Redundant allow-newer warnings

[TeofilC]

Summary

When upgrading to a new version of a compiler, it's normal to add a bunch of allow-newers to one's cabal.project file. Over time these grow to be redundant as packages raise their upper bounds.

I then find myself iteratively going through my cabal.project file and testing whether each of these is still needed. This is quite labour intensive.

Suggestion

Add a warning if an allow-newer is no longer needed. Ideally this would point out the line in the cabal.project file where the override is specified. Then I could just react to these warnings when updating my index-state and I wouldn't have to manually check.

[ulysses4ever]

It sounds like a great feature to me: I have found my self in this same situation not once. Thanks for proposing!

/cc @sebright

[sebright]

I think that it would be very difficult to determine whether each --allow-newer is still needed with 100% certainty with the solver, but we could implement the feature in other ways. Here are two ideas:

1. If package A depends on package B with --allow-newer, check whether the bounds of the latest version of package A allow the latest version of package B.
2. Run the solver. Then check whether each --allow-newer was used or the dependency is within bounds.

I wonder if anyone is using --allow-newer to get a better install plan when a solution was already possible. It that case, they might want to suppress the warning. (EDIT: One benefit of option (2) is that it is less likely to change the install plan.)