add a command to check if an alternative version or constraint for a package would have worked

[dhellmann]

When we build a collection of wheels but can't resolve a co-installable set, it would help to have a command to put into the graph file to check if we had introduced a constraint or added another version to the build whether that would have produced an installable set of packages.

This is going to be tricky to write.

[LalatenduMohanty]

One use case comes to my mind is when

A pkg depends on D (any version of D)
B pkg depends on D (any version of D)
C pkg depends on D (any version of D , however works with specific version of D)

However we have not set any constraint for D, so the build of the collection (with A, B, C) fails. But if we can figure-out that adding a constraint for a version D using the command , it will help in general. @dhellmann does this sounds like the usecase you had in mind? Or do you have other use-cases?

[dhellmann]

I think the case you describe isn't something we could figure out without more information added to C's dependencies.

I was thinking of cases that would be solved by pip's backtracking behavior. If A, B, and C resolve to 3 different versions of D but none of those 3 versions of D could be used by all of A, B, and C, pip will backtrack to look at older versions of D to see if they would work. We could try to do something similar, looking for versions of D that exist on pypi and that would have worked for all of A, B, and C. We could do it as a standalone command using a graph file as input, and we could add it to the bootstrap command for when we try to generate constraints but can't.