On Thu, 14 May 2026 at 12:09, Tomasz Kamiński <[email protected]> wrote:
>
> The components that used __type_identity in C++20 mode (due source
> compatibility with other standard) lead to instantiation of separate
> class template from std::type_identity for each used type. This
> patch makes __type_identity an alias to type_identity if later is
"the latter" instead of "later"
> available.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/type_traits (__type_identity, __type_identity_t)
> [__cpp_lib_type_identity]: Define as alias to type_identity
> and it's nested type respectively.
"its" without the apostrophe
> ---
> Testing on x86_64-linux. Waiting for full test to finish.
> OK for trunk when all tests passes.
>
> libstdc++-v3/include/std/type_traits | 36 ++++++++++++++++++----------
> 1 file changed, 23 insertions(+), 13 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/type_traits
> b/libstdc++-v3/include/std/type_traits
> index 1c03c22e8ee..667725b1b01 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -164,6 +164,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> using __conditional_t
> = typename __conditional<_Cond>::template type<_If, _Else>;
>
> +#ifdef __cpp_lib_type_identity // C++ >= 20
> + /** * Identity metafunction.
I'm not sure why there's an extra '*' here (it was in the original
comment below) but it can be removed.
OK with that change, thanks.
> + * @since C++20
> + * @{
> + */
> + template<typename _Tp>
> + struct type_identity { using type = _Tp; };
> +
> + template<typename _Tp>
> + using type_identity_t = typename type_identity<_Tp>::type;
> + /// @}
> +
> + /// @cond undocumented
> + template <typename _Tp>
> + using __type_identity = type_identity<_Tp>;
> +
> + template<typename _Tp>
> + using __type_identity_t = typename type_identity<_Tp>::type;
> + /// @endcond
> +#else
> /// @cond undocumented
> template <typename _Type>
> struct __type_identity
> @@ -171,7 +191,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> template<typename _Tp>
> using __type_identity_t = typename __type_identity<_Tp>::type;
> + /// @endcond
> +#endif
>
> + /// @cond undocumented
> namespace __detail
> {
> // A variadic alias template that resolves to its first argument.
> @@ -3955,19 +3978,6 @@ template<typename _Ret, typename _Fn, typename...
> _Args>
> /// @}
> #endif // __cpp_lib_remove_cvref
>
> -#ifdef __cpp_lib_type_identity // C++ >= 20
> - /** * Identity metafunction.
> - * @since C++20
> - * @{
> - */
> - template<typename _Tp>
> - struct type_identity { using type = _Tp; };
> -
> - template<typename _Tp>
> - using type_identity_t = typename type_identity<_Tp>::type;
> - /// @}
> -#endif
> -
> #ifdef __cpp_lib_unwrap_ref // C++ >= 20
> /** Unwrap a reference_wrapper
> * @since C++20
> --
> 2.54.0
>
