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 --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
index 2a741bf700067824ebc631456630443d39444a58..9f3b99231b3c5ed0d3051d1ab0b3ef69bac5239c 100644 (file)
--- 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 4b9002e0917c90c2976e4524b5030144ddee77f2..f79d95db7a832242c03d869c7e1e73b19c135fa3 100644 (file)
--- 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