Improve bad access error for std::expected<T, rfl::Error> (#558)

Author: hos-b

include/rfl/Result.hpp

@@ -445,4 +445,22 @@ inline Unexpected<Error> error(const Error& _err) {
 
 }  // namespace rfl
 
+#ifdef REFLECTCPP_USE_STD_EXPECTED
+template <>
+class std::bad_expected_access<rfl::Error> : public bad_expected_access<void> {
+ public:
+  explicit constexpr bad_expected_access(rfl::Error er) : err_(std::move(er)) {}
+  const char* what() const noexcept override { return err_.what().c_str(); }
+
+  template <typename Self>
+  [[nodiscard]]
+  auto error(this Self&& self) noexcept {
+    return std::forward<Self>(self).err_;
+  }
+
+ private:
+  rfl::Error err_;
+};
 #endif
+
+#endif

tests/generic/test_bad_access_error.cpp

@@ -0,0 +1,40 @@
+#ifdef REFLECTCPP_USE_STD_EXPECTED
+#include <expected>
+#endif
+
+#include <rfl.hpp>
+#include <gtest/gtest.h>
+
+namespace test_exceptions {
+
+struct Person {
+  rfl::Rename<"firstName", std::string> first_name;
+  rfl::Rename<"lastName", std::string> last_name = "Flanders";
+};
+
+TEST(generic, test_exceptions) {
+  const std::string error_message = "unified bad access message!";
+  #ifdef REFLECTCPP_USE_STD_EXPECTED
+  rfl::Result<Person> person = std::unexpected(rfl::Error(error_message));
+#else
+  rfl::Result<Person> person(rfl::error(error_message));
+#endif
+
+  try {
+    person.value();
+    FAIL() << "Expected an exception!";
+  }
+#ifdef REFLECTCPP_USE_STD_EXPECTED
+  catch (std::bad_expected_access<rfl::Error> const& err) {
+    EXPECT_EQ(err.what(), error_message);
+  }
+#else
+  catch (std::runtime_error const& err) {
+    EXPECT_EQ(err.what(), error_message);
+  }
+#endif
+  catch (...) {
+    FAIL() << "Invalid exception type!";
+  }
+}
+}  // namespace test_exceptions