Wrong checking of overload for keyword-only args

[dmoisset]

While writing some stubs I found an issue that I simplified to the following:

from typing import overload

@overload
def f(*, out: str, flag: bool=False) -> str: ...

@overload
def f(*, flag: bool=False) -> bool: ...

reveal_type(f())  # I expect bool, get Any
reveal_type(f(flag=False))  # I expect bool, get Any
reveal_type(f(out="xxx"))   # I expect str, get str

The checker shows a

error: Overloaded function signatures 1 and 2 overlap with incompatible return types

but actually the signatures do not overlap (there's no call that can match both the signatures).

Something that's also confusing is that if I change the order of the arguments like this

# Move the "out" argument to the end
@overload
def f(*, flag: bool=False, out: str) -> str: ...
@overload
def f(*, flag: bool=False) -> bool: ...

I stop getting the "signature overlap" error, but I still get the "Any" type on the first two example calls. Given that these arguments are keyword only their positions shouldn't matter at all

(Comes from python/typing#248 )

[euresti]

Very weirdly it's the kwonly that's causing issues:

This code has no issues:

@overload
def f(x: int) -> int: ...
@overload
def f(x: str) -> str: ...

But make those kwonly and boom:

@overload
def f(*, x: int) -> int: ...   # error: Overloaded function signatures 1 and 2 overlap with incompatible return types
@overload
def f(*, x: str) -> str: ...

[ilevkivskyi]

The Any return is now fixed, but the overlap error still appears, so this is a false positive.

[Michael0x2a]

This took way way longer to resolve then I originally thought it would take, but the above examples are working as expected now in master after f61c2ba.

There's still a few todos related to handling edge cases w.r.t. star args, optional args, and such in #5163 that needs to land before we can really call this done, but I'm going to close this mostly so I can prune the list of overload-related things I have left to do.