Side-effect of `ABC.__subclasshook__` / `_abc_instancecheck` ▶ `isinstance(..., Parent)` calls `__subclasscheck__` of uninvolved Child class -> RecursionError

Bug report

Bug description:

I realized there is strange error of __subclasshook__, or possibly _abc_instancecheck which creates a side-effect that __subclasshook__ is called on a Child class that is not even queried.

What is even more strange this error only occurs when the crucial statement B is called in a AB situation, just B or BAB is fine.

# At the end of his snippet are 3 test sets, Only the first causes an error the other 2 are fine.
import sys
sys.setrecursionlimit(59)
from abc import ABC


class Base(ABC): ...

class _ParentCheck(Base):

    @classmethod
    def __subclasshook__(cls, subclass):
        if not issubclass(subclass, Base):
            return NotImplemented
        if not hasattr(subclass, "f"):
            return False
        return True


class Parent(Base):
    f: int

    @classmethod
    def construct(cls, foo: int):
        class Child(cls, _ParentCheck):
            f = foo
        
        
        return Child


# Test Set 1: Error

assert issubclass(Parent.construct(1), Parent)  # Statement A
assert not isinstance(lambda: None, Parent)  # Statement B

# Test Set 2: OK

assert not isinstance(lambda: None, Parent)  # Statement B

# Test Set 3: OK

assert not isinstance(lambda: None, Parent)  # Statement B
assert issubclass(Parent.construct(1), Parent)  # Statement A
assert not isinstance(lambda: None, Parent)  # Statement B

CPython versions tested on:

3.14

Operating systems tested on:

Linux

首先在 CPython 3.14 上執行提供的重現程式，重點關注 ABC.subclasshook、_abc_instancecheck，以及 Parent 與 _ParentCheck 之間的互動。當 Test Set 1 不再引發 RecursionError，且 Test Sets 2 和 3 繼續通過時，即表示完成。