We already know the underlying_type.

We just need to pass it to DEFINE_BITMASK_OPERATORS macro

include/flatbuffers/flatbuffers.h

@@ -1485,21 +1485,17 @@ volatile __attribute__((weak)) const char *flatbuffer_version_string =
 
 #endif  // !defined(_WIN32) && !defined(__CYGWIN__)
 
-#define DEFINE_BITMASK_OPERATORS(E)\
+#define DEFINE_BITMASK_OPERATORS(E, T)\
     inline E operator | (E lhs, E rhs){\
-        using T = std::underlying_type<E>::type;\
         return E(T(lhs) | T(rhs));\
     }\
     inline E operator & (E lhs, E rhs){\
-        using T = std::underlying_type<E>::type;\
         return E(T(lhs) & T(rhs));\
     }\
     inline E operator ^ (E lhs, E rhs){\
-        using T = std::underlying_type<E>::type;\
         return E(T(lhs) ^ T(rhs));\
     }\
     inline E operator ~ (E lhs){\
-        using T = std::underlying_type<E>::type;\
         return E(~T(lhs));\
     }\
     inline E operator |= (E &lhs, E rhs){\

src/idl_gen_cpp.cpp

@@ -184,7 +184,7 @@ static void GenEnum(const Parser &parser, EnumDef &enum_def,
   code += GenEnumVal(enum_def, maxv->name, parser.opts) + "\n";
   code += "};\n";
   if (parser.opts.scoped_enums && enum_def.attributes.Lookup("bit_flags"))
-    code += "DEFINE_BITMASK_OPERATORS(" + enum_def.name + ")\n";
+    code += "DEFINE_BITMASK_OPERATORS(" + enum_def.name + ", " +  GenTypeBasic(enum_def.underlying_type, false) + ")\n";
   code += "\n";
 
   // Generate a generate string table for enum values.