Weird error message for incompatible override when using a TypeVar for self

[AlexWaygood]

Bug Report

Minimal repro:

from typing import TypeVar

Self = TypeVar("Self")

class Flag:
    def __or__(self: Self, other: Self) -> Self: ...
    __ror__ = __or__

class IntFlag(int, Flag):
    def __or__(self: Self, other: int) -> Self: ...
    def __ror__(self: Self, other: int) -> Self: ...

mypy output:

main.py:11: error: Signature of "__ror__" incompatible with supertype "Flag"
main.py:11: note:      Superclass:
main.py:11: note:          def [Self] __or__(self, Self, Self) -> Self
main.py:11: note:      Subclass:
main.py:11: note:          def __ror__(self, int) -> IntFlag

Mypy playground link here: https://mypy-play.net/?mypy=latest&python=3.10&gist=26b60881cd02e608bb9e555db4c3f627

Expected Behavior

I'm not sure an error is even appropriate here: no incompatible override is reported for __or__.

Even if an error is appropriate here, however, the error message is weird and garbled. According to the error message, mypy seems to think that __ror__ in the superclass expects three arguments (it only expects two). It also seems to think the method in the subclass will return IntFlag (not necessarily true; the return type will be whatever self is, and self could be any subtype of IntFlag).

The function that creates this error message is here:

mypy/mypy/messages.py

Line 824 in 9b4bce9

def signature_incompatible_with_supertype(

Mypy version used

• 0.950.
• This doesn't seem to be a recent regression: I can reproduce as far back as 0.920, which is the earliest mypy version that has the verbose error messages.
• I've also reproduced the bug on mypy master.

[AlexWaygood]

I think the root cause of the bug here is probably the fact that mypy erroneously believes __ror__ in the base class to be a callable instance variable, rather than a method:

from typing import TypeVar

Self = TypeVar("Self")

class Flag:
    def __or__(self: Self, other: Self) -> Self: ...
    __ror__ = __or__

reveal_type(Flag.__or__)  # Revealed type is "def [Self] (Self`-1, Self`-1) -> Self`-1"
reveal_type(Flag.__ror__)  # error: Access to generic instance variables via class is ambiguous  # Revealed type is "def [Self] (Any, Any) -> Any"

https://mypy-play.net/?mypy=latest&python=3.10&gist=daa1934028156c5298f443fc65c1afd4