Resolve #2593: Lift global database restrictions

[phadej]

I'm not sure whether cabal-install should really check that we have recent enough GHC.

I tried this shortly with composition package, cabal repl session:

λ *Data.Composition > let x f g = f .: g
λ *Data.Composition > :t x
x :: (c -> d) -> (a -> b -> c) -> a -> b -> d
λ *Data.Composition > :t String

<interactive>:1:1: Not in scope: data constructor ‘String’

[dcoutts]

Yeah, I think it should check the ghc version, otherwise it still fails in an unpleasent way on older ghc.

[phadej]

I'll add the check, I'll believe trac that the functionality is supported from GHC 7.6.1 and up.

[dcoutts]

Cheers.

[dcoutts]

Great. We'll also need to change the conversion of the db stack into ghc/ghc-pkg flags, as it has the same restriction. We've got someone at ZuriHac working on that now, so we can combine those and commit them together.

[phadej]

👍

[snoyberg]

LGTM, and would be very nice to have.

[23Skidoo]

Looks good, so I see no reason not to merge. The second part can be done later.

[snoyberg]

Awesome, thanks @23Skidoo!

[snoyberg]

And of course thanks @phadej for writing it :)

[23Skidoo]

Opened #2772 for the remaining part.

@snoyberg Thanks for the heads-up!