[Bug libstdc++/77727] New: Unwrapping std::optional constructor is not working for non-transferable object

["tomaszkam at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77727

            Bug ID: 77727
           Summary: Unwrapping std::optional constructor is not working
                    for non-transferable object
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tomaszkam at gmail dot com
  Target Milestone: ---

The following code does not compile:
#include <optional>

struct NonTransferable
{
    NonTransferable(int) {}
    NonTransferable(NonTransferable&&) = delete;
    NonTransferable& operator=(NonTransferable&&) = delete;
};

int main()
{
    std::optional<int> oi;
    std::optional<NonTransferable> ot(std::move(oi));

    std::optional<int> oi2(10);
    std::optional<NonTransferable> ot2(oi2);
}

Both the construction of the ot and ot2 shall be well-formed according to
specification from http://cplusplus.github.io/LWG/lwg-active.html#2756 and
call:
optional<T>::optional(optional<U>&&);
optional<T>::optional(optional<U> const&);
constructors respectively as:
is_constructible_v<NonTransferable, int>
is_constructible_v<NonTransferable, int const&>
are true, and the NonTransferable class cannot be constructed/converted from
optional<int>.