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

[Patrick Palka]

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
>       >
>       >
> 
> 
>