[Add test case] Bug in return type of `deque.copy()` in generic type

[NiklasRosenstein]

Bug Report

Mypy reasons incorrectly about the return type of collections.deque.copy() if the item type is a generic with a type parameter inside a generic class.

To Reproduce

from collections import deque
from typing import Any, Callable, Generic, List, Protocol, TypeVar

T = TypeVar("T")

def test(item_type: T) -> None:
 a: deque[List[T]] = deque()
 b: deque[List[T]] = a.copy() # ok


class Test(Generic[T]):
 def test(self) -> None:
 a: deque[List[T]] = deque()
 b: deque[List[T]] = a.copy() # error: Incompatible types in assignment (expression has type "deque[List[List[T]]]", variable has type "deque[List[T]]") [assignment]

Expected Behavior

The return type in the above example should be List[T] not List[List[T]].

Your Environment

• Mypy version used: 0.942

• Mypy command-line flags: mypy test.py

• Mypy configuration options from mypy.ini (and other config files):

  [tool.mypy]
  strict = true
  namespace_packages = true
  ignore_missing_imports = true
  show_error_codes = true
  exclude = "tests"

• Python version used: 3.10.2

• Operating system and version: OSX 12.2.1 Monterey

[JelleZijlstra]

deque.copy in typeshed is just def copy(self: Self) -> Self:, so I do think this is a bug in mypy. Does it reproduce on current master with #12590 applied?

[AlexWaygood]

deque.copy in typeshed is just def copy(self: Self) -> Self:, so I do think this is a bug in mypy. Does it reproduce on current master with #12590 applied?

I can reproduce with 0.942, but not with mypy master. The "first good commit" is indeed 20b0b9b -- so yep, #12590 fixed this.

[JelleZijlstra]

Great, let's add a test case for this one too.