Type inference regression with sort and cmp_to_key

[JukkaL]

This code fragment started generating errors after 0.780 release:

from functools import cmp_to_key

x = [(1, '')]

def f(): pass

reveal_type(cmp_to_key)

x.sort(key=cmp_to_key(lambda x, y: f())) # Cannot infer type argument 1 of "cmp_to_key"

It might be related to python/typeshed#4192.

This happens only about 3 times in a big Dropbox internal codebase, and this is infrequent enough that I don't consider it release-blocking. I'm marking it as high-priority since this is a regression.

[hauntsaninja]

Can confirm that python/typeshed#4192 is the trigger for this to start generating errors. (Just pushed python/typeshed#4492 which improves the return type of cmp_to_key, but doesn't affect this).

Here's a self contained, mostly minimised repro:

from typing import *

T = TypeVar("T")

def ctk(f: Callable[[T, T], int]) -> Callable[[T], Any]: ...

@overload
def sortedx(__iterable: Iterable[str], *,
 key: None = ...) -> List[str]: ...
@overload
def sortedx(__iterable: Iterable[T], *,
 key: Callable[[T], int]) -> List[T]: ...

def sortedx(*args, **kwargs): ...

reveal_type(sortedx([1,2,3], key=ctk(lambda x, y: 5)))

If you remove the overload, the error goes away.

[ilevkivskyi]

Both original and self-contained examples now pass correctly.