[_fe_analyzer_shared] Handle empty types in exhaustiveness

This handles Never, and as a consequence, empty sealed types in
exhaustiveness. Since these contain no value, we should not try to
exhaust them. This avoid invalid non-exhaustive errors in these cases.

Included is handling of a scrutinee with invalid type as Never in the
CFE, thus avoid cascading non-exhaustive or unreachable errors.

Change-Id: Ife097731bb5399c42bf83f1d77aff773346adfb6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/293682
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/_fe_analyzer_shared/lib/src/exhaustiveness/exhaustive.dart

@@ -109,6 +109,10 @@ class _Checker {
       List<StaticType> stack = [firstValuePattern.type];
       while (stack.isNotEmpty) {
         StaticType type = stack.removeAt(0);
+        if (type.isSubtypeOf(StaticType.neverType)) {
+          // Don't try to exhaust the Never type.
+          continue;
+        }
         if (type.isSealed) {
           Witness? result = _filterByType(
               contextType,

pkg/_fe_analyzer_shared/test/exhaustiveness/data/empty.dart

@@ -49,3 +49,38 @@ emptyEnum(E e) {
     
   };
 }
+
+sealed class Empty {}
+
+emptySealed(Empty empty) => /*
+ checkingOrder={Empty},
+ type=Empty
+*/
+    switch (empty) {
+      
+    };
+
+emptyNever(Never never) => /*type=Never*/ switch (never) {  };
+
+emptyUnresolved(
+        Unresolved
+            unresolved) => /*cfe.type=Never*/ /*analyzer.
+ checkingOrder={Object?,Object,Null},
+ error=non-exhaustive:Object(),
+ subtypes={Object,Null},
+ type=Object?
+*/
+    switch (unresolved) {
+      
+    };
+
+nonEmptyUnresolved(
+        Unresolved
+            unresolved) => /*cfe.type=Never*/ /*analyzer.
+ checkingOrder={Object?,Object,Null},
+ subtypes={Object,Null},
+ type=Object?
+*/
+    switch (unresolved) {
+      _ /*cfe.space=∅*/ /*analyzer.space=()*/ => 0,
+    };

pkg/_fe_analyzer_shared/test/exhaustiveness/data/generic_extension_members.dart

@@ -2,7 +2,17 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-class A<T> {}
+class I<T> {}
+
+class J<T> extends I<T> {}
+
+class A<T> extends J<T> {}
+
+extension<T> on I<T> {
+  num get member {
+    return T == int ? 0.5 : 1;
+  }
+}
 
 extension<T> on A<T> {
   void member(T t) {}
@@ -10,36 +20,27 @@ extension<T> on A<T> {
 
 exhaustiveInferred(
         A<num>
-            a) => /*cfe.
- fields={A<int>.member:void Function(int),A<num>.member:void Function(num)},
- type=<invalid>
-*/ /*analyzer.
- checkingOrder={Object?,Object,Null},
- error=non-exhaustive:Object(),
- fields={A<int>.member:void Function(int),A<num>.member:void Function(num)},
- subtypes={Object,Null},
- type=Object?
-*/
-    switch (o) {
+            a) => /*
+             fields={A<int>.member:void Function(int),A<num>.member:void Function(num)},
+             type=A<num>
+            */
+    switch (a) {
       A<int>(
         :var member
-      ) /*cfe.space=<invalid>(A<int>.member: void Function(int) (void Function(int)))*/ /*analyzer.space=A<int>(A<int>.member: void Function(int) (void Function(int)))*/ =>
+      ) /*space=A<int>(A<int>.member: void Function(int) (void Function(int)))*/ =>
         0,
       A<num>(
         :var member
-      ) /*cfe.
-     error=unreachable,
-     space=<invalid>(A<num>.member: void Function(num) (void Function(num)))
-    */ /*analyzer.space=A<num>(A<num>.member: void Function(num) (void Function(num)))*/ =>
+      ) /*space=A<num>(A<num>.member: void Function(num) (void Function(num)))*/ =>
         1,
     };
 
