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

[Yrong]

Friendly ping~

Yrong <yronglin777@gmail.com>于2025年8月26日 周二21:46写道：

> 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
>> >       >
>> >       >
>> >
>> >
>> >
>
>
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