {
-  lowered final self::ExtensionType12a /* = Never */ #this = n;
+static inline-class-member method ExtensionType11|set#property(lowered final self::ExtensionType11 /* = Null */ #this, core::String value) → void {}
+static inline-class-member method ExtensionType12a|constructor#(Never n) → self::ExtensionType12a /* = invalid-type */ {
+  lowered final self::ExtensionType12a /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12a|constructor#_#new#tearOff(Never n) → self::ExtensionType12a /* = Never */
+static inline-class-member method ExtensionType12a|constructor#_#new#tearOff(Never n) → self::ExtensionType12a /* = invalid-type */
  return self::ExtensionType12a|constructor#(n);
-static inline-class-member method ExtensionType12b|constructor#(Never n) → self::ExtensionType12b /* = Never */ {
-  lowered final self::ExtensionType12b /* = Never */ #this = n;
+static inline-class-member method ExtensionType12b|constructor#(imp::ClassL n) → self::ExtensionType12b /* = imp::ClassL */ {
+  lowered final self::ExtensionType12b /* = imp::ClassL */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12b|constructor#_#new#tearOff(Never n) → self::ExtensionType12b /* = Never */
+static inline-class-member method ExtensionType12b|constructor#_#new#tearOff(imp::ClassL n) → self::ExtensionType12b /* = imp::ClassL */
  return self::ExtensionType12b|constructor#(n);
-static inline-class-member method ExtensionType12b|set#property(lowered final self::ExtensionType12b /* = Never */ #this, core::bool value) → void {}
-static inline-class-member method ExtensionType12c|constructor#(Never n) → self::ExtensionType12c /* = Never */ {
-  lowered final self::ExtensionType12c /* = Never */ #this = n;
+static inline-class-member method ExtensionType12b|set#property(lowered final self::ExtensionType12b /* = imp::ClassL */ #this, core::bool value) → void {}
+static inline-class-member method ExtensionType12c|constructor#(imp::ClassM n) → self::ExtensionType12c /* = imp::ClassM */ {
+  lowered final self::ExtensionType12c /* = imp::ClassM */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12c|constructor#_#new#tearOff(Never n) → self::ExtensionType12c /* = Never */
+static inline-class-member method ExtensionType12c|constructor#_#new#tearOff(imp::ClassM n) → self::ExtensionType12c /* = imp::ClassM */
  return self::ExtensionType12c|constructor#(n);
-static inline-class-member method ExtensionType12c|get#property(lowered final self::ExtensionType12c /* = Never */ #this) → core::bool
+static inline-class-member method ExtensionType12c|get#property(lowered final self::ExtensionType12c /* = imp::ClassM */ #this) → core::bool
  return true;
-static inline-class-member method ExtensionType12d|constructor#(Never n) → self::ExtensionType12d /* = Never */ {
-  lowered final self::ExtensionType12d /* = Never */ #this = n;
+static inline-class-member method ExtensionType12d|constructor#(Never n) → self::ExtensionType12d /* = invalid-type */ {
+  lowered final self::ExtensionType12d /* = invalid-type */ #this = n;
  return #this;
 }
-static inline-class-member method ExtensionType12d|constructor#_#new#tearOff(Never n) → self::ExtensionType12d /* = Never */
+static inline-class-member method ExtensionType12d|constructor#_#new#tearOff(Never n) → self::ExtensionType12d /* = invalid-type */
  return self::ExtensionType12d|constructor#(n);
-static inline-class-member method ExtensionType12d|get#property(lowered final self::ExtensionType12d /* = Never */ #this) → core::bool
+static inline-class-member method ExtensionType12d|get#property(lowered final self::ExtensionType12d /* = invalid-type */ #this) → core::bool
  return true;
-static inline-class-member method ExtensionType12d|set#property(lowered final self::ExtensionType12d /* = Never */ #this, core::bool value) → void {}
+static inline-class-member method ExtensionType12d|set#property(lowered final self::ExtensionType12d /* = invalid-type */ #this, core::bool value) → void {}
 static inline-class-member method ExtensionType13a|constructor#(core::int i) → self::ExtensionType13a /* = core::int */ {
  lowered final self::ExtensionType13a /* = core::int */ #this = i;
  return #this;
--- a/pkg/front_end/testcases/extension_types/issue53824.dart
+++ b/pkg/front_end/testcases/extension_types/issue53824.dart
@ -0,0 +1,7 @@
+// Copyright (c) 2023, 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 type E1(Never foo) {} // Error.
+extension type E2<X extends Never>(X foo) {} // Error.
+extension type E3<X extends Y, Y extends Never>(X foo) {} // Error.
--- a/pkg/front_end/testcases/extension_types/issue53824.dart.strong.expect
