Property setter parameter type-checked against getter return type instead of setter parameter type #12900

Bug Report
As shown in the code under To Reproduce, mypy checks the value that is set on an attribute not against that attribute's setter's parameter type (Iterable[Member]), but against the getter's return type (List[Member]). While it may not be common for a setter to accept a supertype of the type returned by the getter, or even a completely unrelated and incompatible type, there are valid reasons to do so and mypy should not make the assumption that both types are identical.

To Reproduce

from typing import List, Iterable


class Member:
    pass


class Group:
    @property
    def members(self) -> List[Member]:
        pass

    @members.setter
    def members(self, members: Iterable[Member]) -> None:
        pass


group = Group()
group.members = {Member(), Member()}

# mypy error: test.py:19: error: Incompatible types in assignment (expression has type "Set[Member]", variable has type "List[Member]")

Expected Behavior

No errors raised by mypy at all.

Actual Behavior

mypy erraneously flags the type of the value set on the attribute as incompatible.

Your Environment

• Mypy version used: 0.950
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files):

  check_untyped_defs = True
  no_implicit_optional = True
  

• Python version used: 3.9
• Operating system and version: Ubuntu 21.10