Difference in behaviour between overload and TypeVars when using functools.update_wrapper

[hajarmitage]

Bug Report

When calling functools.update_wrapper(f, g) where f has a type of TypeVar('T', Type1, Type2), mypy errors incorrectly, saying it expects Callable[[Never], Never] in that spot.

To Reproduce
https://mypy-play.net/?mypy=latest&python=3.12&gist=8b79db2fe7eb042e15122536a393dd43

Expected Behavior

I think f1 and f2 are identical functions. I expect both checks to succeed (as they did on mypy 1.9.0), or possibly both to fail (but if this is an error, I don't see why...).

Actual Behavior

update_wrapper(f1, g) succeeds and update_wrapper(f2, g) errors, with the message Argument 1 to "update_wrapper" has incompatible type "Callable[[T], T]"; expected "Callable[[Never], Never]" [arg-type]

Your Environment

• Mypy version used: 1.9.0 -> 1.10.0
• Python version used: 3.10, 3.12

[burrowsa]

I also see this issue. Is there a fix or work around?

[hauntsaninja]

If you want a workaround, you could use TypeVar(..., bound=int | str) (in general I think value constrained TypeVars are weird and get used in places when a bound is better)

[wcorrea-ec]

I'm not sure, but I think the new mypy is correct.

A variable of type T may be a str or an int. So f2's signature is equivalent to def f2(x: str | int) -> str | int. I could pass either type and I could get either type in return, not necessarily the one I passed in. That's incompatible with g, which only accepts and returns int.

Also, I think the type annotation for f1 is incorrect. If you ask mypy to reveal the type, you'll see: "Overload(def (x: builtins.str) -> builtins.str, def (x: builtins.int) -> builtins.int)". So the type of f1 is Callable[[str], str] | Callable[[int], int], not Callable[[str|int], [str|int]], which is the type of f2.