Tested aarch64-linux. Pushed to trunk. -- >8 --
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. --- 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 -- 2.44.0