test: more tests

Author: X1aomu

tests/test_expected/monadic_tests.cpp

@@ -1,4 +1,5 @@
 #include <string>
+#include <type_traits>
 
 #include <catch2/catch_all.hpp>
 
@@ -30,3 +31,59 @@ TEST_CASE("and_then()", "[monadic]")
         static_assert(std::is_same_v<std::remove_cv_t<decltype(newVal)>, Expected2>);
     }
 }
+
+TEST_CASE("void-T and_then()", "[monadic, void-T]")
+{
+    using T        = std::string;
+    using Expected = expected<void, T>;
+
+    { // non-void
+        using Expected2 = expected<double, T>;
+
+        Expected e {};
+
+        auto const newVal = e.and_then([]() { return Expected2 {}; });
+
+        static_assert(std::is_same_v<std::remove_cv_t<decltype(newVal)>, Expected2>);
+    }
+    { // void
+        using Expected2 = expected<void, T>;
+
+        Expected e {};
+
+        auto const newVal = e.and_then([]() -> Expected2 { return Expected2 {}; });
+
+        static_assert(std::is_same_v<std::remove_cv_t<decltype(newVal)>, Expected2>);
+    }
+}
+
+TEST_CASE("or_else()", "[monadic]")
+{
+    using T        = std::string;
+    using Expected = expected<T, int>;
+
+    { // non-void
+        using Expected2 = expected<T, double>;
+
+        Expected e {};
+
+        auto const newVal = e.or_else([](auto&&) { return Expected2 {}; });
+
+        static_assert(std::is_same_v<std::remove_cv_t<decltype(newVal)>, Expected2>);
+    }
+}
+
+TEST_CASE("void-T or_else()", "[monadic, void-T]")
+{
+    using Expected = expected<void, int>;
+
+    { // non-void
+        using Expected2 = expected<void, double>;
+
+        Expected e {};
+
+        auto const newVal = e.or_else([](auto&&) { return Expected2 {}; });
+
+        static_assert(std::is_same_v<std::remove_cv_t<decltype(newVal)>, Expected2>);
+    }
+}