1.11 regression: "generic" mapping failing with confusing error message

Bug Report

I've boiled down a minimal example -- this pattern is used by pyupgrade (and reorder-python-imports) to register ast functions and has been working with mypy since version 0.710 -- but was broken with the last update sadly

To Reproduce

this is a simplified version of pyupgrade/_data.py -- FUNCS acts as a mapping from ast types to their callback functions

import ast
import collections
from typing import Callable, Protocol, TypeVar

AST_T = TypeVar('AST_T', bound=ast.AST)
ASTFunc = Callable[[AST_T], None]

FUNCS = collections.defaultdict(list)

def register(tp: type[AST_T]) -> Callable[[ASTFunc[AST_T]], ASTFunc[AST_T]]:
    def register_decorator(func: ASTFunc[AST_T]) -> ASTFunc[AST_T]:
        FUNCS[tp].append(func)
        return func
    return register_decorator


class ASTCallbackMapping(Protocol):
    def __getitem__(self, tp: type[AST_T]) -> list[ASTFunc[AST_T]]: ...

def visit(funcs: ASTCallbackMapping) -> None: ...


def f() -> None:
    visit(FUNCS)

Expected Behavior

the behaviour pre-mypy-1.11:

(no errors)

Actual Behavior

$ mypy t.py 
t.py:24: error: Argument 1 to "visit" has incompatible type "defaultdict[type[AST_T], list[Callable[[AST_T], None]]]"; expected "ASTCallbackMapping"  [arg-type]
t.py:24: note: Following member(s) of "defaultdict[type[AST_T], list[Callable[[AST_T], None]]]" have conflicts:
t.py:24: note:     Expected:
t.py:24: note:         def [AST_T: AST] __getitem__(self, type[AST_T], /) -> list[Callable[[AST_T], None]]
t.py:24: note:     Got:
t.py:24: note:         def __getitem__(self, type[AST_T], /) -> list[Callable[[AST_T], None]]
Found 1 error in 1 file (checked 1 source file)

the error message seems suspicious because I think those two are the same ?

Your Environment

• Mypy version used: 1.11.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10.12 -- though really anything modern

Tái hiện hồi quy bằng ví dụ tối giản trong t.py với mypy 1.11. So sánh hành vi tương thích của generic protocol và defaultdict, sử dụng pyupgrade/_data.py làm ví dụ động lực. Công việc được xem là hoàn tất khi mapping hợp lệ được chấp nhận mà không có lỗi kiểu không tương thích gây hiểu nhầm và hồi quy được bao phủ bởi một test phù hợp.