[PATCH] libstdc++: Use new __is_destructible built-in in <type_traits>

[Tomasz Kaminski]

On Mon, Jun 2, 2025 at 10:56 PM Jonathan Wakely <jwakely@redhat.com> wrote:

> libstdc++-v3/ChangeLog:
>
>         * include/std/type_traits (is_destructible, is_destructible_v):
>         Define using new built-in.
>         (is_nothrow_destructible, is_nothrow_destructible_v): Likewise.
>         (is_trivially_destructible, is_trivially_destructible_v):
>         Likewise.
> ---
>
> Thanks to Jason's latest fixes, the new built-in now allows us to
> replace the complicated class templates that implement these traits.
>
> Tested x86_64-linux.
>
LGTM. I wouldn't think about updating variable traits.

>
>  libstdc++-v3/include/std/type_traits | 42 ++++++++++++++++++++++++++--
>  1 file changed, 40 insertions(+), 2 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/type_traits
> b/libstdc++-v3/include/std/type_traits
> index 6bf355d97cc9..c8907fe4d382 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -1039,6 +1039,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>    // Destructible and constructible type properties.
>
> +#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_destructible)
> +  /// is_destructible
> +  template<typename _Tp>
> +    struct is_destructible
> +    : public __bool_constant<__is_destructible(_Tp)>
> +    { };
> +#else
>    // In N3290 is_destructible does not say anything about function
>    // types and abstract types, see LWG 2049. This implementation
>    // describes function types as non-destructible and all complete
> @@ -1090,7 +1097,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>  static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
>         "template argument must be a complete class or an unbounded
> array");
>      };
> +#endif
>
> +#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_nothrow_destructible)
> +  /// is_nothrow_destructible
> +  template<typename _Tp>
> +    struct is_nothrow_destructible
> +    : public __bool_constant<__is_nothrow_destructible(_Tp)>
> +    { };
> +#else
>    /// @cond undocumented
>
>    // is_nothrow_destructible requires that is_destructible is
> @@ -1144,6 +1159,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>  static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
>         "template argument must be a complete class or an unbounded
> array");
>      };
> +#endif
>
>    /// @cond undocumented
>    template<typename _Tp, typename... _Args>
> @@ -1451,6 +1467,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         "template argument must be a complete class or an unbounded
> array");
>      };
>
> +#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_trivially_destructible)
> +  /// is_trivially_destructible
> +  template<typename _Tp>
> +    struct is_trivially_destructible
> +    : public __bool_constant<__is_trivially_destructible(_Tp)>
> +    { };
> +#else
>    /// is_trivially_destructible
>    template<typename _Tp>
>      struct is_trivially_destructible
> @@ -1460,7 +1483,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>  static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
>         "template argument must be a complete class or an unbounded
> array");
>      };
> -
> +#endif
>
>    /// has_virtual_destructor
>    template<typename _Tp>
> @@ -3581,8 +3604,13 @@ template <typename _Tp>
>    inline constexpr bool is_move_assignable_v
>      = __is_assignable(__add_lval_ref_t<_Tp>, __add_rval_ref_t<_Tp>);
>
> +#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_destructible)
> +template <typename _Tp>
> +  inline constexpr bool is_destructible_v = __is_destructible(_Tp);
> +#else
>  template <typename _Tp>
>    inline constexpr bool is_destructible_v = is_destructible<_Tp>::value;
> +#endif
>
>  template <typename _Tp, typename... _Args>
>    inline constexpr bool is_trivially_constructible_v
> @@ -3609,7 +3637,11 @@ template <typename _Tp>
>      = __is_trivially_assignable(__add_lval_ref_t<_Tp>,
>                                 __add_rval_ref_t<_Tp>);
>
> -#if __cpp_concepts
> +#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_trivially_destructible)
> +template <typename _Tp>
> +  inline constexpr bool is_trivially_destructible_v
> +    = __is_trivially_destructible(_Tp);
> +#elif __cpp_concepts
>  template <typename _Tp>
>    inline constexpr bool is_trivially_destructible_v = false;
>
> @@ -3654,9 +3686,15 @@ template <typename _Tp>
>    inline constexpr bool is_nothrow_move_assignable_v
>      = __is_nothrow_assignable(__add_lval_ref_t<_Tp>,
> __add_rval_ref_t<_Tp>);
>
> +#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_nothrow_destructible)
> +template <typename _Tp>
> +  inline constexpr bool is_nothrow_destructible_v
> +    = __is_nothrow_destructible(_Tp);
> +#else
>  template <typename _Tp>
>    inline constexpr bool is_nothrow_destructible_v =
>      is_nothrow_destructible<_Tp>::value;
> +#endif
>
>  template <typename _Tp>
>    inline constexpr bool has_virtual_destructor_v
> --
> 2.49.0
>
>
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