LANGUAGE ConstraintKinds not needed to use ConstraintKinds?

[Evan Laforge]

I recently upgraded to ghc 8 and started using stacks via
ImplicitParams.  For that I wind up using 'type Stack = (?stack ::
CallStack)' and so ContraintKinds (I see that in the future GHC will
do this by default).

So now I can have a file like:

module T where
import qualified Log as Log

f :: Log.Stack => IO ()
f = Log.warn "blah blah"

I noticed that now haskell-src-exts refuses to parse this file, saying
'MultiParamTypeClasses language extension is not enabled.'.

I assume it's a bug with haskell-src-exts in that it should require
LANGUAGE ConstraintKinds instead, but then GHC itself doesn't want
ConstraintKinds.  Instead, it wants FlexibleContexts.  From the docs,
FlexibleContexts seems to be about the contexts in instance heads.

Is this intentional?  I'll go ahead and make a bug for
haskell-src-exts, but the ghc behaviour here seems odd as well.  What
extension should haskell-src-exts require to parse this?