+++ b/pkg/front_end/testcases/extension_types/issue53824.dart.strong.expect
@ -0,0 +1,51 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:5:25: Error: The representation type can't be a bottom type.
+// extension type E1(Never foo) {} // Error.
+//                         ^^^
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:6:38: Error: The representation type can't be a bottom type.
+// extension type E2<X extends Never>(X foo) {} // Error.
+//                                      ^^^
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:7:51: Error: The representation type can't be a bottom type.
+// extension type E3<X extends Y, Y extends Never>(X foo) {} // Error.
+//                                                   ^^^
+//
+import self as self;
+
+extension type E1(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → Never;
+  constructor • = self::E1|constructor#;
+  constructor tearoff • = self::E1|constructor#_#new#tearOff;
+}
+extension type E2<X extends Never>(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → X;
+  constructor • = self::E2|constructor#;
+  constructor tearoff • = self::E2|constructor#_#new#tearOff;
+}
+extension type E3<X extends Y = Never, Y extends Never>(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → X;
+  constructor • = self::E3|constructor#;
+  constructor tearoff • = self::E3|constructor#_#new#tearOff;
+}
+static inline-class-member method E1|constructor#(Never foo) → self::E1 /* = invalid-type */ {
+  lowered final self::E1 /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E1|constructor#_#new#tearOff(Never foo) → self::E1 /* = invalid-type */
+  return self::E1|constructor#(foo);
+static inline-class-member method E2|constructor#<X extends Never>(self::E2|constructor#::X foo) → self::E2<self::E2|constructor#::X> /* = invalid-type */ {
+  lowered final self::E2<self::E2|constructor#::X> /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E2|constructor#_#new#tearOff<X extends Never>(self::E2|constructor#_#new#tearOff::X foo) → self::E2<self::E2|constructor#_#new#tearOff::X>% /* = invalid-type */
+  return self::E2|constructor#<self::E2|constructor#_#new#tearOff::X>(foo);
+static inline-class-member method E3|constructor#<X extends self::E3|constructor#::Y = Never, Y extends Never>(self::E3|constructor#::X foo) → self::E3<self::E3|constructor#::X, self::E3|constructor#::Y> /* = invalid-type */ {
+  lowered final self::E3<self::E3|constructor#::X, self::E3|constructor#::Y> /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E3|constructor#_#new#tearOff<X extends self::E3|constructor#_#new#tearOff::Y = Never, Y extends Never>(self::E3|constructor#_#new#tearOff::X foo) → self::E3<self::E3|constructor#_#new#tearOff::X, self::E3|constructor#_#new#tearOff::Y>% /* = invalid-type */
+  return self::E3|constructor#<self::E3|constructor#_#new#tearOff::X, self::E3|constructor#_#new#tearOff::Y>(foo);
--- a/pkg/front_end/testcases/extension_types/issue53824.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/issue53824.dart.strong.transformed.expect
@ -0,0 +1,51 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:5:25: Error: The representation type can't be a bottom type.
+// extension type E1(Never foo) {} // Error.
+//                         ^^^
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:6:38: Error: The representation type can't be a bottom type.
+// extension type E2<X extends Never>(X foo) {} // Error.
+//                                      ^^^
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:7:51: Error: The representation type can't be a bottom type.
+// extension type E3<X extends Y, Y extends Never>(X foo) {} // Error.
+//                                                   ^^^
+//
+import self as self;
+
+extension type E1(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → Never;
+  constructor • = self::E1|constructor#;
+  constructor tearoff • = self::E1|constructor#_#new#tearOff;
+}
+extension type E2<X extends Never>(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → X;
+  constructor • = self::E2|constructor#;
+  constructor tearoff • = self::E2|constructor#_#new#tearOff;
+}
+extension type E3<X extends Y = Never, Y extends Never>(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → X;
+  constructor • = self::E3|constructor#;
+  constructor tearoff • = self::E3|constructor#_#new#tearOff;
+}
+static inline-class-member method E1|constructor#(Never foo) → self::E1 /* = invalid-type */ {
+  lowered final self::E1 /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E1|constructor#_#new#tearOff(Never foo) → self::E1 /* = invalid-type */
