False positive when using `Self` as the return type with multiple base classes

[Cnoor0171]

Bug Report

If you try to derive from multiple base classes that both define the same function returning Self, mypy incorrectly warns that the definitions are incompatible.

To Reproduce

# https://mypy-play.net/?mypy=latest&python=3.12&gist=e5d3734df862e8f3ada5c11c076cae57
# main.py
from typing import Self

class MyBase1:
    def foo(self) -> Self:
        return self

class MyBase2:
    def foo(self) -> Self:
        return self

class MyDerived(MyBase1, MyBase2):
    pass


MyBase1().foo()  # Should be Valid. Has type MyBase1
MyBase1().foo()  # Should be Valid. Has type MyBase2
MyDerived().foo()  # Should be Valid. Has type MyDerived

Expected Behavior

Shouldn't report any errors. In the context of the line class MyDerived(MyBase1, MyBase2): both MyBase1 and MyBase2 returns Self aka MyDerived

Actual Behavior

$ mypy main.py
main.py:13: error: Definition of "foo" in base class "MyBase1" is incompatible with definition in base class "MyBase2"  [misc]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 1.11.1 (compiled: yes)
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.12

[Cnoor0171]

A similar problem if the methods are classmethods

from typing import Self

class MyBase1:
    @classmethod
    def foo(cls) -> Self:
        return cls()

class MyBase2:
    @classmethod
    def foo(cls) -> Self:
        return cls()

class MyDerived(MyBase1, MyBase2):
    pass


MyBase1.foo()  # Should be Valid. Has type MyBase1
MyBase1.foo()  # Should be Valid. Has type MyBase2
MyDerived.foo()  # Should be Valid. Has type MyDerived

Interestingly, mypy is also able to derive the correct type of MyDerived.foo() or MyDerived().foo() if you do reveal_type on them.

[sterliakov]

Missed this ticket - also resolved by #18465 and passes on current master.