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

[Yihan Wang]

libstdc++-v3/ChangeLog:

        * include/std/expected:
        * 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         | 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>)
          && 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>);
-- 
2.39.5