Add fake-depends

[phadej]

Describe the feature request

fake-depends is a field like build-depends which would constraint solver, but otherwise do nothing.

It's sometimes (rarely, but nevertheless) needed to add a constraint, but the dependency is not (at least directly) used anywhere. It would be great if .cabal format had a way to explicitly specify that.

One (bad) example is to use fake-depends: QuickCheck with build-depends: tasty-quickcheck. You can import everything through tasty-quickcheck re-exports, but you may need to constraint underlying QuickCheck. Arguably tasty-quickcheck shouldn't re-export anything, or at least properly version own (re)exports. But here we are.

See export-depends in #11052 for another semantically more precise dependency specification field.

[tbidne]

Similar example: A dependency d has an incorrect bound on t (e.g. no upper bound), hence the user of d may currently need to add a dependency on t -- just to tighten the bound -- even though t is purely transitive. I recall having to do this when aeson-2 came out.

Bikeshedding: Could also be called solver-depends, solving-depends.

[sebright]

I think this is the same feature as #9558.

[geekosaur]

I'm not convinced this belongs in cabal. For sure, I actually make use of the first example (did so 2 days ago, in fact), but I just used build-depends and disabled -Wall since my App.hs was main = putStrLn "hi". The other example is best covered by using cabal.project.local while petitioning the Hackage trustees for a metadata update on the broken package; doing it in a cabal file not only distributes the pain to victims, it hides the symptoms of the actual problem.

[phadej]

@geekosaur

Suggesting -Wno-.... is not an option. The .cabal format should allow us to express things in a way so we don't need to disable legit warnings. (Of course ignoring a warning is an option, but there SHOULD be a way to do things "correctly").

using cabal.project.local is not an option for packages to be uploaded on Hackage. We "bundle" the extra constraints into uploaded metadata. And this issue is exactly about being able to do that.

(Don't mix Cabal and cabal-install issue. cabal.project.local won't work for stack users / Stackage curators).