+  return self::E1|constructor#(foo);
+static inline-class-member method E2|constructor#<X extends Never>(self::E2|constructor#::X foo) → self::E2<self::E2|constructor#::X> /* = invalid-type */ {
+  lowered final self::E2<self::E2|constructor#::X> /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E2|constructor#_#new#tearOff<X extends Never>(self::E2|constructor#_#new#tearOff::X foo) → self::E2<self::E2|constructor#_#new#tearOff::X>% /* = invalid-type */
+  return self::E2|constructor#<self::E2|constructor#_#new#tearOff::X>(foo);
+static inline-class-member method E3|constructor#<X extends self::E3|constructor#::Y = Never, Y extends Never>(self::E3|constructor#::X foo) → self::E3<self::E3|constructor#::X, self::E3|constructor#::Y> /* = invalid-type */ {
+  lowered final self::E3<self::E3|constructor#::X, self::E3|constructor#::Y> /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E3|constructor#_#new#tearOff<X extends self::E3|constructor#_#new#tearOff::Y = Never, Y extends Never>(self::E3|constructor#_#new#tearOff::X foo) → self::E3<self::E3|constructor#_#new#tearOff::X, self::E3|constructor#_#new#tearOff::Y>% /* = invalid-type */
+  return self::E3|constructor#<self::E3|constructor#_#new#tearOff::X, self::E3|constructor#_#new#tearOff::Y>(foo);
--- a/pkg/front_end/testcases/extension_types/issue53824.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/extension_types/issue53824.dart.textual_outline.expect
@ -0,0 +1,3 @@
+extension type E1(Never foo) {}
+extension type E2<X extends Never>(X foo) {}
+extension type E3<X extends Y, Y extends Never>(X foo) {}
--- a/pkg/front_end/testcases/extension_types/issue53824.dart.textual_outline_modelled.expect
+++ b/pkg/front_end/testcases/extension_types/issue53824.dart.textual_outline_modelled.expect
@ -0,0 +1,3 @@
+extension type E1(Never foo) {}
+extension type E2<X extends Never>(X foo) {}
+extension type E3<X extends Y, Y extends Never>(X foo) {}
--- a/pkg/front_end/testcases/extension_types/issue53824.dart.weak.expect
+++ b/pkg/front_end/testcases/extension_types/issue53824.dart.weak.expect
@ -0,0 +1,51 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:5:25: Error: The representation type can't be a bottom type.
+// extension type E1(Never foo) {} // Error.
+//                         ^^^
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:6:38: Error: The representation type can't be a bottom type.
+// extension type E2<X extends Never>(X foo) {} // Error.
+//                                      ^^^
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:7:51: Error: The representation type can't be a bottom type.
+// extension type E3<X extends Y, Y extends Never>(X foo) {} // Error.
+//                                                   ^^^
+//
+import self as self;
+
+extension type E1(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → Never;
+  constructor • = self::E1|constructor#;
+  constructor tearoff • = self::E1|constructor#_#new#tearOff;
+}
+extension type E2<X extends Never>(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → X;
+  constructor • = self::E2|constructor#;
+  constructor tearoff • = self::E2|constructor#_#new#tearOff;
+}
+extension type E3<X extends Y = Never, Y extends Never>(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → X;
+  constructor • = self::E3|constructor#;
+  constructor tearoff • = self::E3|constructor#_#new#tearOff;
+}
+static inline-class-member method E1|constructor#(Never foo) → self::E1 /* = invalid-type */ {
+  lowered final self::E1 /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E1|constructor#_#new#tearOff(Never foo) → self::E1 /* = invalid-type */
+  return self::E1|constructor#(foo);
+static inline-class-member method E2|constructor#<X extends Never>(self::E2|constructor#::X foo) → self::E2<self::E2|constructor#::X> /* = invalid-type */ {
+  lowered final self::E2<self::E2|constructor#::X> /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E2|constructor#_#new#tearOff<X extends Never>(self::E2|constructor#_#new#tearOff::X foo) → self::E2<self::E2|constructor#_#new#tearOff::X>% /* = invalid-type */
+  return self::E2|constructor#<self::E2|constructor#_#new#tearOff::X>(foo);
+static inline-class-member method E3|constructor#<X extends self::E3|constructor#::Y = Never, Y extends Never>(self::E3|constructor#::X foo) → self::E3<self::E3|constructor#::X, self::E3|constructor#::Y> /* = invalid-type */ {
+  lowered final self::E3<self::E3|constructor#::X, self::E3|constructor#::Y> /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E3|constructor#_#new#tearOff<X extends self::E3|constructor#_#new#tearOff::Y = Never, Y extends Never>(self::E3|constructor#_#new#tearOff::X foo) → self::E3<self::E3|constructor#_#new#tearOff::X, self::E3|constructor#_#new#tearOff::Y>% /* = invalid-type */
