[gcc r14-9643] libstdc++: Add __is_in_place_index_v helper and use it in <variant>

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:3763fb8970d7515a4a3be2152604140965303031

commit r14-9643-g3763fb8970d7515a4a3be2152604140965303031
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Mar 22 11:47:44 2024 +0000

    libstdc++: Add __is_in_place_index_v helper and use it in <variant>
    
    We already have __is_in_place_type_v for in_place_type_t so adding an
    equivalent for in_place_index_t allows us avoid a class template
    instantiation for the __not_in_place_tag constraint on the most
    commonly-used std::variant::variant(T&&) constructor.
    
    For in_place_type_t we also have a __is_in_place_type class template
    defined in terms of the variable template, but that isn't actually used
    anywhere. I'm not adding an equivalent for the new variable template,
    because that wouldn't be used either.
    
    For GCC 15 we should remove the unused __is_in_place_tag and
    __is_in_place_type class templates.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/utility.h (__is_in_place_index_v): New variable
            template.
            * include/std/variant (__not_in_place_tag): Define in terms of
            variable templates not a class template.

Diff:
---
 libstdc++-v3/include/bits/utility.h | 6 ++++++
 libstdc++-v3/include/std/variant    | 3 ++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
index 2a741bf7000..9f3b99231b3 100644
--- a/libstdc++-v3/include/bits/utility.h
+++ b/libstdc++-v3/include/bits/utility.h
@@ -223,6 +223,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Tp>
     using __is_in_place_type = bool_constant<__is_in_place_type_v<_Tp>>;
 
+  template<typename>
+    inline constexpr bool __is_in_place_index_v = false;
+
+  template<size_t _Nm>
+    inline constexpr bool __is_in_place_index_v<in_place_index_t<_Nm>> = true;
+
 #endif // C++17
 
 #if _GLIBCXX_USE_BUILTIN_TRAIT(__type_pack_element)
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 4b9002e0917..f79d95db7a8 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1414,7 +1414,8 @@ namespace __variant
 
       template<typename _Tp>
 	static constexpr bool __not_in_place_tag
-	  = !__is_in_place_tag<__remove_cvref_t<_Tp>>::value;
+	  = !__is_in_place_type_v<__remove_cvref_t<_Tp>>
+	      && !__is_in_place_index_v<__remove_cvref_t<_Tp>>;
 
     public:
 #if __cpp_concepts