test: add tests for LWG-3886

Author: X1aomu

tests/test_expected/lwg_3886_tests.cpp

@@ -0,0 +1,72 @@
+#include <catch2/catch_all.hpp>
+
+#include <zeus/expected.hpp>
+
+using namespace zeus;
+
+namespace
+{
+
+struct ConstructModeDistinction
+{
+    enum ConstructedMode
+    {
+        kDefault,
+        kCopy,
+        kMove,
+        kCopyAssign,
+        kMoveAssign,
+    };
+
+    constexpr ConstructModeDistinction() noexcept
+        : mode(kDefault)
+    {
+    }
+    constexpr ConstructModeDistinction(ConstructModeDistinction const&) noexcept
+        : mode(kCopy)
+    {
+    }
+    constexpr ConstructModeDistinction(ConstructModeDistinction&&) noexcept
+        : mode(kMove)
+    {
+    }
+    constexpr ConstructModeDistinction& operator=(ConstructModeDistinction&&) noexcept
+    {
+        mode = kMoveAssign;
+        return *this;
+    }
+    constexpr ConstructModeDistinction& operator=(ConstructModeDistinction const&) noexcept
+    {
+        mode = kCopyAssign;
+        return *this;
+    }
+
+    ConstructedMode mode;
+};
+
+} // namespace
+
+SCENARIO("Monad mo' problems", "[LWG-3886]")
+{
+    using T = const ConstructModeDistinction;
+    using E = int;
+
+    // FIXME LWG-3891
+    //SECTION("constructor")
+    //{
+    //    expected<T, E> const e({});
+    //    CHECK(e.value().mode == T::kMove);
+    //}
+    //SECTION("assignment")
+    //{
+    //    expected<T, E> e {zeus::unexpect};
+    //    e = {};
+    //    CHECK(e.value().mode == T::kMove);
+    //}
+
+    SECTION("value_or()")
+    {
+        expected<T, E> const e {zeus::unexpect};
+        CHECK(e.value_or({}).mode == T::kMove);
+    }
+}