+  return self::E3|constructor#<self::E3|constructor#_#new#tearOff::X, self::E3|constructor#_#new#tearOff::Y>(foo);
--- a/pkg/front_end/testcases/extension_types/issue53824.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/extension_types/issue53824.dart.weak.modular.expect
@ -0,0 +1,51 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:5:25: Error: The representation type can't be a bottom type.
+// extension type E1(Never foo) {} // Error.
+//                         ^^^
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:6:38: Error: The representation type can't be a bottom type.
+// extension type E2<X extends Never>(X foo) {} // Error.
+//                                      ^^^
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:7:51: Error: The representation type can't be a bottom type.
+// extension type E3<X extends Y, Y extends Never>(X foo) {} // Error.
+//                                                   ^^^
+//
+import self as self;
+
+extension type E1(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → Never;
+  constructor • = self::E1|constructor#;
+  constructor tearoff • = self::E1|constructor#_#new#tearOff;
+}
+extension type E2<X extends Never>(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → X;
+  constructor • = self::E2|constructor#;
+  constructor tearoff • = self::E2|constructor#_#new#tearOff;
+}
+extension type E3<X extends Y = Never, Y extends Never>(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → X;
+  constructor • = self::E3|constructor#;
+  constructor tearoff • = self::E3|constructor#_#new#tearOff;
+}
+static inline-class-member method E1|constructor#(Never foo) → self::E1 /* = invalid-type */ {
+  lowered final self::E1 /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E1|constructor#_#new#tearOff(Never foo) → self::E1 /* = invalid-type */
+  return self::E1|constructor#(foo);
+static inline-class-member method E2|constructor#<X extends Never>(self::E2|constructor#::X foo) → self::E2<self::E2|constructor#::X> /* = invalid-type */ {
+  lowered final self::E2<self::E2|constructor#::X> /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E2|constructor#_#new#tearOff<X extends Never>(self::E2|constructor#_#new#tearOff::X foo) → self::E2<self::E2|constructor#_#new#tearOff::X>% /* = invalid-type */
+  return self::E2|constructor#<self::E2|constructor#_#new#tearOff::X>(foo);
+static inline-class-member method E3|constructor#<X extends self::E3|constructor#::Y = Never, Y extends Never>(self::E3|constructor#::X foo) → self::E3<self::E3|constructor#::X, self::E3|constructor#::Y> /* = invalid-type */ {
+  lowered final self::E3<self::E3|constructor#::X, self::E3|constructor#::Y> /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E3|constructor#_#new#tearOff<X extends self::E3|constructor#_#new#tearOff::Y = Never, Y extends Never>(self::E3|constructor#_#new#tearOff::X foo) → self::E3<self::E3|constructor#_#new#tearOff::X, self::E3|constructor#_#new#tearOff::Y>% /* = invalid-type */
+  return self::E3|constructor#<self::E3|constructor#_#new#tearOff::X, self::E3|constructor#_#new#tearOff::Y>(foo);
--- a/pkg/front_end/testcases/extension_types/issue53824.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/extension_types/issue53824.dart.weak.outline.expect
@ -0,0 +1,45 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:5:25: Error: The representation type can't be a bottom type.
+// extension type E1(Never foo) {} // Error.
+//                         ^^^
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:6:38: Error: The representation type can't be a bottom type.
+// extension type E2<X extends Never>(X foo) {} // Error.
+//                                      ^^^
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:7:51: Error: The representation type can't be a bottom type.
+// extension type E3<X extends Y, Y extends Never>(X foo) {} // Error.
+//                                                   ^^^
+//
+import self as self;
+
+extension type E1(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → Never;
+  constructor • = self::E1|constructor#;
+  constructor tearoff • = self::E1|constructor#_#new#tearOff;
+}
+extension type E2<X extends Never>(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → X;
+  constructor • = self::E2|constructor#;
+  constructor tearoff • = self::E2|constructor#_#new#tearOff;
+}
+extension type E3<X extends Y = Never, Y extends Never>(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → X;
