[Haskell-cafe] Enabling GADTs breaks Rank2Types code compilation - Why?

[dm-list-haskell-cafe at scs.stanford.edu]

I'm using GHC 7.0.2 and running into a compiler error that I cannot
understand.  Can anyone shed light on the issue for me?  The code does
not make use of GADTs and compiles just fine without them.  But when I
add a {-# LANGUAGE GADTs #-} pragma, it fails to compile.

Here is the code:

====

{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE GADTs #-}

mkUnit :: (forall t. t -> m t) -> m ()
mkUnit f = f ()

data Const b a = Const { unConst :: b }

sillyId :: r -> r
sillyId r = unConst (mkUnit mkConst_r) -- This line can't compile with GADTs
    where mkConst_r t = mkConst r t
          mkConst :: b -> t -> Const b t
          mkConst b _ = Const b

====

And the error I get is:

    Couldn't match type `t0' with `t'
      because type variable `t' would escape its scope
    This (rigid, skolem) type variable is bound by
      a type expected by the context: t -> Const r t
    The following variables have types that mention t0
      mkConst_r :: t0 -> Const r t0
        (bound at /u/dm/hack/hs/gadt.hs:11:11)
    In the first argument of `mkUnit', namely `mkConst_r'
    In the first argument of `unConst', namely `(mkUnit mkConst_r)'
    In the expression: unConst (mkUnit mkConst_r)

I've found several workarounds for the issue, but I'd like to
understand what the error message means and why it is caused by GADTs.

Thanks in advance for any help.

David