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

[Jonathan Wakely]

On Tue, 3 Jun 2025 at 10:54, Tomasz Kaminski wrote:
>
>
>
> 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.

Implementing the variable templates directly in terms of the built-in
means not instantiating any class template for those variables, which
is a small but significant speedup.


>>
>>
>>  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
>>