On Wed, 13 Aug 2025, Patrick Palka wrote:
> Thanks for the patch! Looks good to me for the most part.
>
> On Fri, 8 Aug 2025, Yihan Wang wrote:
>
> > libstdc++-v3/ChangeLog:
> >
> > * include/std/expected:
>
> This ChangeLog entry should be filled in, e.g.
>
> * include/std/expected (expected::expected(_Up&&)): Add
> missing constraint as per LWG 4222.
>
> > * testsuite/20_util/expected/lwg4222.cc: New test.
> >
> > Signed-off-by: Yihan Wang <yronglin...@gmail.com>
> > ---
> > libstdc++-v3/include/std/expected | 1 +
> > .../testsuite/20_util/expected/lwg4222.cc | 15 +++++++++++++++
> > 2 files changed, 16 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..2b200ea0589 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>)
>
> Seems this line is overly indented causing it to not be aligned with the rest.
>
> > && 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..a260cfef3dd
> > --- /dev/null
> > +++ b/libstdc++-v3/testsuite/20_util/expected/lwg4222.cc
> > @@ -0,0 +1,15 @@
> > +// { dg-do compile { target c++23 } }
> > +
> > +// LWG 4222. 'expected' constructor from a single value missing a
> > constraint
> > +
> > +#include <expected>
> > +#include <type_traits>
> > +
> > +struct T {
> > + explicit T(auto) {}
> > +};
> > +struct E {
> > + E(int) {}
> > +};
> > +
> > +static_assert(!std::is_constructible_v<std::expected<T, E>,
> > std::unexpect_t>);
Maybe we should also test constructing from const unexpect_t and
unexpect_t& and variations thereof.
> > --
> > 2.39.5
> >
> >
>
