Spurious "Provides-Extra:" generated in metadata

[wimglenn]

In the recently released version of setuptools, when installing my distribution copyingmock, it claims that it provides the extras=['']. Of course, I don't provide the empty-string extra, and the correct metadata should be extras=[]. This seems to be related to using the conditional deps feature, somehow:

install_requires=["mock ; python_version<'3.3'"],

To reproduce the issue, you can get the sdist:

$ pip download copyingmock==0.2 --no-binary copyingmock

Then use git bisect with this script:

#!/usr/bin/env python
# check.py
import json
import subprocess
import sys
from zipfile import ZipFile

if __name__ == '__main__':
    subprocess.check_call(['pip', 'wheel', '--no-deps', '--no-index', './copyingmock-0.2.tar.gz'])
    whl = ZipFile('copyingmock-0.2-py2.py3-none-any.whl')
    metadata = json.loads(whl.read('copyingmock-0.2.dist-info/metadata.json'))
    print(metadata['extras'])

    if metadata['extras']:
        sys.exit(1)
    else:
        sys.exit(0)

I did bisect and found

$ git bisect start
$ git bisect bad tags/v38.6.0
$ git bisect good tags/v28.8.0
Bisecting: 375 revisions left to test after this (roughly 9 steps)
[2cd7f22eb62b413d65fe4208d66f7e2738f71a56] Correctly honor exist_ok. Ref #1083.
$ git bisect run ./check.py
...

b2ea3c4 is the first bad commit.