annotationlib.ForwardRef.__hash__ raises TypeError on an unhashable value

annotationlib.ForwardRef defines both __eq__ and __hash__, but __hash__
raises TypeError when the forward reference holds an unhashable value, even
though two such forward references compare equal. This violates the data model
(objects that compare equal must be hashable and hash equal) and breaks storing
forward references in sets or as dict keys, which typing tools do.

Reproducer

from annotationlib import Format, get_annotations

class Unhashable:
    __hash__ = None

obj = Unhashable()

def f(a: undefined | obj): ...

fr1 = get_annotations(f, format=Format.FORWARDREF)["a"]
fr2 = get_annotations(f, format=Format.FORWARDREF)["a"]
assert fr1 == fr2
hash(fr1)   # TypeError: unhashable type: 'Unhashable'

obj resolves as a global, so the FORWARDREF format stores it in the forward
reference's __extra_names__. The same happens when the forward reference's
__owner__ is unhashable, for example a class whose metaclass sets
__hash__ = None.

Root cause

ForwardRef.__hash__ already hashes the unhashable __globals__ and __cell__
fields by identity (id()), but __owner__ and __extra_names__ are hashed
directly. __extra_names__ holds arbitrary values embedded in an annotation
that the FORWARDREF format could not render as a plain name, so those values
may be unhashable.

Regression

This is a sibling of #143831, which fixed the same class of bug for __cell__.
The traceback in that report already pointed at the __extra_names__ line.

Affected versions

3.14+ (where annotationlib was added).

Linked PRs

• gh-152360

从 annotationlib.ForwardRef.hash 开始，将 globals 和 cell 的处理方式与 owner 和 extra_names 进行比较。查看相关 issue #143831，并复现此处所示的失败；当包含不可哈希值的相等 forward references 能够在不产生 TypeError 的情况下被一致地进行哈希时，即表示完成。