Fix unsupported type detection when serializing polymorphic System.Type instances

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/UnsupportedTypeConverterFactory.cs

@@ -14,7 +14,7 @@ public override bool CanConvert(Type type)
 
             return
                 // There's no safe way to construct a Type from untrusted user input.
-                type == typeof(Type) ||
+                typeof(Type).IsAssignableFrom(type) ||
                 // (De)serialization of SerializationInfo is already disallowed due to Type being disallowed
                 // (the two ctors on SerializationInfo take a Type, and a Type member is present when serializing).
                 // Explicitly disallowing this type provides a clear exception when ctors with

src/libraries/System.Text.Json/tests/Common/UnsupportedTypesTests.cs

@@ -72,6 +72,7 @@ async Task RunTest<T>(string json)
         [Fact]
         public async Task SerializeUnsupportedType()
         {
+            // TODO refactor to Xunit theory
             await RunTest(typeof(int));
             await RunTest(new SerializationInfo(typeof(Type), new FormatterConverter()));
             await RunTest((IntPtr)123);
@@ -96,7 +97,6 @@ async Task RunTest<T>(T value)
                 Assert.Contains("$", exAsStr);
 
                 ClassWithType<T> obj = new ClassWithType<T> { Prop = value };
-
                 ex = await Assert.ThrowsAsync<NotSupportedException>(async () => await Serializer.SerializeWrapper(obj));
                 exAsStr = ex.ToString();
                 Assert.Contains(fullName, exAsStr);
@@ -124,6 +124,20 @@ async Task RunTest<T>(T value)
                     serialized = await Serializer.SerializeWrapper(obj, new JsonSerializerOptions { IgnoreNullValues = true });
                     Assert.Equal(@"{}", serialized);
                 }
+
+#if !BUILDING_SOURCE_GENERATOR_TESTS
+                Type runtimeType = GetNullableOfTUnderlyingType(value.GetType(), out bool _);
+
+                ex = await Assert.ThrowsAsync<NotSupportedException>(async () => await Serializer.SerializeWrapper<object>(value));
+                exAsStr = ex.ToString();
+                Assert.Contains(runtimeType.FullName, exAsStr);
+                Assert.Contains("$", exAsStr);
+
+                ClassWithType<object> polyObj = new ClassWithType<object> { Prop = value };
+                ex = await Assert.ThrowsAsync<NotSupportedException>(async () => await Serializer.SerializeWrapper(polyObj));
+                exAsStr = ex.ToString();
+                Assert.Contains(runtimeType.FullName, exAsStr);
+#endif
             }
         }