Passing generic callable to function fails

[saulshanabrook]

I am trying to type a function that takes in a function, and a value, and returns the arguments of the function. For example, this is how it would look supporting two argument functions:

T = typing.TypeVar("T")
U = typing.TypeVar("U")
V = typing.TypeVar("V")
X = typing.TypeVar("X")


def extract_args(fn: typing.Callable[[T, U], V], ret: V) -> typing.Tuple[T, U]:
    ...

This works great when the callable I pass in is not generic, like this:

def hi(a: int, b: str) -> str:
    ...

reveal_type(extract_args(hi, "return"))
# Revealed type is 'Tuple[builtins.int*, builtins.str*]'

However, if I try to make the function generic, it doesn't seem to work:

def hi_generic(a: int, b: X) -> X:
    ...


reveal_type(extract_args(hi_generic, "return"))
# Argument 1 to "extract_args" has incompatible type "Callable[[int, X], X]"; expected "Callable[[int, X], str]"
# Revealed type is 'Tuple[builtins.int*, X`-1]'

I would expect it to infer fn has type Callable[[int, str], str] since I am passing in the return value of type str so the second arg also has to be a str.

This might be the same issue as #3193.

If anyone has a work around for how I could get a function like this to type, they would be much appreciated!

[ilevkivskyi]

Thanks for reporting! Actually both are duplicates of #1317 (already high priority).