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

[Patrick Palka]

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
> 
>