implement LWG-4031

README.md

@@ -22,6 +22,7 @@ Implemented LWG Issues:
 - [x] [LWG-4026](https://wg21.link/lwg4026) Assignment operators of `std::expected` should propagate triviality
 - [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
 
 Enhancements:
 

include/zeus/expected.hpp

@@ -235,14 +235,16 @@ template<>
 class bad_expected_access<void> : public std::exception
 {
 public:
-    virtual const char *what() const noexcept override { return "Bad expected access"; }
+    [[nodiscard]] const char *what() const noexcept override { return "Bad expected access"; }
 
 protected:
-    bad_expected_access()                                       = default;
-    bad_expected_access(const bad_expected_access &)            = default;
-    bad_expected_access(bad_expected_access &&)                 = default;
-    bad_expected_access &operator=(const bad_expected_access &) = default;
-    bad_expected_access &operator=(bad_expected_access &&)      = default;
+    // LWG-4031
+    bad_expected_access() noexcept                                       = default;
+    bad_expected_access(const bad_expected_access &) noexcept            = default;
+    bad_expected_access(bad_expected_access &&) noexcept                 = default;
+    bad_expected_access &operator=(const bad_expected_access &) noexcept = default;
+    bad_expected_access &operator=(bad_expected_access &&) noexcept      = default;
+    ~bad_expected_access() override                                      = default;
 };
 
 template<class E>

tests/test_expected/CMakeLists.txt

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

tests/test_expected/lwg_4031_tests.cpp

@@ -0,0 +1,24 @@
+#include <catch2/catch_all.hpp>
+
+#include <zeus/expected.hpp>
+
+using namespace zeus;
+
+namespace
+{
+// Exposing all members for testing
+struct test_bad_expected_access : public bad_expected_access<void>
+{
+    using bad_expected_access<void>::bad_expected_access;
+};
+}
+
+TEST_CASE("bad_expected_access<void> special members are noexcept", "[LWG-4031]")
+{
+    STATIC_REQUIRE(std::is_nothrow_default_constructible_v<test_bad_expected_access>);
+    STATIC_REQUIRE(std::is_nothrow_copy_constructible_v<test_bad_expected_access>);
+    STATIC_REQUIRE(std::is_nothrow_move_constructible_v<test_bad_expected_access>);
+    STATIC_REQUIRE(std::is_nothrow_copy_assignable_v<test_bad_expected_access>);
+    STATIC_REQUIRE(std::is_nothrow_move_assignable_v<test_bad_expected_access>);
+    STATIC_REQUIRE(std::is_nothrow_destructible_v<test_bad_expected_access>);
+}