Issue with decorator and generic type arguments

[saulshanabrook]

I seem to be getting some odd incorrect types when I wrap a method in a decorator and that method takes generic parameters.

Here is a minimal example that should pass type checking, but fails on the last line with Incompatible return value type (got "Abstraction[V, V]", expected "Abstraction[T, V]"):

from __future__ import annotations


import typing

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


def decorator(f: T) -> T:
    ...


class Abstraction(typing.Generic[T, U]):
    @decorator
    def __add__(self, other: Abstraction[U, V]) -> Abstraction[T, V]:
        ...


def fn() -> Abstraction[T, V]:
    return Abstraction[T, U]() + Abstraction[U, V]()

And here is a slightly more realistic version of the same example, with the same error:

from __future__ import annotations


import typing

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


def decorator(f: T) -> T:
    ...


class Abstraction(typing.Generic[T, U]):
    def __init__(self, fn: typing.Callable[[T], U]):
        ...

    @decorator
    def __add__(self, other: Abstraction[U, V]) -> Abstraction[T, V]:
        ...


def fn(f1: typing.Callable[[T], U], f2: typing.Callable[[U], V]) -> Abstraction[T, V]:
    return Abstraction(f1) + Abstraction(f2)

I tested this with the last master commit:

$ mypy --version
mypy 0.720+dev.48916e63403645730a584d6898fbe925d513a841

I tried it with the new semantic analyzer on and off and I get the same result.

EDIT: Not that if I do not use the decorator both of these examples type check properly.

[ilevkivskyi]

I am not sure this is a duplicate, but most likely related to #1317 (this is one of the oldest high-priority issues).

[AlexWaygood]

The false-positive error is no longer emitted in mypy 0.750+