 exhaustiveTyped(
         A<num>
             a) => /*cfe.
- fields={A<int>.member:void Function(int),A<num>.member:void Function(num)},
- type=<invalid>
-*/ /*analyzer.
+             fields={A<int>.member:void Function(int),A<num>.member:void Function(num)},
+             type=Never
+            */ /*analyzer.
  checkingOrder={Object?,Object,Null},
  error=non-exhaustive:Object(),
  fields={A<int>.member:void Function(int),A<num>.member:void Function(num)},
@@ -49,23 +50,23 @@ exhaustiveTyped(
     switch (o) {
       A<int>(
         :void Function(int) member
-      ) /*cfe.space=<invalid>(A<int>.member: void Function(int) (void Function(int)))*/ /*analyzer.space=A<int>(A<int>.member: void Function(int) (void Function(int)))*/ =>
+      ) /*cfe.space=Never(A<int>.member: void Function(int) (void Function(int)))*/ /*analyzer.space=A<int>(A<int>.member: void Function(int) (void Function(int)))*/ =>
         0,
       A<num>(
         :void Function(num) member
       ) /*cfe.
-     error=unreachable,
-     space=<invalid>(A<num>.member: void Function(num) (void Function(num)))
-    */ /*analyzer.space=A<num>(A<num>.member: void Function(num) (void Function(num)))*/ =>
+       error=unreachable,
+       space=Never(A<num>.member: void Function(num) (void Function(num)))
+      */ /*analyzer.space=A<num>(A<num>.member: void Function(num) (void Function(num)))*/ =>
         1,
     };
 
 unreachable(
         A<num>
             a) => /*cfe.
- fields={A<int>.member:void Function(int),A<num>.member:void Function(num)},
- type=<invalid>
-*/ /*analyzer.
+             fields={A<int>.member:void Function(int),A<num>.member:void Function(num)},
+             type=Never
+            */ /*analyzer.
  checkingOrder={Object?,Object,Null},
  error=non-exhaustive:Object(),
  fields={A<int>.member:void Function(int),A<num>.member:void Function(num)},
@@ -75,14 +76,14 @@ unreachable(
     switch (o) {
       A<num>(
         :var member
-      ) /*cfe.space=<invalid>(A<num>.member: void Function(num) (void Function(num)))*/ /*analyzer.space=A<num>(A<num>.member: void Function(num) (void Function(num)))*/ =>
+      ) /*cfe.space=Never(A<num>.member: void Function(num) (void Function(num)))*/ /*analyzer.space=A<num>(A<num>.member: void Function(num) (void Function(num)))*/ =>
         1,
       A<int>(
         :var member
       ) /*cfe.
-     error=unreachable,
-     space=<invalid>(A<int>.member: void Function(int) (void Function(int)))
-    */ /*analyzer.
+       error=unreachable,
+       space=Never(A<int>.member: void Function(int) (void Function(int)))
+      */ /*analyzer.
      error=unreachable,
      space=A<int>(A<int>.member: void Function(int) (void Function(int)))
     */
@@ -93,9 +94,9 @@ unreachable(
 nonExhaustiveRestricted(
         A<num>
             a) => /*cfe.
- fields={A<int>.member:void Function(int),A<num>.member:void Function(num)},
- type=<invalid>
-*/ /*analyzer.
+             fields={A<int>.member:void Function(int),A<num>.member:void Function(num)},
+             type=Never
+            */ /*analyzer.
  checkingOrder={Object?,Object,Null},
  error=non-exhaustive:Object(),
  fields={A<int>.member:void Function(int),A<num>.member:void Function(num)},
@@ -105,14 +106,14 @@ nonExhaustiveRestricted(
     switch (o) {
       A<num>(
         :void Function(num) member
-      ) /*cfe.space=<invalid>(A<num>.member: void Function(num) (void Function(num)))*/ /*analyzer.space=A<num>(A<num>.member: void Function(num) (void Function(num)))*/ =>
+      ) /*cfe.space=Never(A<num>.member: void Function(num) (void Function(num)))*/ /*analyzer.space=A<num>(A<num>.member: void Function(num) (void Function(num)))*/ =>
         1,
       A<int>(
         :var member
       ) /*cfe.
-     error=unreachable,
-     space=<invalid>(A<int>.member: void Function(int) (void Function(int)))
-    */ /*analyzer.
+       error=unreachable,
+       space=Never(A<int>.member: void Function(int) (void Function(int)))
+      */ /*analyzer.
      error=unreachable,
      space=A<int>(A<int>.member: void Function(int) (void Function(int)))
     */
@@ -135,3 +136,17 @@ intersection(o) {
           A<num>(member: var member2):
   }
 }
+
+// TODO(johnniwinther): This should be exhaustive.
+num exhaustiveMixed(
+        I<num>
+            i) => /*
+ error=non-exhaustive:I<num>(member: double()),
+ fields={I<num>.member:num,J<num>.member:num},
+ type=I<num>
+*/
+    switch (i) {
+      I<num>(:int member) /*space=I<num>(I<num>.member: int (num))*/ => member,
+      J<num>(:double member) /*space=J<num>(J<num>.member: double (num))*/ =>
+        member,
+    };

