Dec: unwrap member pointer in detectFamily

It is required to correctly build JumpTable for classes with custom
decoding rule - we need to get the underlying type to understand what
types are acceptable to decode.

Author: drewdzzz

src/mpp/Dec.hpp

@@ -102,9 +102,12 @@ template <class BUF, class T>
 constexpr auto detectFamily()
 {
 	using U = unwrap_t<T>;
-	static_assert(!std::is_const_v<U> || tnt::is_tuplish_v<U>,
+	static_assert(!std::is_const_v<U> || tnt::is_tuplish_v<U> ||
+		      std::is_member_pointer_v<U>,
 		      "Can't decode to constant type");
-	if constexpr (is_raw_decoded_v<T, BUF>) {
+	if constexpr (std::is_member_pointer_v<U>) {
+		return detectFamily<BUF, tnt::demember_t<U>>();
+	} else if constexpr (is_raw_decoded_v<T, BUF>) {
 		static_assert(!is_wrapped_family_v<T>);
 		return getFamiliesByRules<all_rules_t>();
 	} else if constexpr (is_wrapped_family_v<T>) {

test/EncDecTest.cpp

@@ -1350,6 +1350,18 @@ test_optional()
 
 	buf.flush();
 
+	TEST_CASE("trivial object");
+	mpp::encode(buf, std::optional<IntegerWrapper>({64}));
+
+	run = buf.begin<true>();
+	std::optional<IntegerWrapper> opt_int_wrapper;
+	ok = mpp::decode(run, opt_int_wrapper);
+	fail_unless(ok);
+	fail_unless(opt_int_wrapper.has_value());
+	fail_unless(opt_int_wrapper.value().i == 64);
+
+	buf.flush();
+
 	TEST_CASE("objects");
 	Body wr;
 	wr.gen();