+  constructor • = self::E3|constructor#;
+  constructor tearoff • = self::E3|constructor#_#new#tearOff;
+}
+static inline-class-member method E1|constructor#(Never foo) → self::E1 /* = invalid-type */
+  ;
+static inline-class-member method E1|constructor#_#new#tearOff(Never foo) → self::E1 /* = invalid-type */
+  return self::E1|constructor#(foo);
+static inline-class-member method E2|constructor#<X extends Never>(self::E2|constructor#::X foo) → self::E2<self::E2|constructor#::X> /* = invalid-type */
+  ;
+static inline-class-member method E2|constructor#_#new#tearOff<X extends Never>(self::E2|constructor#_#new#tearOff::X foo) → self::E2<self::E2|constructor#_#new#tearOff::X>% /* = invalid-type */
+  return self::E2|constructor#<self::E2|constructor#_#new#tearOff::X>(foo);
+static inline-class-member method E3|constructor#<X extends self::E3|constructor#::Y = Never, Y extends Never>(self::E3|constructor#::X foo) → self::E3<self::E3|constructor#::X, self::E3|constructor#::Y> /* = invalid-type */
+  ;
+static inline-class-member method E3|constructor#_#new#tearOff<X extends self::E3|constructor#_#new#tearOff::Y = Never, Y extends Never>(self::E3|constructor#_#new#tearOff::X foo) → self::E3<self::E3|constructor#_#new#tearOff::X, self::E3|constructor#_#new#tearOff::Y>% /* = invalid-type */
+  return self::E3|constructor#<self::E3|constructor#_#new#tearOff::X, self::E3|constructor#_#new#tearOff::Y>(foo);
--- a/pkg/front_end/testcases/extension_types/issue53824.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/issue53824.dart.weak.transformed.expect
@ -0,0 +1,51 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:5:25: Error: The representation type can't be a bottom type.
+// extension type E1(Never foo) {} // Error.
+//                         ^^^
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:6:38: Error: The representation type can't be a bottom type.
+// extension type E2<X extends Never>(X foo) {} // Error.
+//                                      ^^^
+//
+// pkg/front_end/testcases/extension_types/issue53824.dart:7:51: Error: The representation type can't be a bottom type.
+// extension type E3<X extends Y, Y extends Never>(X foo) {} // Error.
+//                                                   ^^^
+//
+import self as self;
+
+extension type E1(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → Never;
+  constructor • = self::E1|constructor#;
+  constructor tearoff • = self::E1|constructor#_#new#tearOff;
+}
+extension type E2<X extends Never>(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → X;
+  constructor • = self::E2|constructor#;
+  constructor tearoff • = self::E2|constructor#_#new#tearOff;
+}
+extension type E3<X extends Y = Never, Y extends Never>(invalid-type foo) {
+  abstract inline-class-member representation-field get foo() → X;
+  constructor • = self::E3|constructor#;
+  constructor tearoff • = self::E3|constructor#_#new#tearOff;
+}
+static inline-class-member method E1|constructor#(Never foo) → self::E1 /* = invalid-type */ {
+  lowered final self::E1 /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E1|constructor#_#new#tearOff(Never foo) → self::E1 /* = invalid-type */
+  return self::E1|constructor#(foo);
+static inline-class-member method E2|constructor#<X extends Never>(self::E2|constructor#::X foo) → self::E2<self::E2|constructor#::X> /* = invalid-type */ {
+  lowered final self::E2<self::E2|constructor#::X> /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E2|constructor#_#new#tearOff<X extends Never>(self::E2|constructor#_#new#tearOff::X foo) → self::E2<self::E2|constructor#_#new#tearOff::X>% /* = invalid-type */
+  return self::E2|constructor#<self::E2|constructor#_#new#tearOff::X>(foo);
+static inline-class-member method E3|constructor#<X extends self::E3|constructor#::Y = Never, Y extends Never>(self::E3|constructor#::X foo) → self::E3<self::E3|constructor#::X, self::E3|constructor#::Y> /* = invalid-type */ {
+  lowered final self::E3<self::E3|constructor#::X, self::E3|constructor#::Y> /* = invalid-type */ #this = foo;
+  return #this;
+}
+static inline-class-member method E3|constructor#_#new#tearOff<X extends self::E3|constructor#_#new#tearOff::Y = Never, Y extends Never>(self::E3|constructor#_#new#tearOff::X foo) → self::E3<self::E3|constructor#_#new#tearOff::X, self::E3|constructor#_#new#tearOff::Y>% /* = invalid-type */
+  return self::E3|constructor#<self::E3|constructor#_#new#tearOff::X, self::E3|constructor#_#new#tearOff::Y>(foo);
--- a/pkg/kernel/lib/core_types.dart
+++ b/pkg/kernel/lib/core_types.dart
@ -6,7 +6,7 @@ library kernel.core_types;

