Overloading a function with a parameter that can be either SomeType[Any] or Any always results in the return type Any

Edit: I initially observed this behaviour for Callable[..., Any] and Any, but it now seems like this happens for any SomeType[Any] and Any, hence we've edited the title. For a more extensive explanation, scroll down to post 8.

I noticed that overloading a function with a Callable / Any parameter doesn't work as I'd expect. For example:

from typing import Any, Callable, overload

@overload
def foo(a: Callable) -> int: ...
@overload
def foo(a: Any) -> Any: ...

def foo(a):
    pass

a: str = foo(lambda x: x)

I'd expect the mypy error Incompatible types in assignment (expression has type "int", variable has type "str"), but there's no error here.

Of note, if I replace Callable by something else, it works fine:

from typing import Any, overload

@overload
def foo(a: int) -> int: ...
@overload
def foo(a: Any) -> Any: ...

def foo(a):
    pass

a: str = foo(1)
# mypy error: Incompatible types in assignment (expression has type "int", variable has type "str")

Finally, if I instead replace Any by something else, it also works fine.

from typing import Any, Callable, overload

@overload
def foo(a: Callable) -> int: ...
@overload
def foo(a: int) -> Any: ...

def foo(a):
    pass

a: str = foo(lambda x: x)
# mypy error: Incompatible types in assignment (expression has type "int", variable has type "str")

So it only doesn't work as expected in the first code block. Is that a bug? Or is it expected behaviour somehow?

Commencez par les exemples d’overload dans l’issue et suivez la manière dont mypy choisit entre un overload de type SomeType[Any] et un overload de type Any. Comparez le cas Callable/Any avec les cas int/Any et Callable/int, puis vérifiez que le comportement corrigé signale l’affectation incompatible tout en préservant les cas contrastés existants.