Handle integral with cpp.type for populator

Summary: currently partial specialization is ambiguous. Explicitly disable generic cpp_type partial specialization for integral.

Reviewed By: iahs

Differential Revision: D68084261

fbshipit-source-id: 0a8c8bea60cc78af8f5683029bb8463a9d057b36

thrift/lib/cpp2/reflection/populator.h

@@ -515,9 +515,12 @@ struct populator_methods<type::adapted<Adapter, InnerTag>, T> {
   }
 };
 
-// cpp.type is handled transparently by each implementation
+// non-integral cpp.type is handled transparently by each implementation
 template <typename T, typename Tag>
-struct populator_methods<type::cpp_type<T, Tag>, T>
+struct populator_methods<
+    type::cpp_type<T, Tag>,
+    T,
+    std::enable_if_t<!type::is_a_v<Tag, type::integral_c>>>
     : populator_methods<Tag, T> {};
 
 /**