Concatenate[str, bool, Q] spuriously duplicated to Concatenate[str, bool, str, bool, Q]

[andersk]

Mypy gives a false positive on this valid code. The error message seems to indicate that it’s spuriously duplicated the str, bool parameters to str, bool, str, bool somewhere.

from typing import Callable, Concatenate, ParamSpec

P = ParamSpec("P")
Q = ParamSpec("Q")

def foo(f: Callable[P, int]) -> Callable[P, int]:
    return f

def bar(f: Callable[Concatenate[str, bool, Q], int]) -> Callable[Concatenate[str, bool, Q], int]:
    return foo(f)

Mypy playground link

main.py:10: error: Incompatible return value type (got "Callable[[str, bool, str, bool, **Q], int]", expected "Callable[[str, bool, **Q], int]")
main.py:10: error: Argument 1 to "foo" has incompatible type "Callable[[str, bool, **Q], int]"; expected "Callable[[str, bool, str, bool, **Q], int]"
Found 2 errors in 1 file (checked 1 source file)

[andersk]

Trying to eta-expand foo(f) results in other weird “incompatible type” errors—one where the LHS and RHS display as identical, and two where the RHS displays as a list of types.

from typing import Callable, Concatenate, ParamSpec

P = ParamSpec("P")
Q = ParamSpec("Q")

def foo(f: Callable[P, int]) -> Callable[P, int]:
    return f

def bar(f: Callable[Concatenate[str, bool, Q], int]) -> Callable[Concatenate[str, bool, Q], int]:
    def g(s: str, b: bool, *args: Q.args, **kwargs: Q.kwargs) -> int:
        return foo(f)(s, b, *args, **kwargs)

    return g

main.py:11: error: Argument 1 to "foo" has incompatible type "Callable[[str, bool, **Q], int]"; expected "Callable[[str, bool, **Q], int]"
main.py:11: error: Argument 1 has incompatible type "str"; expected "[str, bool, **Q.args]"
main.py:11: error: Argument 2 has incompatible type "bool"; expected "[str, bool, **Q.args]"
Found 3 errors in 1 file (checked 1 source file)

[PIG208]

Relevant code where each ParamSpec argument eventually gets expanded twice