Test that function types use mutual subtyping for type parameter bounds.

For dart-lang/language#807

Change-Id: Ife86fa35fda20347437391d742a4432518d7898a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/135350
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/generated/type_system.dart

@@ -1916,8 +1916,6 @@ class Dart2TypeSystem extends TypeSystem {
     // The bounds of type parameters must be equal.
     var freshTypeFormalTypes =
         FunctionTypeImpl.relateTypeFormals(f, g, (t, s, _, __) {
-      // Type parameter bounds are invariant.
-      // TODO(scheglov) We do this for top types, but the spec says explicitly.
       return isSubtypeOf2(t, s) && isSubtypeOf2(s, t);
     });
     if (freshTypeFormalTypes == null) {

pkg/analyzer/test/src/dart/element/subtype_test.dart

@@ -1712,6 +1712,24 @@ class SubtypeTest extends _SubtypingTestBase {
     );
   }
 
+  test_functionType_73() {
+    var E0 = typeParameter('E0', bound: objectNone);
+    var E1 = typeParameter('E1', bound: futureOrNone(objectNone));
+
+    isSubtype(
+      functionTypeNone(
+        typeFormals: [E0],
+        returnType: voidNone,
+      ),
+      functionTypeNone(
+        typeFormals: [E1],
+        returnType: voidNone,
+      ),
+      strT0: 'void Function<E0 extends Object>()',
+      strT1: 'void Function<E1 extends FutureOr<Object>>()',
+    );
+  }
+
   test_functionType_generic_nested() {
     var E0 = typeParameter('E0');
     var F0 = typeParameter('F0');