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(Imported from Trac #788, reported by @dcoutts on 2011-01-11)
It is a deliberate decision that packages cannot depend on package + flag combinations, only on packages. The point is that flags are not supposed to change the API of a package.
This needs to be enforced.
Consider a real example (from Chart package: http://hackage.haskell.org/packages/archive/Chart/0.14/Chart.cabal)
build-depends: gtk >= 0.9.11
The solution is that Chart should be prevented from conditionally exposing modules. We should add a QA check that looks for exposed modules that are conditional on a flag. It is annoying but somewhat less bad for modules to change between platforms.
Should it be a hard failure or just a warning?