typing: Unexpected result with value of instance of class inherited from typing.NamedTuple

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-03-14.22:24:50.423>
labels = ['3.7', 'type-bug', 'library', 'docs']
title = 'typing: Unexpected result with value of instance of class inherited from typing.NamedTuple'
updated_at = <Date 2018-03-20.22:04:40.986>
user = None

bugs.python.org fields:

activity = <Date 2018-03-20.22:04:40.986>
actor = 'levkivskyi'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Library (Lib)']
creation = <Date 2018-03-14.22:24:50.423>
creator = '\xd0\x95\xd0\xb2\xd0\xb3\xd0\xb5\xd0\xbd\xd0\xb8\xd0\xb9 \xd0\x9c\xd0\xb0\xd1\x85\xd0\xbc\xd1\x83\xd0\xb4\xd0\xbe\xd0\xb2'
dependencies = []
files = ['47487']
hgrepos = []
issue_num = 33077
keywords = []
message_count = 10.0
messages = ['313843', '313848', '313916', '313918', '313919', '313920', '313925', '313945', '313957', '314168']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'rhettinger', 'eric.smith', 'docs@python', 'levkivskyi', '\xd0\x95\xd0\xb2\xd0\xb3\xd0\xb5\xd0\xbd\xd0\xb8\xd0\xb9 \xd0\x9c\xd0\xb0\xd1\x85\xd0\xbc\xd1\x83\xd0\xb4\xd0\xbe\xd0\xb2']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33077'
versions = ['Python 3.6', 'Python 3.7']