On Wed, 3 Sept 2025 at 19:09, Jonathan Wakely wrote: > > On Tue, 19 Aug 2025 at 16:17, Patrick Palka wrote: > > > > LGTM! Perhaps we want to backport this, not sure how far back > > (std::expected was implemented in GCC 12). > > OK for trunk and 13/14/15 - thanks for the contribution
It doesn't apply cleanly to gcc-14 unless I backport r15-2307-gdca6a9a940e46d for https://cplusplus.github.io/LWG/issue3836 first. I think that's probably OK to backport for std::expected, but what do we think about backporting the std::optional part of that too? > > > > > 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 <yronglin...@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 > > > > > > > >
