specs for gen_server module

[mheiber]

Is your feature request related to a problem? Please describe.

gen_server:call/2, call/3, cast, etc. don't have specs, afaict, which makes type-checking these calls more difficult.
There are spec-like things written in the XML used to generate documentation. These could be adapted as a starting point for psecs.

Describe the solution you'd like

specs, which unfortunately would have to have a lot of term()s, because the type language doesn't allow us to express the ways in which the return of (for example)
call(ServerRef, Request) -> Reply is dependent on callback implementations in the process that corresponds to ServerRef.

Describe alternatives you've considered

• no specs–type checkers would have to have their own logic for connecting behavior functions to callbacks
• AND/OR extending the type language to express the relationship between behavior types and callback types. Adding term()y specs now doesn't preclude a more precise solution in future.

Additional context

Happy to make a PR–I have some of these typed up already, based on adapting specs from the xml used to generate documentation.

[garazdawi]

Related pr #2375

[RaimoNiskanen]

Should be fixed by PR #5751