Type erased with TypeAlias on higher ordered functions

[tyoung-patreon]

Bug Report

Mypy appears to lose type information simply by using a TypeAlias with generics. I'm trying to define a re-usable type for Decorator throughout my codebase so that I don't have to recreate Callable[[Callable[P, A]], Callable[P, A]] and can instead just use Decorator[P, A]. However, when I do this the type is silently replaced with Never.

To Reproduce

from typing import Callable
from typing import TypeVar
from typing_extensions import ParamSpec
from typing_extensions import TypeAlias

A = TypeVar("A")
P = ParamSpec("P")

Decorator: TypeAlias = Callable[[Callable[P, A]], Callable[P, A]]


def my_decorator(f: Callable[P, A]) -> Callable[P, A]:
    return f


def my_decorator_factory() -> Decorator[P, A]:
    return my_decorator


@my_decorator_factory()
def x(i: int) -> int:
    return i


reveal_type(my_decorator)
reveal_type(my_decorator_factory)
reveal_type(my_decorator_factory())
reveal_type(x)

# Revealed type is "def [P, A] (f: def (*P.args, **P.kwargs) -> A`-2) -> def (*P.args, **P.kwargs) -> A`-2"
# Revealed type is "def [P, A] () -> def (def (*P.args, **P.kwargs) -> A`-2) -> def (*P.args, **P.kwargs) -> A`-2"
# Revealed type is "def (def (*Never, **Never) -> Never) -> def (*Never, **Never) -> Never"
# Revealed type is "def (*Never, **Never) -> Never"

Expected Behavior

I expect mypy to to reveal that x has a type of Revealed type is "def (i: builtins.int) -> builtins.int". This is what I get when I do not use the TypeAlias for Decorator

Actual Behavior

Instead I get that Revealed type is "def (*Never, **Never) -> Never".

Your Environment

• Mypy version used: 1.7.0
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.9

[ilevkivskyi]

Duplicate of #3924