__text_signature__ parser silently drops arguments with certain unsupported default forms

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-01-20.11:21:25.305>
labels = ['extension-modules']
title = '__text_signature__ parser silently drops arguments with certain unsupported default forms'
updated_at = <Date 2021-01-20.11:21:25.305>
user = 'https://github.com/anntzer'

bugs.python.org fields:

activity = <Date 2021-01-20.11:21:25.305>
actor = 'Antony.Lee'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Extension Modules']
creation = <Date 2021-01-20.11:21:25.305>
creator = 'Antony.Lee'
dependencies = []
files = []
hgrepos = []
issue_num = 42976
keywords = []
message_count = 1.0
messages = ['385335']
nosy_count = 1.0
nosy_names = ['Antony.Lee']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue42976'
versions = []