P0340R3 SFINAE-Friendly underlying_type (#284)

Resolves #11. Implemented unconditionally.

stl/inc/type_traits

@@ -1151,13 +1151,19 @@ template <size_t _Len, class... _Types>
 using aligned_union_t = typename aligned_union<_Len, _Types...>::type;
 
 // STRUCT TEMPLATE underlying_type
-template <class _Ty>
-struct underlying_type { // determine underlying type for enum
+template <class _Ty, bool = is_enum_v<_Ty>>
+struct _Underlying_type {
     using type = __underlying_type(_Ty);
 };
 
 template <class _Ty>
-using underlying_type_t = typename underlying_type<_Ty>::type;
+struct _Underlying_type<_Ty, false> {};
+
+template <class _Ty>
+struct underlying_type : _Underlying_type<_Ty> {}; // determine underlying type for enum
+
+template <class _Ty>
+using underlying_type_t = typename _Underlying_type<_Ty>::type;
 
 // STRUCT TEMPLATE rank
 template <class _Ty>

stl/inc/yvals_core.h

@@ -150,6 +150,7 @@
 // P0063R3 C11 Standard Library
 // P0074R0 owner_less<>
 // P0092R1 <chrono> floor(), ceil(), round(), abs()
+// P0340R3 SFINAE-Friendly underlying_type
 // P0414R2 shared_ptr<T[]>, shared_ptr<T[N]>
 // P0418R2 atomic compare_exchange memory_order Requirements
 // P0435R1 Overhauling common_type