Type alias expansion sometimes interferes with type variable binding

[ilevkivskyi]

Consider the following two examples, I think they should be equivalent, since logically expansion of type aliases should happen before any other steps. (At least this would be consistent and natural for those familiar with C macros):

from typing import TypeVar, Callable
T = TypeVar('T')

def test1() -> Callable[[T], T]: ...
reveal_type(test1) # Revealed type is 'def () -> def [T] (T`-1) -> T`-1'
reveal_type(test1()) # Revealed type is 'def [T] (T`-1) -> T`-1'

F = Callable[[T], T]
def test2() -> F[T]: ...
reveal_type(test2) # Revealed type is 'def [T] () -> def (T`-1) -> T`-1'
reveal_type(test2()) # Revealed type is 'def (<nothing>) -> <nothing>'

Here test1 works as expected, while if I use an alias something strange happens with test2. Namely notice a subtle difference between their types.

def () -> def [T] (T`-1) -> T`-1
and
def [T] () -> def (T`-1) -> T`-1

I have noticed few similar scenarios, they all seem to be related to Callable.

[gvanrossum]

Sounds like the code that made -> Callable[[T], T] possible in the first place still has some bugs...

[mvolfik]

I have an issue which is, I believe, related to this:

from typing import TypeVar, Callable

T = TypeVar("T")
Function = Callable[[T], T]

a: Function         = ...
b: Function[T]      = ...
c: Callable[[T], T] = ...

(save as test.pyi, or replace the ellipsis with lambda x: x, function created in function etc.)

Mypy always disallows a, with --strict disallows b and always allows c. Tested with 0.800 and 0.810+dev.9e0e23e97653dab3a558d34340486e7a66f7d6f0.

How much is this related, should I file a separate issue? I would expect either a or b (no idea which one though) to be entirely correct

When I was looking for duplicates, #8922 also looked related