[PATCH v3] libstdc++: Implement LWG4222 'expected' constructor from a single value missing a constraint

[Yrong]

Got it, thanks!

Patrick Palka <ppalka@redhat.com>于2025年8月23日 周六02:59写道：

> On Fri, 22 Aug 2025, Yrong wrote:
>
> > friendly ping~
> >
> > Yrong <yronglin777@gmail.com> 于2025年8月19日周二 23:58写道：
> >       Thanks for the review! But I don't have a commit access, could you
> help me to commit this patch? Many thanks!
>
> The patch will be pushed by one of us one it's approved by Jonathan
> or another maintainer.
>
> >
> >       >   Perhaps we want to backport this, not sure how far back
> >
> >       Is there anything I need to do about this?
>
> Nope, you're all set, we'll decide that at some point based on
> the risk/benefit of the patch, and let you know.  Thanks again for the
> patch!
>
> >
> >       Best regards,Yihan
> >
> > Patrick Palka <ppalka@redhat.com> 于2025年8月19日周二 23:16写道：
> >       LGTM!  Perhaps we want to backport this, not sure how far back
> >       (std::expected was implemented in GCC 12).
> >
> >       On Sat, 16 Aug 2025, Yihan Wang wrote:
> >
> >       > libstdc++-v3/ChangeLog:
> >       >
> >       >       * include/std/expected: Add missing constraint as per LWG
> 4222.
> >       >       * testsuite/20_util/expected/lwg4222.cc: New test.
> >       >
> >       > Signed-off-by: Yihan Wang <yronglin777@gmail.com>
> >       > ---
> >       >  libstdc++-v3/include/std/expected             |  1 +
> >       >  .../testsuite/20_util/expected/lwg4222.cc     | 39
> +++++++++++++++++++
> >       >  2 files changed, 40 insertions(+)
> >       >  create mode 100644
> libstdc++-v3/testsuite/20_util/expected/lwg4222.cc
> >       >
> >       > diff --git a/libstdc++-v3/include/std/expected
> b/libstdc++-v3/include/std/expected
> >       > index 60f1565f15b..4eaaab693e1 100644
> >       > --- a/libstdc++-v3/include/std/expected
> >       > +++ b/libstdc++-v3/include/std/expected
> >       > @@ -474,6 +474,7 @@ namespace __expected
> >       >        template<typename _Up = remove_cv_t<_Tp>>
> >       >       requires (!is_same_v<remove_cvref_t<_Up>, expected>)
> >       >         && (!is_same_v<remove_cvref_t<_Up>, in_place_t>)
> >       > +       && (!is_same_v<remove_cvref_t<_Up>, unexpect_t>)
> >       >         && is_constructible_v<_Tp, _Up>
> >       >         && (!__expected::__is_unexpected<remove_cvref_t<_Up>>)
> >       >         &&
> __expected::__not_constructing_bool_from_expected<_Tp, _Up>
> >       > diff --git a/libstdc++-v3/testsuite/20_util/expected/lwg4222.cc
> b/libstdc++-v3/testsuite/20_util/expected/lwg4222.cc
> >       > new file mode 100644
> >       > index 00000000000..5c107792456
> >       > --- /dev/null
> >       > +++ b/libstdc++-v3/testsuite/20_util/expected/lwg4222.cc
> >       > @@ -0,0 +1,39 @@
> >       > +// { dg-do run { target c++23 } }
> >       > +
> >       > +// LWG 4222. 'expected' constructor from a single value missing
> a constraint
> >       > +
> >       > +#include <expected>
> >       > +#include <type_traits>
> >       > +#include <testsuite_hooks.h>
> >       > +
> >       > +struct T {
> >       > +  explicit T(auto) {}
> >       > +};
> >       > +struct E {
> >       > +  E(int) {}
> >       > +};
> >       > +
> >       > +struct V {
> >       > + explicit constexpr V(std::unexpect_t) {}
> >       > +};
> >       > +
> >       > +static_assert(!std::is_constructible_v<std::expected<T, E>,
> std::unexpect_t>);
> >       > +static_assert(!std::is_constructible_v<std::expected<T, E>,
> std::unexpect_t &>);
> >       > +static_assert(!std::is_constructible_v<std::expected<T, E>,
> std::unexpect_t &&>);
> >       > +static_assert(!std::is_constructible_v<std::expected<T, E>,
> const std::unexpect_t>);
> >       > +static_assert(!std::is_constructible_v<std::expected<T, E>,
> const std::unexpect_t &>);
> >       > +static_assert(!std::is_constructible_v<std::expected<T, E>,
> const std::unexpect_t &&>);
> >       > +
> >       > +constexpr bool test() {
> >       > +  std::expected<V, int> e1(std::in_place, std::unexpect);
> >       > +  VERIFY( e1.has_value() );
> >       > +  std::expected<int, V> e2(std::unexpect, std::unexpect);
> >       > +  VERIFY( !e2.has_value() );
> >       > +  return true;
> >       > +}
> >       > +
> >       > +int main() {
> >       > +  test();
> >       > +  static_assert(test());
> >       > +  return 0;
> >       > +}
> >       > --
> >       > 2.39.5
> >       >
> >       >
> >
> >
> >
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://gcc.gnu.org/pipermail/libstdc++/attachments/20250826/3f0231a6/attachment-0001.htm>