[gcc r13-2351] libstdc++: Add specializations for some variable templates

[Jonathan Wakely]

https://gcc.gnu.org/g:33005a4be49466880fe3f5a9317bccc8c1ed423b

commit r13-2351-g33005a4be49466880fe3f5a9317bccc8c1ed423b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Sep 1 12:32:09 2022 +0100

    libstdc++: Add specializations for some variable templates
    
    This avoids having to instantiate a class template when we can detect
    the true cases easily with a partial specialization.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/type_traits (is_lvalue_reference_v)
            (is_rvalue_reference_v, is_reference_v, is_const_v)
            (is_volatile_v): Define using partial specializations instead
            of instantiating class templates.

Diff:
---
 libstdc++-v3/include/std/type_traits | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 52cca8bf3af..e4b9b59ce08 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -3153,11 +3153,13 @@ template <typename _Tp, size_t _Num>
 template <typename _Tp>
   inline constexpr bool is_pointer_v = is_pointer<_Tp>::value;
 template <typename _Tp>
-  inline constexpr bool is_lvalue_reference_v =
-    is_lvalue_reference<_Tp>::value;
+  inline constexpr bool is_lvalue_reference_v = false;
 template <typename _Tp>
-  inline constexpr bool is_rvalue_reference_v =
-    is_rvalue_reference<_Tp>::value;
+  inline constexpr bool is_lvalue_reference_v<_Tp&> = true;
+template <typename _Tp>
+  inline constexpr bool is_rvalue_reference_v = false;
+template <typename _Tp>
+  inline constexpr bool is_rvalue_reference_v<_Tp&&> = true;
 template <typename _Tp>
   inline constexpr bool is_member_object_pointer_v =
     is_member_object_pointer<_Tp>::value;
@@ -3173,7 +3175,11 @@ template <typename _Tp>
 template <typename _Tp>
   inline constexpr bool is_function_v = is_function<_Tp>::value;
 template <typename _Tp>
-  inline constexpr bool is_reference_v = is_reference<_Tp>::value;
+  inline constexpr bool is_reference_v = false;
+template <typename _Tp>
+  inline constexpr bool is_reference_v<_Tp&> = true;
+template <typename _Tp>
+  inline constexpr bool is_reference_v<_Tp&&> = true;
 template <typename _Tp>
   inline constexpr bool is_arithmetic_v = is_arithmetic<_Tp>::value;
 template <typename _Tp>
@@ -3187,9 +3193,13 @@ template <typename _Tp>
 template <typename _Tp>
   inline constexpr bool is_member_pointer_v = is_member_pointer<_Tp>::value;
 template <typename _Tp>
-  inline constexpr bool is_const_v = is_const<_Tp>::value;
+  inline constexpr bool is_const_v = false;
+template <typename _Tp>
+  inline constexpr bool is_const_v<const _Tp> = true;
+template <typename _Tp>
+  inline constexpr bool is_volatile_v = false;
 template <typename _Tp>
-  inline constexpr bool is_volatile_v = is_volatile<_Tp>::value;
+  inline constexpr bool is_volatile_v<volatile _Tp> = true;
 template <typename _Tp>
   inline constexpr bool is_trivial_v = is_trivial<_Tp>::value;
 template <typename _Tp>