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 <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
>
>