 import 'ast.dart';
 import 'library_index.dart';
-import 'type_algebra.dart';
+import 'type_algebra.dart' as type_algebra;

 /// Provides access to the classes and libraries in the core libraries.
 class CoreTypes {
@ -1160,7 +1160,8 @@ class CoreTypes {
    // TOP(T*) is true iff TOP(T) or OBJECT(T).
    if (type.declaredNullability == Nullability.nullable ||
        type.declaredNullability == Nullability.legacy) {
-      DartType nonNullableType = unwrapNullabilityConstructor(type, this);
+      DartType nonNullableType =
+          type_algebra.unwrapNullabilityConstructor(type);
      if (!identical(type, nonNullableType)) {
        return isTop(nonNullableType) || isObject(nonNullableType);
      }
@ -1202,29 +1203,7 @@ class CoreTypes {
  /// https://github.com/dart-lang/language/blob/master/resources/type-system/upper-lower-bounds.md#helper-predicates
  @pragma("vm:prefer-inline")
  bool isBottom(DartType type) {
-    if (type is InterfaceType) return false;
-    return _isBottom(type);
-  }
-
-  bool _isBottom(DartType type) {
-    if (type is InvalidType) return false;
-
-    // BOTTOM(Never) is true.
-    if (type is NeverType && type.nullability == Nullability.nonNullable) {
-      return true;
-    }
-
-    // BOTTOM(X&T) is true iff BOTTOM(T).
-    if (type is IntersectionType && type.isPotentiallyNonNullable) {
-      return isBottom(type.right);
-    }
-
-    // BOTTOM(X extends T) is true iff BOTTOM(T).
-    if (type is TypeParameterType && type.isPotentiallyNonNullable) {
-      return isBottom(type.parameter.bound);
-    }
-
-    return false;
+    return type_algebra.isBottom(type);
  }

