Fix for invalid implicit downcast in InterfaceType.allSupertypes.

R=brianwilkerson@google.com

Change-Id: Ic4480fbab8ea8da6e9e8bb03ad95af66b3622618
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155821
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: scheglov

pkg/analyzer/CHANGELOG.md

@@ -2,6 +2,7 @@
 * Move `asInstanceOf(ClassElement)` to `DartType`, so that it is also
   supported for `TypeParameterType` when its bound implements the
   requested interface.
+* Fixed invalid implicit downcast in `InterfaceType.allSupertypes`.
 
 ## 0.39.14
 * Removed `defaultSdkDirectory()` and `getSdkProperty()` from internal

pkg/analyzer/lib/src/dart/element/type.dart

@@ -719,7 +719,9 @@ class InterfaceTypeImpl extends TypeImpl implements InterfaceType {
   @override
   List<InterfaceType> get allSupertypes {
     var substitution = Substitution.fromInterfaceType(this);
-    return element.allSupertypes.map(substitution.substituteType).toList();
+    return element.allSupertypes
+        .map((t) => substitution.substituteType(t) as InterfaceType)
+        .toList();
   }
 
   @override

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

@@ -1282,6 +1282,23 @@ class FunctionTypeImplTest extends AbstractTypeTest {
 
 @reflectiveTest
 class InterfaceTypeImplTest extends AbstractTypeTest {
+  void test_allSupertypes() {
+    void check(InterfaceType type, List<String> expected) {
+      var actual = type.allSupertypes.map((e) {
+        return e.getDisplayString(
+          withNullability: true,
+        );
+      }).toList()
+        ..sort();
+      expect(actual, expected);
+    }
+
+    check(objectNone, []);
+    check(numNone, ['Comparable<num>', 'Object']);
+    check(intNone, ['Comparable<num>', 'Object', 'num']);
+    check(listNone(intQuestion), ['Iterable<int?>', 'Object']);
+  }
+
   test_asInstanceOf_explicitGeneric() {
     // class A<E> {}
     // class B implements A<C> {}