[libc++] Simplify unwrap_ref_decay a bit (llvm#121623)

Author: philnik777

libcxx/include/__type_traits/unwrap_ref.h

@@ -29,6 +29,9 @@ struct __unwrap_reference<reference_wrapper<_Tp> > {
   using type _LIBCPP_NODEBUG = _Tp&;
 };
 
+template <class _Tp>
+using __unwrap_ref_decay_t = typename __unwrap_reference<__decay_t<_Tp> >::type;
+
 #if _LIBCPP_STD_VER >= 20
 template <class _Tp>
 struct unwrap_reference : __unwrap_reference<_Tp> {};
@@ -40,19 +43,9 @@ template <class _Tp>
 struct unwrap_ref_decay : unwrap_reference<__decay_t<_Tp> > {};
 
 template <class _Tp>
-using unwrap_ref_decay_t = typename unwrap_ref_decay<_Tp>::type;
+using unwrap_ref_decay_t = __unwrap_ref_decay_t<_Tp>;
 #endif // _LIBCPP_STD_VER >= 20
 
-template <class _Tp>
-struct __unwrap_ref_decay
-#if _LIBCPP_STD_VER >= 20
-    : unwrap_ref_decay<_Tp>
-#else
-    : __unwrap_reference<__decay_t<_Tp> >
-#endif
-{
-};
-
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___TYPE_TRAITS_UNWRAP_REF_H

libcxx/include/__utility/pair.h

@@ -532,11 +532,9 @@ swap(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y) noexcept(noexcept(__x
 #endif
 
 template <class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI
-_LIBCPP_CONSTEXPR_SINCE_CXX14 pair<typename __unwrap_ref_decay<_T1>::type, typename __unwrap_ref_decay<_T2>::type>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<__unwrap_ref_decay_t<_T1>, __unwrap_ref_decay_t<_T2> >
 make_pair(_T1&& __t1, _T2&& __t2) {
-  return pair<typename __unwrap_ref_decay<_T1>::type, typename __unwrap_ref_decay<_T2>::type>(
-      std::forward<_T1>(__t1), std::forward<_T2>(__t2));
+  return pair<__unwrap_ref_decay_t<_T1>, __unwrap_ref_decay_t<_T2> >(std::forward<_T1>(__t1), std::forward<_T2>(__t2));
 }
 
 template <class _T1, class _T2>

libcxx/include/tuple

@@ -1125,9 +1125,9 @@ inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 tuple<_Tp&...> tie(_T
 }
 
 template <class... _Tp>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 tuple<typename __unwrap_ref_decay<_Tp>::type...>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 tuple<__unwrap_ref_decay_t<_Tp>...>
 make_tuple(_Tp&&... __t) {
-  return tuple<typename __unwrap_ref_decay<_Tp>::type...>(std::forward<_Tp>(__t)...);
+  return tuple<__unwrap_ref_decay_t<_Tp>...>(std::forward<_Tp>(__t)...);
 }
 
 template <class... _Tp>