std::optional specialization for reference types

Question

Why std::optional (std::experimental::optional in libc++ at the moment) does not have specialization for reference types (compared

Answer 1

If I would hazard a guess, it would be because of this sentence in the specification of std::experimental::optional. (Section 5.2, p1)

A program that necessitates the instantiation of template optional for a reference type, or for possibly cv-qualified types in_place_t or nullopt_t is ill-formed.