"reloc-assign" is just assignment by value: operator=(T). See my email just now.
While the copy-and-swap (in this case, reloc-and-swap) idiom works fine for this assignment operator, for gsl::not_null a memberwise implementation would also work, indicating that this assignment operator should be a defaultable special member function:
not_null& operator=(not_null) = default;
In turn, this indicates that the spelling for the relocation operator should simply be T(T) - that is, it should have the syntax of a constructor. Since a by-value constructor is currently invalid, this is free syntax.