implement LWG-4222

README.md

@@ -23,6 +23,7 @@ Implemented LWG Issues:
 - [x] [LWG-3877](https://wg21.link/lwg3877) incorrect constraints on const-qualified monadic overloads for `std::expected`
 - [x] [LWG-3886](https://wg21.link/lwg3886) Monad mo' problems
 - [x] [LWG-4031](https://wg21.link/lwg4031) `bad_expected_access<void>` member functions should be noexcept
+- [x] [LWG-4222](https://wg21.link/lwg4222) `expected` constructor from a single value missing a constraint
 
 Enhancements:
 

include/zeus/expected.hpp

@@ -291,7 +291,8 @@ using enable_forward_t = std::enable_if_t<
     !std::is_same_v<expected<T, E>, expected_detail::remove_cvref_t<U>> &&                   //
     !expected_detail::is_specialization_v<expected_detail::remove_cvref_t<U>, unexpected> && //
     (!std::is_same_v<std::remove_cv_t<T>, bool> ||                                           // LWG-3836
-     !expected_detail::is_specialization_v<expected_detail::remove_cvref_t<U>, expected>)    //
+     !expected_detail::is_specialization_v<expected_detail::remove_cvref_t<U>, expected>) && //
+    !std::is_same_v<expected_detail::remove_cvref_t<U>, unexpect_t>                          // LWG-4222
     >;
 
 template<class T, class E, class U, class G, class UF, class GF>

tests/test_expected/CMakeLists.txt

@@ -7,6 +7,7 @@ set(SOURCES
     equality_tests.cpp
     lwg_3886_tests.cpp
     lwg_4031_tests.cpp
+    lwg_4222_tests.cpp
 )
 
 find_package(Catch2 3 REQUIRED)

tests/test_expected/lwg_4222_tests.cpp

@@ -0,0 +1,30 @@
+#include <catch2/catch_all.hpp>
+
+#include <zeus/expected.hpp>
+
+using namespace zeus;
+
+namespace
+{
+
+struct ConstructibleFromUnexpect
+{
+    ConstructibleFromUnexpect() = default;
+    explicit ConstructibleFromUnexpect([[maybe_unused]] unexpect_t ut) {}
+};
+
+} // namespace
+
+TEST_CASE("expected<T, E>(unexpect) is not ambiguous when T is constructible from unexpect_t", "[LWG-4222]")
+{
+    expected<ConstructibleFromUnexpect, int> e(unexpect);
+    CHECK_FALSE(e.has_value());
+    CHECK(e.error() == int {});
+}
+
+TEST_CASE("expected<T, E>(unexpect, args) still works when T is constructible from unexpect_t", "[LWG-4222]")
+{
+    expected<ConstructibleFromUnexpect, int> e(unexpect, 42);
+    CHECK_FALSE(e.has_value());
+    CHECK(e.error() == 42);
+}