Missing `swap` for `gsl::not_null` for move-only types about gsl

[dmitrykobets-msft]

Hi @christianbrugger, thanks for the question; I'll bring it up in the next maintainers' sync.

[hsutter]

Maintainers call: Yes, there should be a homogeneous swap. We should add a member not_null<T>::swap(not_null<T>&) and specialize template<typename T> std::swap(not_null<T>&, not_null<T>&).

[mbeutel]

and specialize template<typename T> std::swap(not_null<T>&, not_null<T>&).

Wait, is specializing function templates from namespace std something we do? I thought that was outlawed as a consequence of P0551. The current draft only mentions that class templates from std may be specialized.

Edit: the cppreference.com documentation on std::swap discusses this:

Specializations

std::swap may be specialized in namespace std for program- defined types, but such specializations are not found by ADL (the namespace std is not the associated namespace for the program-defined type).
(until C++20)

The expected way to make a program-defined type swappable is to provide a non-member function swap in the same namespace as the type: see Swappable for details.

[hsutter]

Thanks! I think you're correct, we should overload (not specialize) in gsl:: (not std::).