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

[Patrick Palka]

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 <yronglin777@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
> > 
> > 
>