Improve error message when constraints conflict with requirements.

[jsirois]

Right now straight pip gives:

$ pip download pex==2.1.28 --constraint <(echo pex==2.1.27)
ERROR: Could not find a version that satisfies the requirement pex==2.1.28
ERROR: No matching distribution found for pex==2.1.28

There is no extra useful information to gather even in the verbose logs.

Pex could use the full requirements / constraints parsing it is now capable of and either fail fast or else hold information in reserve anticipating Pip failing. Pairs of requirement / constraint could be compared to make sure they were compatible. This would require something like https://pypi.org/project/python-ranges/ but that lib is python 3.6+.

[jsirois]

Upgrading vendored Pip will fix this: pypa/pip#9300; so this is just blocked on dropping Python 2 support in Pex.

[jsirois]

I'm going to close this as fixed since you can now use --pip-version ¹ to select a newer Pip with support for these better error messages. This does not help Python>=2.7,<3.8, but for all other cases:

:; cat constraints.txt
pex==2.1.27

:; pex --python python3.8 pex==2.1.28 --constraint constraints.txt --pip-version latest
pid 518077 -> /home/jsirois/.cache/pex/venvs/0/878245defe5cf7850888db6ca8ac0dfd371cb319/05ac1c1e24762083e2c9af13d44377083b7e8668/bin/python -sE /home/jsirois/.cache/pex/venvs/0/878245defe5cf7850888db6ca8ac0dfd371cb319/05ac1c1e24762083e2c9af13d44377083b7e8668/pex --disable-pip-version-check --no-python-version-warning --exists-action a --no-input --isolated -q --cache-dir /home/jsirois/.cache/pex/pip/0/24.2/pip_cache --log /tmp/pex-pip-log.t44qpfyg/pip.log download --dest /home/jsirois/.cache/pex/downloads/0/resolver_download.wgg7nj6u/home.jsirois..pyenv.versions.3.8.20.bin.python3.8 --constraint constraints.txt pex==2.1.28 --retries 5 --timeout 15 exited with 1 and STDERR:
pip: ERROR: Cannot install pex==2.1.28 because these package versions have conflicting dependencies.
pip: ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/topics/dependency-resolution/#dealing-with-dependency-conflicts
pip:
pip:  The conflict is caused by:
pip:      The user requested pex==2.1.28
pip:      The user requested (constraint) pex==2.1.27
pip:
pip:  To fix this you could try to:
pip:  1. loosen the range of package versions you've specified
pip:  2. remove package versions to allow pip to attempt to solve the dependency conflict

Footnotes

1. The support for Pip fixes like this was spec'd in Pull in Pip fixes. #1805, implemented in Add support for Pip 22.2.2. #1893 and released in https://github.com/pex-tool/pex/releases/tag/v2.1.104 ↩