pkg/_fe_analyzer_shared/test/exhaustiveness/data/never_members.dart

@@ -13,9 +13,30 @@ membersMethod(o) {
   */
       switch (o) {
     Typedef(:var hashCode) /*space=Never(hashCode: ∅)*/ => hashCode,
-    Typedef(:var runtimeType) /*space=Never(runtimeType: ∅)*/ => runtimeType,
-    Typedef(:var toString) /*space=Never(toString: ∅)*/ => toString(),
-    Typedef(:var noSuchMethod) /*space=Never(noSuchMethod: ∅)*/ => noSuchMethod,
+    Typedef(
+      :var runtimeType
+    ) /*
+     error=unreachable,
+     space=Never(runtimeType: ∅)
+    */
+      =>
+      runtimeType,
+    Typedef(
+      :var toString
+    ) /*
+     error=unreachable,
+     space=Never(toString: ∅)
+    */
+      =>
+      toString(),
+    Typedef(
+      :var noSuchMethod
+    ) /*
+     error=unreachable,
+     space=Never(noSuchMethod: ∅)
+    */
+      =>
+      noSuchMethod,
     _ /*space=()*/ => null,
   };
 }
@@ -69,7 +90,6 @@ exhaustiveNoSuchMethod(Typedef o) {
 
 nonExhaustiveRestrictedValue(Typedef o) {
   return /*
-   error=non-exhaustive:Never(hashCode: int())/Never(),
    fields={hashCode:int},
    type=Never
   */
@@ -80,7 +100,6 @@ nonExhaustiveRestrictedValue(Typedef o) {
 
 nonExhaustiveRestrictedType(Typedef o) {
   return /*
-   error=non-exhaustive:Never(noSuchMethod: dynamic Function(Invocation) _)/Never(),
    fields={noSuchMethod:dynamic Function(Invocation)},
    type=Never
   */
@@ -94,14 +113,34 @@ nonExhaustiveRestrictedType(Typedef o) {
 
 unreachableMethod(Typedef o) {
   return /*
-   error=non-exhaustive:Never(hashCode: int(), noSuchMethod: dynamic Function(Invocation) _, runtimeType: Type(), toString: String Function() _)/Never(),
    fields={hashCode:int,noSuchMethod:dynamic Function(Invocation),runtimeType:Type,toString:String Function()},
    type=Never
   */
       switch (o) {
     Typedef(:var hashCode) /*space=Never(hashCode: ∅)*/ => hashCode,
-    Typedef(:var runtimeType) /*space=Never(runtimeType: ∅)*/ => runtimeType,
-    Typedef(:var toString) /*space=Never(toString: ∅)*/ => toString(),
-    Typedef(:var noSuchMethod) /*space=Never(noSuchMethod: ∅)*/ => noSuchMethod,
+    Typedef(
+      :var runtimeType
+    ) /*
+     error=unreachable,
+     space=Never(runtimeType: ∅)
+    */
+      =>
+      runtimeType,
+    Typedef(
+      :var toString
+    ) /*
+     error=unreachable,
+     space=Never(toString: ∅)
+    */
+      =>
+      toString(),
+    Typedef(
+      :var noSuchMethod
+    ) /*
+     error=unreachable,
+     space=Never(noSuchMethod: ∅)
+    */
+      =>
+      noSuchMethod,
   };
 }

pkg/front_end/lib/src/fasta/kernel/constant_evaluator.dart

@@ -1537,7 +1537,11 @@ class ConstantsTransformer extends RemovingTransformer {
       required bool hasDefault,
       required bool mustBeExhaustive,
       required bool isSwitchExpression}) {
-    StaticType type = exhaustivenessCache.getStaticType(expressionType);
+    StaticType type = exhaustivenessCache.getStaticType(
+        // Treat invalid types as empty.
+        expressionType is InvalidType
+            ? const NeverType.nonNullable()
+            : expressionType);
     List<Space> cases = [];
     PatternConverter patternConverter = new PatternConverter(
         exhaustivenessCache, staticTypeContext,

pkg/front_end/test/spell_checking_list_code.txt

@@ -504,6 +504,7 @@ exchanging
 execute
 executor
 executors
+exhaust
 exhausted
 exhaustively
 exhaustiveness