Mypy fails to infer type parameters when composing generic functions with themselves

[LeeeeT]

Bug Report

Mypy incorrectly infers Never types when the same generic function is used in multiple argument positions of a higher-order generic function, preventing proper composition of identical generic functions.

To Reproduce

from collections.abc import Callable

def compose[A, B, C](first: Callable[[A], B], second: Callable[[B], C]) -> Callable[[A], C]: ...

def id[T](value: T) -> list[T]: ...  # any generic would do

compose(id, id)
# Argument 1 to "compose" has incompatible type "Callable[[T], list[T]]"; expected "Callable[[Never], Never]"
# Argument 2 to "compose" has incompatible type "Callable[[T], list[T]]"; expected "Callable[[Never], Never]"

Expected Behavior

1. The first id function should be inferred as Callable[[T], list[T]]
2. The second id function should be inferred as Callable[[list[T]], list[list[T]]]
3. The resulting composition should be Callable[[T], list[list[T]]]

Actual Behavior

Mypy fails to infer type parameters.

Your Environment

• Mypy version used: 1.15.0
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.13

[sterliakov]

Related/dupe of #5040 (except for the "typevar argument with default" bit which is tracked in 3737), but this is written much better and focuses on one problem - I'm inclined to close the older one as a dupe of this.

[ilevkivskyi]

Oh, this is annoying. It works if I do something like this:

def id2[T](value: T) -> list[T]: ...
reveal_type(compose(id, id2))

So this means this is another place where we need to "freshen" type variables. But I think it is better to detect such cases, and only create new independent variable for second etc. occurrence of each function.