Re: [PATCH RFC] c++: mangle function template constraints

[Jonathan Wakely <jwakely@redhat.com>]

On Wed, 22 Nov 2023 at 14:50, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On Mon, 20 Nov 2023 at 02:56, Jason Merrill wrote:
> >
> > Tested x86_64-pc-linux-gnu.  Are the library bits OK?  Any comments before I
> > push this?
>
> The library parts are OK.
>
> The variable template is_trivially_copyable_v just uses
> __is_trivially_copyable so should be just as efficient, and the change
> to <bit> is fine.
>
> The variable template is_trivially_destructible_v instantiates the
> is_trivially_destructible type trait, which instantiates
> __is_destructible_safe and __is_destructible_impl, which is probably
> why we used the built-in directly in <variant>. But that's an
> acceptable overhead to avoid using the built-in in a mangled context,
> and it would be good to optimize the variable template anyway, as a
> separate change.

For C++20 we could do:

#if __cpp_concepts
template <typename _Tp>
  inline constexpr bool is_trivially_destructible_v = false;
template <typename _Tp> requires (_Tp& __t) { __t.~_Tp(); }
  inline constexpr bool is_trivially_destructible_v<_Tp>
    = __has_trivial_destructor(_Tp);
#else
template <typename _Tp>
  inline constexpr bool is_trivially_destructible_v =
    is_trivially_destructible<_Tp>::value;
#endif

But that won't help C++17.