8000 (🐞) Explicit type parameter specification with method reference screws up assignability · Issue #13041 · python/mypy · GitHub
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KotlinIsland opened this issue Jun 30, 2022 · 0 comments · Fixed by #18943
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(🐞) Explicit type parameter specification with method reference screws up assignability #13041

KotlinIsland opened this issue Jun 30, 2022 · 0 comments · Fixed by #18943
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bug mypy got something wrong

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@KotlinIsland
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from typing import _T as T, Callable, Generic, TypeVar

class A(Generic[T]):
    def f(self) -> None: ...

c: Callable[[A[int]], None]
c = A.f
c = A[int].f  # error: Incompatible types in assignment (expression has type "(A[T]) -> None", variable has type "(A[int]) -> None")

expression has type "(A[T]) -> None"

Uhhhh, I don't think it does...

Mypy in Real Life

class B(A[int]): ...

# some higher order method
foo(list_of_b, B.f)  # error
@KotlinIsland KotlinIsland added the bug mypy got something wrong label Jun 30, 2022
@ilevkivskyi ilevkivskyi mentioned this issue May 19, 2025
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cdce8p pushed a commit to cdce8p/mypy that referenced this issue May 31, 2025
@ilevkivskyi @hauntsaninja @cdce8p
Use checkmember.py to check protocol subtyping (python#18943)
2b175b2 
Fixes python#18024
Fixes python#18706
Fixes python#17734
Fixes python#15097
Fixes python#14814
Fixes python#14806
Fixes python#14259
Fixes python#13041
Fixes python#11993
Fixes python#9585
Fixes python#9266
Fixes python#9202
Fixes python#5481

This is a fourth "major" PR toward
python#7724. This is one is
watershed/crux of the whole series (but to set correct expectations,
there are almost a dozen smaller follow-up/clean-up PRs in the
pipeline).

The core of the idea is to set current type-checker as part of the
global state. There are however some details:
* There are cases where we call `is_subtype()` before type-checking. For
now, I fall back to old logic in this cases. In follow up PRs we may
switch to using type-checker instances before type checking phase (this
requires some care).
* This increases typeops import cycle by a few modules, but
unfortunately this is inevitable.
* This PR increases potential for infinite recursion in protocols. To
mitigate I add: one legitimate fix for `__call__`, and one temporary
hack for `freshen_all_functions_type_vars` (to reduce performance
impact).
* Finally I change semantics for method access on class objects to match
the one in old `find_member()`. Now we will expand type by instance, so
we have something like this:
  ```python
  class B(Generic[T]):
      def foo(self, x: T) -> T: ...
  class C(B[str]): ...
  reveal_type(C.foo)  # def (self: B[str], x: str) -> str
  ```
FWIW, I am not even 100% sure this is correct, it seems to me we _may_
keep the method generic. But in any case what we do currently is
definitely wrong (we infer a _non-generic_ `def (x: T) -> T`).

---------

Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
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Use checkmember.py to check protocol subtyping ilevkivskyi/mypy
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