`isinstance(obj, Hashable)` raises `TypeError` when both `obj` and `type(obj)` are unhashable

Bug report

Bug description:

Assumption: isinstance(anything, any_type) should never raise.

The problem appears to be in _abc.c function _abc__abc_subclasscheck_impl (known at runtime as _abc._abc_subclasscheck()), which reads (in part):

    /* 1. Check cache. */
    incache = _in_weak_set(impl->_abc_cache, subclass);

The implementation assumes subclass is hashable. That's almost always true of classes, but not guaranteed.

Reproduction case:

from __future__ import annotations
from collections.abc import Hashable

class UnhashableMeta(type):
    def __eq__(self, other: object) -> bool:
        return super().__eq__(other)

class UnhashableClass(metaclass=UnhashableMeta):
    def __eq__(self, other: object) -> bool:
        return super().__eq__(other)

# any non-hashable instance of a non-hashable class
#         ┌────────┴────────┐
isinstance(UnhashableClass(), Hashable)

Traceback for (cpython) Python 3.13.1:

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File ".../bug.py", line 14, in <module>
    isinstance(UnhashableClass(), Hashable)
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen abc>", line 119, in __instancecheck__
  File "<frozen abc>", line 123, in __subclasscheck__
TypeError: unhashable type: 'UnhashableMeta'

Tracebacks are nearly identical with Python 3.8, 3.9, 3.10, 3.11, and 3.12, as well as Python 3.14.0a4.

CPython versions tested on:

3.13, 3.12, 3.11, 3.10, 3.9, 3.14

Operating systems tested on:

macOS

Apri Modules/_abc.c e inizia da _abc__abc_subclasscheck_impl, in particolare dalla chiamata a _in_weak_set descritta nell’issue. Esegui su CPython la riproduzione fornita con UnhashableMeta e UnhashableClass, quindi verifica che isinstance(UnhashableClass(), Hashable) non sollevi più un’eccezione e che la regressione sia coperta da un test appropriato.