Make mypy --warn-no-return clean

[ddfisher]

I made up a way to handle calling a function that itself always raises.

The only errors that remain are in checker.py, and it's not entirely clear to me what to do about them. A bunch of functions are declared as returning Type when they just don't, but they can't simply be changed because they're overriding functions from a superclass. See e.g.

mypy/mypy/checker.py

Line 271 in b024ac3

def visit_overloaded_func_def(self, defn: OverloadedFuncDef) -> Type:

[gvanrossum]

Regarding those visitors, maybe the visitor superclass can be changed to make those visitors return Optional[T], and the subclasses can choose to override those methods using either -> None or -> (whatever) based on their behavior?

[gvanrossum]

This is super confusing. The function raises, but is also declared to return an exception, and its callers raise whatever it returns, but those raise statements are never raised because the called function raises. Wouldn't it be simpler to make this function actually return the exception and have the call sites actually raise the exception it returns?

[ddfisher]

Yeah, that's much better.

[ddfisher]

That fix won't work. It might work if we were running under strict Optional (though we might explicitly forbid stuff like that; I'm not sure), but it definitely won't otherwise (because None is Void). :/

[ddfisher]

Looking some more at the code, I think the right solution might be to make TypeChecker inherit from NodeVisitor[Type] and have some functions explicitly return None (which is the case in some of them already).

[rwbarton]

Yeah that seems fine for now; fixing that issue in the long run is one of the motivations of #1783.