Cannot determine type of "__gt__" error on decorated __lt__ method where decorator is an assigned alias of another decorator

Okay, this one has me stumped.

I am getting a Cannot determine type of "__gt__" error on my definition of __lt__. Yes. You read that right. The error complains of __gt__, but happens on def __lt__ …. If I swap the declaration order (such that def __gt__ … appears before def __lt__ …), then I get the error Cannot determine type of "__lt__" error on my definition of __gt__, so at least it's…symmetrical.

The problem only emerges when I use an alias for a decorator created by assignment.

To Reproduce

# test_cast.py
from operator import __gt__, __lt__
from typing import Callable, TypeVar, Union

_T = TypeVar("_T")

def identity(__: _T) -> _T:
    return __

maybe_not_available_decorator = identity  # <-- problematic decorator alias

# try:
#     from spicy import spicy_decorator as maybe_not_available_decorator
# except ImportError:
#     pass

class Spam:
    def __init__(self, value: int):
        super().__init__()
        self._value = value

    @identity  # <- function
    def __lt__(self, other: Union[int, "Spam"]) -> bool:  # <-- this is fine
        if isinstance(other, Spam):
            return __lt__(self._value, other._value)
        else:
            return NotImplemented

    @identity  # <- function
    def __gt__(self, other: Union[int, "Spam"]) -> bool:  # <-- so is this
        if isinstance(other, Spam):
            return __gt__(self._value, other._value)
        else:
            return NotImplemented

class Ham:
    def __init__(self, value: int):
        super().__init__()
        self._value = value

    @maybe_not_available_decorator  # <- alias
    def __lt__(self, other: Union[int, "Ham"]) -> bool:  # <-- Cannot determine type of "__gt__"
        if isinstance(other, Ham):
            return __lt__(self._value, other._value)
        else:
            return NotImplemented

    @maybe_not_available_decorator  # <- alias
    def __gt__(self, other: Union[int, "Ham"]) -> bool:  # <-- this is fine for some reason
        if isinstance(other, Ham):
            return __gt__(self._value, other._value)
        else:
            return NotImplemented

Your Environment

% mypy --config-file=/dev/null test_case.py
/dev/null: No [mypy] section in config file
test_case.py:42: error: Cannot determine type of "__gt__"
Found 1 error in 1 file (checked 1 source file)
% mypy --version
mypy 0.910

Beginne mit dem Reproducer in test_case.py und führe mypy --config-file=/dev/null aus, um den gemeldeten Fehler für die dekorierten Ham-Methoden zu bestätigen. Vergleiche den identity-Decorator mit seinem zugewiesenen Alias und verfolge anschließend mypys Analyse von Decorators und speziellen Methoden. Als abgeschlossen gilt die Aufgabe, wenn die Alias-Form keinen unbegründeten Cannot determine type-Fehler erzeugt und das bestehende Verhalten weiterhin intakt ist.