Backtracking when package has conflicting runtime dependencies

Description

The oldest-supported-numpy package version 0.13 has conflicting requirements in its setup.cfg: https://github.com/scipy/oldest-supported-numpy/blob/v0.13/setup.cfg#L43-L50

Those two lines conflict on s390x and Python 3.8:

    numpy==1.17.5; python_version=='3.8' and platform_machine=='s390x' and platform_python_implementation != 'PyPy'
    numpy==1.17.3; python_version=='3.8' and (platform_machine!='arm64' or platform_system!='Darwin') and platform_machine!='aarch64' and platform_python_implementation != 'PyPy'

When copying those directly in a requirements file, you get the expected error:

$ python -c "import platform; print(platform.machine())"
s390x
$ cat requirements-build.txt 
    numpy==1.17.5; python_version=='3.8' and platform_machine=='s390x' and platform_python_implementation != 'PyPy'
    numpy==1.17.3; python_version=='3.8' and (platform_machine!='arm64' or platform_system!='Darwin') and platform_machine!='aarch64' and platform_python_implementation != 'PyPy'

$ pip install -r requirements-build.txt 
Collecting numpy==1.17.5
  Using cached numpy-1.17.5.zip (6.4 MB)
  Preparing metadata (setup.py) ... done
ERROR: Cannot install numpy==1.17.3 and numpy==1.17.5 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested numpy==1.17.5
    The user requested numpy==1.17.3

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/user_guide/#fixing-conflicting-dependencies

However, when installing said package, pip silently downgrades the package to the previous version without conflicting requirements:

$ pip install oldest-supported-numpy      
Collecting oldest-supported-numpy
  Using cached oldest_supported_numpy-0.13-py3-none-any.whl (3.8 kB)
Collecting numpy==1.17.5
  Using cached numpy-1.17.5.zip (6.4 MB)
  Preparing metadata (setup.py) ... done
Collecting oldest-supported-numpy
  Using cached oldest_supported_numpy-0.12-py3-none-any.whl (3.8 kB)
Collecting numpy==1.17.3
  Using cached numpy-1.17.3.zip (6.4 MB)
  Preparing metadata (setup.py) ... done
Building wheels for collected packages: numpy
[etc.]

It should at least display a warning about the unexpected downgrade (or perhaps better, error out).

Expected behavior

No response

pip version

21.3.1

Python version

3.8.10

OS

Ubuntu 20.04.3

How to Reproduce

See description above.

Output

No response

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[pitrou]

Just for the record, in my case a simple warning would have been satisfactory. The problem was being unable to easily diagnose the issue, since pip was doing something totally unexpected without saying anything about it (and since multiple packages were involved, finding the root cause wasn’t trivial).

[pfmoore]

OK, for a given set of supported environments. That’s a very good approach, but something only the individual project can do, which I agree is the correct solution here. My comment was more about the backend doing the validation at build time, not the project itself.