Re: [committed] libstdc++: Simplify constraints for std::any construction [PR104242]

[Jonathan Wakely <jwakely@redhat.com>]

Oops, I meant to change the commit summary line to Un-simplify, since
it was reverting the "Simplify constaints ..." commit.


On Fri, 18 Mar 2022 at 15:43, Jonathan Wakely via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Tested powerpc64le-linux, pushed to trunk,
>
> -- >8 --
>
> Partially revert r12-4190-g6da36b7d0e43b6f9281c65c19a025d4888a25b2d
> because using __and_<..., is_copy_constructible<T>> when T is incomplete
> results in an error about deriving from is_copy_constructible<T> when
> that is incomplete. I don't know how to fix that, so this simply
> restores the previous constraint which worked in this case (even though
> I think it's technically undefined to use is_copy_constructible<T> with
> incomplete T). This doesn't restore exactly what we had before, but uses
> the is_copy_constructible_v and __is_in_place_type_v variable templates
> instead of the ::value member.
>
> libstdc++-v3/ChangeLog:
>
>         PR libstdc++/104242
>         * include/std/any (any(T&&)): Revert change to constraints.
>         * testsuite/20_util/any/cons/104242.cc: New test.
> ---
>  libstdc++-v3/include/std/any                      |  4 ++--
>  libstdc++-v3/testsuite/20_util/any/cons/104242.cc | 12 ++++++++++++
>  2 files changed, 14 insertions(+), 2 deletions(-)
>  create mode 100644 libstdc++-v3/testsuite/20_util/any/cons/104242.cc
>
> diff --git a/libstdc++-v3/include/std/any b/libstdc++-v3/include/std/any
> index bdad76239b4..a6770e8f235 100644
> --- a/libstdc++-v3/include/std/any
> +++ b/libstdc++-v3/include/std/any
> @@ -185,8 +185,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      /// Construct with a copy of @p __value as the contained object.
>      template <typename _Tp, typename _VTp = _Decay_if_not_any<_Tp>,
>               typename _Mgr = _Manager<_VTp>,
> -             typename = _Require<__not_<__is_in_place_type<_VTp>>,
> -                                 is_copy_constructible<_VTp>>>
> +             enable_if_t<is_copy_constructible_v<_VTp>
> +                         && !__is_in_place_type_v<_VTp>, bool> = true>
>        any(_Tp&& __value)
>        : _M_manager(&_Mgr::_S_manage)
>        {
> diff --git a/libstdc++-v3/testsuite/20_util/any/cons/104242.cc b/libstdc++-v3/testsuite/20_util/any/cons/104242.cc
> new file mode 100644
> index 00000000000..8d5868b7ff9
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/20_util/any/cons/104242.cc
> @@ -0,0 +1,12 @@
> +// { dg-do compile { target c++17 } }
> +
> +// PR libstdc++/104242 - Class with constructor from std::any is not copyable
> +
> +#include <any>
> +#include <type_traits>
> +
> +struct A {
> +    A(const A&) = default;
> +    explicit A(std::any value);
> +};
> +static_assert(std::is_copy_constructible_v<A>);
> --
> 2.34.1
>