singledispatchmethod raises an error when relying on a forward declaration

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-10-09.20:11:43.528>
labels = ['type-bug', 'library', '3.9', '3.10']
title = 'singledispatchmethod raises an error when relying on a forward declaration'
updated_at = <Date 2022-01-11.09:46:19.220>
user = 'https://github.com/glyph'

bugs.python.org fields:

activity = <Date 2022-01-11.09:46:19.220>
actor = 'AlexWaygood'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-10-09.20:11:43.528>
creator = 'glyph'
dependencies = []
files = []
hgrepos = []
issue_num = 41987
keywords = ['patch']
message_count = 16.0
messages = ['378346', '378350', '378353', '378361', '379896', '379911', '380797', '380799', '380800', '380803', '380832', '380843', '380845', '380852', '409572', '410269']
nosy_count = 9.0
nosy_names = ['gvanrossum', 'glyph', 'lukasz.langa', 'ethan smith', 'joernheissler', 'BTaskaya', 'mental', 'ryansobol', 'AlexWaygood']
pr_nums = ['23216']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41987'
versions = ['Python 3.9', 'Python 3.10']