Changed strict-equality behavior with dict/Mapping

[JukkaL]

When using mypy 1.6, this example produces no errors:

# mypy: strict-equality

from typing import Mapping

def f(x: dict[str, int | None], y: dict[str, str | None]) -> bool:
    return x == y

def g(x: Mapping[str, int], y: str) -> bool:
    return x == y

However, after #16237, it started generating these errors:

t.py:6: error: Non-overlapping equality check (left operand type: "dict[str, int | None]", right operand type: "dict[str, str | None]")  [comparison-overlap]
t.py:9: error: Non-overlapping equality check (left operand type: "Mapping[str, int]", right operand type: "str")  [comparison-overlap]

I'm not sure if the new behavior is better or not, but I suspect that the change in behavior may have been an unintended consequence of #16237, so it seems worth investigating what is going on.

cc @ilevkivskyi

[ilevkivskyi]

This is actually intentional. For my len() PR I needed to disable the len() check if a tuple subclass has a custom (i.e. user-defined) __len__() (and/or a user-defined __bool__() for cases like assert some_tuple). But then it turned out the existing helper custom_special_method() didn't work, because it considered special methods defined in typing as user-defined (it only allowed builtins as not user-defined), and tuple gets its __len__() from typing. But also it turns out dict gets its __eq__() from typing as well, so we actually never checked unsafe overlap for dictionaries/mappings (because we always considered equality as user-defined there).

FWIW I think the new behavior is consistent with how we handle e.g. lists etc where we get

Non-overlapping equality check (left operand type: "list[int | None]", right operand type: "list[str | None]")

I propose closing this.

[JukkaL]

Makes sense! Thanks for looking into this.