Special semantics for generator context managers

[bswck]

@ImogenBits

I don’t think this is something that type checkers should handle. The contextmanager decorator expects a generator function and it’s getting exactly that. For this to be a type checker’s responsibility, the type system would have to differentiate between generators that yield once and ones that yield some other number of times.

I see where you’re coming from!

I think it is necessary to differentiate between these because of e.g. Support suppressing generator context managers - #2 by bswck that leads to silent false positives.

Generator context managers seem special enough to specialcase them.