When using --use-feature=2020-resolver - Pip fails installing some packages from site-packages in virtualenv

Hi, I tried looking for a similar issue but I couldn’t find anything, although this issue is related with #8731

Environment

pip version: 20.2.4 Python version:3.7 OS: any Dockerfile that illustrates this issue:

FROM python:3.7
RUN pip install virtualenv requests argparse==1.4.0
RUN /usr/local/bin/virtualenv --system-site-packages tst
RUN /tst/bin/pip3.7 install --upgrade pip==20.2.4 && /tst/bin/pip3.7 install  --use-feature=2020-resolver --proxy 127.0.0.1 --retries 0 --timeout 1 argparse==1.4.0

Description When trying to install some packages like argparse the installation is failing if there is no connection to a remote repo that contains that package.

Using the same example from #8731, the issue seems to be happening only with some packages like argparse and not with requests for example.

Expected behavior Pip installs argparse correctly in the virtualenv in the above example.

Output

ERROR: Could not find a version that satisfies the requirement argparse==1.4.0
ERROR: No matching distribution found for argparse==1.4.0

[pradyunsg]

@pfmoore @uranusjr I think it’s OK to call this a downstream issue. If you think so too, I think @uranusjr’s PR covers this TBH.

[pradyunsg]

Closing this seems to be resolved. 😃