Clarification of gsl::not_null supported types, and ease-of-use members

[dmitrykobets-msft]

What is the guidelines' current position on adding "useful" methods to not_null to support ease-of-use for more general "nullable" types?

A recent issue in the ms-gsl repo microsoft/GSL#1039 is asking whether gsl::not_null can be made to support operator() for std::function.

Two similar issues existed in the ms-gsl repo in the past, but were closed due to not_null being only for pointers-to-objects and for standard smart pointers.

• Using not_null with function pointers microsoft/GSL#626
• not_null and std::function microsoft/GSL#325

However, it seems that the guidelines now define gsl::not_null more generally as

not_null // T is usually a pointer type (e.g., not_null<int*> and not_null<owner<Foo*>>) that must not be nullptr. T can be any type for which ==nullptr is meaningful.

[hsutter]

Editors call: No. not_null is currently intended for pointers to single objects.