refactor: better constraint for out<T>

include/jvalidate/detail/out.h

@@ -5,12 +5,11 @@
 #include <variant>
 
 namespace jvalidate::detail {
-template <typename T> class out;
-
 constexpr struct discard_out_t {
 } discard_out;
 
-template <typename T> class out {
+template <typename T>
+requires(std::is_same_v<T, std::decay_t<T>>) class out {
 private:
   T * ref_ = nullptr;