a decorator that modifies another decorator gives an incorrect type error

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Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request?

Bug.

• Please insert below the code you are checking with mypy, or a mock-up repro if the source is private. We would appreciate if you try to simplify your case to a minimal repro.

from typing import Any, Callable, TypeVar

_FirstDecoratee = TypeVar("_FirstDecoratee", bound=Callable[..., Any])
_SecondDecoratee = TypeVar("_SecondDecoratee", bound=Callable[..., Any])

def basic_decorator(decoratee: _FirstDecoratee) -> _FirstDecoratee:
    "decorate a function"
    return decoratee

def decorator_transformer(
    decorator: Callable[[_SecondDecoratee], _SecondDecoratee]
) -> Callable[[_SecondDecoratee], _SecondDecoratee]:
    "take a decorator and return a version of it with the same signature"
    def modified_decorator(decoratee: _SecondDecoratee) -> _SecondDecoratee:
        return decoratee
    return modified_decorator

@basic_decorator
def unstacked() -> None:
    "no decorator stack: no error"

@decorator_transformer(basic_decorator)
def stacked() -> None:
    "decorator stack: this should be the same, but it's an error"

• What is the actual behavior/output?

t.py:26: error: Argument 1 has incompatible type "Callable[[], None]"; expected "_FirstDecoratee"

• What is the behavior/output you expect?

No error.

• What are the versions of mypy and Python you are using?

mypy 0.782 Python 3.8.5

• Do you see the same issue after installing mypy from Git master?

Yes (0.790+dev.ffd9d1cdff4af3b482d4dd1f871fd1dc5b39eebb)

• What are the mypy flags you are using? (For example --strict-optional)

warn_redundant_casts=True
warn_unused_ignores=True
strict_optional=True
strict_equality=True
no_implicit_optional=True
disallow_untyped_defs=True
disallow_any_generics=True

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Is this a duplicate of https://github.com/python/mypy/issues/3924 ? Or the issue Jukka alluded to in https://github.com/python/mypy/issues/8978#issuecomment-643375262, "a known very old bug that generic functions when passed as arguments to another generic functions don't work"?

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The workaround seems to be

def decorator_transformer(
    decorator: Callable[[_FirstDecoratee], _FirstDecoratee]
) -> Callable[[_SecondDecoratee], _SecondDecoratee]:

which sacrifices a little bit of strictness but manages to mostly maintain the desired shape