mypy is unable to correctly disambiguate a union when there is a dependent function type

[Jackmin801]

Bug Report

mypy is unable to correctly disambiguate a union when there is a dependent function type.
The call meow(s)(s) is always valid as s is either int or str and should not be able to take on both in the line.
However, the dependency of the return type of meow(s) on s is not remembered and mypy becomes upset.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.11&gist=50bc3d85db8bc5120d881d37b891fd2a

from typing import overload, Callable, Union

@overload
def meow(a: int) -> Callable[[int], int]:
    ...

@overload
def meow(a: str) -> Callable[[str], str]:
    ...

def meow(a):
    return lambda x: x

# OK
a: str = 's'
meow(a)(a)

# Not ok though it should be
s: Union[int, str] = 's'
meow(s)(s)

Expected Behavior

Expected Behavior

Success: no issues found in 1 source file

Actual Behavior

main.py:20: error: Argument 1 has incompatible type "Union[int, str]"; expected "int"  [arg-type]
main.py:20: error: Argument 1 has incompatible type "Union[int, str]"; expected "str"  [arg-type]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.991
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11

[tmke8]

Hm, the problem is that mypy doesn't keep track of how the type of meow(s) was determined. It's just treated as a union type:

s: Union[int, str] = 's'
reveal_type(meow(s))  # Union[Callable[[int], int], Callable [[str], str]]

When you then do meow(s)(s), mypy only sees that meow(s)(x) either takes an int or a str but it's not known which one, so mypy throws an error.

I would predict that mypy will never support this but I can't be sure. In the meantime, you can maybe solve your problem with a generic function:

MyData = TypeVar("MyData", int, str)
def meow(a: MyData) -> Callable[[MyData], MyData]:
    return lambda x: x

[ichard26]

See also: #2008 (it's why mypy doesn't narrow s's type on assignment).