  /// Checks if [type] satisfies the NULL predicate.
--- a/pkg/kernel/lib/type_algebra.dart
+++ b/pkg/kernel/lib/type_algebra.dart
@ -5,7 +5,6 @@
 library kernel.type_algebra;

 import 'ast.dart';
-import 'core_types.dart';
 import 'src/find_type_visitor.dart';
 import 'src/replacement_visitor.dart';

@ -1884,8 +1883,8 @@ class _PrimitiveTypeVerifier implements DartTypeVisitor<bool> {
 /// Some types are nullable even without the application of the nullable type
 /// constructor at the top level, for example, Null or FutureOr<int?>.
 // TODO(cstefantsova): Remove [coreTypes] parameter when NullType is landed.
-DartType unwrapNullabilityConstructor(DartType type, CoreTypes coreTypes) {
-  return type.accept1(const _NullabilityConstructorUnwrapper(), coreTypes);
+DartType unwrapNullabilityConstructor(DartType type) {
+  return type.accept(const _NullabilityConstructorUnwrapper());
 }

 /// Implementation of [unwrapNullabilityConstructor] as a visitor.
@ -1894,81 +1893,79 @@ DartType unwrapNullabilityConstructor(DartType type, CoreTypes coreTypes) {
 /// new implementation of [DartType] visible at compile time.
 // TODO(cstefantsova): Remove CoreTypes as the second argument when NullType is
 // landed.
-class _NullabilityConstructorUnwrapper
-    implements DartTypeVisitor1<DartType, CoreTypes> {
+class _NullabilityConstructorUnwrapper implements DartTypeVisitor<DartType> {
  const _NullabilityConstructorUnwrapper();

  @override
-  DartType visitAuxiliaryType(AuxiliaryType node, CoreTypes coreTypes) {
+  DartType visitAuxiliaryType(AuxiliaryType node) {
    throw new UnsupportedError(
        "Unsupported auxiliary type ${node} (${node.runtimeType}).");
  }

  @override
-  DartType visitDynamicType(DynamicType node, CoreTypes coreTypes) => node;
+  DartType visitDynamicType(DynamicType node) => node;

  @override
-  DartType visitFunctionType(FunctionType node, CoreTypes coreTypes) {
+  DartType visitFunctionType(FunctionType node) {
    return node.withDeclaredNullability(Nullability.nonNullable);
  }

  @override
-  DartType visitRecordType(RecordType node, CoreTypes coreTypes) {
+  DartType visitRecordType(RecordType node) {
    return node.withDeclaredNullability(Nullability.nonNullable);
  }

  @override
-  DartType visitFutureOrType(FutureOrType node, CoreTypes coreTypes) {
+  DartType visitFutureOrType(FutureOrType node) {
    return node.withDeclaredNullability(Nullability.nonNullable);
  }

  @override
-  DartType visitInterfaceType(InterfaceType node, CoreTypes coreTypes) {
+  DartType visitInterfaceType(InterfaceType node) {
    return node.withDeclaredNullability(Nullability.nonNullable);
  }

