Type alias expansion sometimes interferes with type variable binding

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#3,924 6 comments 5 reactions 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
compilers, devtools

Research direction

Start by running the two Python reproductions and comparing their reveal_type output, focusing on Callable aliases and type-variable binding. Trace the type-alias expansion path to determine why test2 binds T at the outer function level; done means equivalent aliases produce equivalent inferred types.

Written by the indexing model from the issue text.

Description

bug priority-0-high topic-calls topic-type-alias topic-type-variables

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.

Dominant language
Python
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