libstdc++: Remove __is_referenceable helper

We only use the __is_referenceable helper in three places now:
add_pointer, add_lvalue_reference, and add_rvalue_reference. But lots of
other traits depend on add_[lr]value_reference, and decay depends on
add_pointer, so removing the instantiation of __is_referenceable helps
compile all those other traits slightly faster.

We can just use void_t<T&> to check for a referenceable type in the
add_[lr]value_reference traits.

Then we can specialize add_pointer for reference types, so that we don't
need to use remove_reference, and then use void_t<T*> for all
non-reference types to detect when we can form a pointer to the type.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (__is_referenceable): Remove.
	(__add_lvalue_reference_helper, __add_rvalue_reference_helper):
	Use __void_t instead of __is_referenceable.
	(__add_pointer_helper): Likewise.
	(add_pointer): Add partial specializations for reference types.

1 files changed, 16 insertions, 21 deletions

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 3041ac3..8b11f31 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -712,18 +712,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   // __void_t (std::void_t for C++11)
   template<typename...> using __void_t = void;
-
-  // Utility to detect referenceable types ([defns.referenceable]).
-
-  template<typename _Tp, typename = void>
-    struct __is_referenceable
-    : public false_type
-    { };
-
-  template<typename _Tp>
-    struct __is_referenceable<_Tp, __void_t<_Tp&>>
-    : public true_type
-    { };
   /// @endcond
 
   // Type properties.
@@ -1024,12 +1012,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     };
 
   /// @cond undocumented
-  template<typename _Tp, bool = __is_referenceable<_Tp>::value>
+  template<typename _Tp, typename = void>
     struct __add_lvalue_reference_helper
     { using type = _Tp; };
 
   template<typename _Tp>
-    struct __add_lvalue_reference_helper<_Tp, true>
+    struct __add_lvalue_reference_helper<_Tp, __void_t<_Tp&>>
     { using type = _Tp&; };
 
   template<typename _Tp>
@@ -1046,12 +1034,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     };
 
   /// @cond undocumented
-  template<typename _Tp, bool = __is_referenceable<_Tp>::value>
+  template<typename _Tp, typename = void>
     struct __add_rvalue_reference_helper
     { using type = _Tp; };
 
   template<typename _Tp>
-    struct __add_rvalue_reference_helper<_Tp, true>
+    struct __add_rvalue_reference_helper<_Tp, __void_t<_Tp&&>>
     { using type = _Tp&&; };
 
   template<typename _Tp>
@@ -1971,14 +1959,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     : public __remove_pointer_helper<_Tp, __remove_cv_t<_Tp>>
     { };
 
-  template<typename _Tp, bool = __or_<__is_referenceable<_Tp>,
-				      is_void<_Tp>>::value>
+  template<typename _Tp, typename = void>
     struct __add_pointer_helper
-    { typedef _Tp     type; };
+    { using type = _Tp; };
 
   template<typename _Tp>
-    struct __add_pointer_helper<_Tp, true>
-    { typedef typename remove_reference<_Tp>::type*     type; };
+    struct __add_pointer_helper<_Tp, __void_t<_Tp*>>
+    { using type = _Tp*; };
 
   /// add_pointer
   template<typename _Tp>
@@ -1986,6 +1973,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     : public __add_pointer_helper<_Tp>
     { };
 
+  template<typename _Tp>
+    struct add_pointer<_Tp&>
+    { using type = _Tp*; };
+
+  template<typename _Tp>
+    struct add_pointer<_Tp&&>
+    { using type = _Tp*; };
+
 #if __cplusplus > 201103L
   /// Alias template for remove_pointer
   template<typename _Tp>