  @override
-  DartType visitExtensionType(ExtensionType node, CoreTypes coreTypes) {
+  DartType visitExtensionType(ExtensionType node) {
    return node.withDeclaredNullability(Nullability.nonNullable);
  }

  @override
-  DartType visitInvalidType(InvalidType node, CoreTypes coreTypes) => node;
+  DartType visitInvalidType(InvalidType node) => node;

  @override
-  DartType visitNeverType(NeverType node, CoreTypes coreTypes) {
+  DartType visitNeverType(NeverType node) {
    return node.withDeclaredNullability(Nullability.nonNullable);
  }

  @override
-  DartType visitNullType(NullType node, CoreTypes coreTypes) => node;
+  DartType visitNullType(NullType node) => node;

  @override
-  DartType visitTypeParameterType(TypeParameterType node, CoreTypes coreTypes) {
+  DartType visitTypeParameterType(TypeParameterType node) {
    return node.withDeclaredNullability(
        TypeParameterType.computeNullabilityFromBound(node.parameter));
  }

  @override
-  DartType visitStructuralParameterType(
-      StructuralParameterType node, CoreTypes coreTypes) {
+  DartType visitStructuralParameterType(StructuralParameterType node) {
    return node.withDeclaredNullability(
        StructuralParameterType.computeNullabilityFromBound(node.parameter));
  }

  @override
-  DartType visitIntersectionType(IntersectionType node, CoreTypes coreTypes) {
+  DartType visitIntersectionType(IntersectionType node) {
    // Intersection types don't have their own nullabilities.
    return node;
  }

  @override
-  DartType visitTypedefType(TypedefType node, CoreTypes coreTypes) {
+  DartType visitTypedefType(TypedefType node) {
    return node.withDeclaredNullability(Nullability.nonNullable);
  }

  @override
-  DartType visitVoidType(VoidType node, CoreTypes coreTypes) => node;
+  DartType visitVoidType(VoidType node) => node;
 }

 abstract class NullabilityAwareTypeVariableEliminatorBase
@ -2413,3 +2410,32 @@ DartType _updateNestedFutureOrNullability(
    return typeToUpdate.withDeclaredNullability(updatedNullability);
  }
 }
+
+/// Returns true if [type] is a bottom type
+///
+/// Some examples of bottom types are `Never`, `X` where `X extends Never`, and
+/// `X & Never`.
+///
+/// For the definition of BOTTOM see the following:
+/// https://github.com/dart-lang/language/blob/master/resources/type-system/upper-lower-bounds.md#helper-predicates
+bool isBottom(DartType type) {
+  if (type is InterfaceType) return false;
+  if (type is InvalidType) return false;
+
+  // BOTTOM(Never) is true.
+  if (type is NeverType && type.nullability == Nullability.nonNullable) {
+    return true;
+  }
+
+  // BOTTOM(X&T) is true iff BOTTOM(T).
+  if (type is IntersectionType && type.isPotentiallyNonNullable) {
+    return isBottom(type.right);
+  }
+
+  // BOTTOM(X extends T) is true iff BOTTOM(T).
+  if (type is TypeParameterType && type.isPotentiallyNonNullable) {
+    return isBottom(type.parameter.bound);
+  }
+
+  return false;
+}
--- a/tests/language/extension_type/extension_type_representation_type_test.dart
+++ b/tests/language/extension_type/extension_type_representation_type_test.dart
@ -17,6 +17,8 @@ extension type V02(void _) {}
 extension type V03(Never _) {}
 //                 ^^^^^
 // [analyzer] COMPILE_TIME_ERROR.EXTENSION_TYPE_REPRESENTATION_TYPE_BOTTOM
+//                       ^
+// [cfe] The representation type can't be a bottom type.
 extension type V04(Null _) {}
 extension type V05(Function _) {}
 extension type V06(Record _) {}