New resolver does not identify a subset of extras

[rrueth]

When installing a package with multiple extras (e.g. foo[a,b,c]), the new Pip resolver fails to identify an installed package with multiple extras (e.g. foo[a,b,c]) that is a superset of a requirement for the package with fewer extras (e.g. foo[a,b]) as valid.

What did you want to do?
I want to denote in a setup.py for my package, foo, that foo requires multiple extras for a specific dependency. For example:

setup(
    install_requires=[
        "lib[a,b]",
    ],
)

Then, in a requirements.txt file my application, bar, I need to install foo, and my application requires lib to be installed with more extras than just what foo needs. In my case lib is a local package that will be installed using -e. So my requirements file might look like this:

-e ../lib[a,b,c]
-e ../foo

Then, I ran pip using:

pip install \
        --use-feature=2020-resolver \
        --no-cache-dir \
        --no-index \
        --find-links="<path_to_a_local_wheel_directory>" \
        -r requirements.txt

Output
Pip failed to recognize that my applications lib[a,b,c] was a superset of foo's dependency on lib[a,b]:

ERROR: Could not find a version that satisfies the requirement lib[a,b] (from foo)
ERROR: No matching distribution found for lib[a,b]

[uranusjr]

I believe this is the same as #8785. The subset thing is a bit misleading; the new resolver just have bugs handling local dependencies with extras in general.

[brainwane]

Pradyun agrees with Tzu-ping. Thank you @rrueth for the bug report! Also, I thought you might want to know that we have also updated our migration guide and the details there may be helpful to you and your colleagues.