additional tests

- implement the test for #976 in Java and .NET (PASSED)
- port `canLeverageIndirectInterfacePolymorphsm` to python (PASSED)

packages/jsii-dotnet-runtime-test/test/Amazon.JSII.Runtime.IntegrationTests/ComplianceTests.cs

@@ -1033,6 +1033,13 @@ public void VariadicCallbacksAreHandledCorrectly()
             Assert.Equal(new double[]{2d, 3d, 4d}, invoker.AsArray(1, 2, 3));
         }
 
+        [Fact(DisplayName = Prefix + nameof(ReturnSubclassThatImplementsInterface976))]
+        public void ReturnSubclassThatImplementsInterface976() 
+        {
+            var obj = SomeTypeJsii976.ReturnReturn();
+            Assert.Equal(obj.Foo, 333);
+        }
+
         private sealed class OverrideVariadicMethod : VariadicMethod
         {
             public override double[] AsArray(double first, params double[] others)

packages/jsii-java-runtime-test/project/src/test/java/software/amazon/jsii/testing/ComplianceTest.java

@@ -1391,6 +1391,12 @@ public void correctlyDeserializesStructUnions() {
         assertTrue(StructUnionConsumer.isStructB(b1));
     }
 
+    @Test
+    public void returnSubclassThatImplementsInterface976() {
+        IReturnJsii976 obj = SomeTypeJsii976.returnReturn();
+        assertEquals(obj.getFoo(), 333);
+    }
+
     static class PartiallyInitializedThisConsumerImpl extends PartiallyInitializedThisConsumer {
         @Override
         public String consumePartiallyInitializedThis(final ConstructorPassesThisOut obj,

packages/jsii-python-runtime/tests/test_compliance.py

@@ -55,7 +55,9 @@
     StructA,
     StructB,
     StructUnionConsumer,
-    SomeTypeJsii976
+    SomeTypeJsii976,
+    AnonymousImplementationProvider,
+    IAnonymousImplementationProvider
 )
 from scope.jsii_calc_lib import IFriendly, EnumFromScopedModule, Number
 
@@ -1013,6 +1015,12 @@ def test_can_pass_nested_struct_as_dict():
         }
     )
 
+def test_can_leverage_indirect_interface_polymorphism():
+    provider = AnonymousImplementationProvider()
+    assert provider.provide_as_class().value == 1337
+    assert provider.provide_as_interface().value == 1337
+    assert provider.provide_as_interface().verb() == "to implement"
+
 # https://github.com/aws/jsii/issues/976
 def test_return_subclass_that_implements_interface_976():
     obj = SomeTypeJsii976.return_return()