Incompleteness of type inference: must quantify over implication constraints

Consider this program (from Iavor)

f x = let g y = let h :: Eq c => c -> ()
                    h z = sig x y z
                in ()
      in fst x

sig :: Eq (f b c) => f () () -> b -> c -> ()
sig _ _ _ = ()

This example is rejected by both Hugs and GHC but I think that it is a well typed program. The Right Type to infer for g is this:

g :: forall b. (forall c. Eq c => Eq (b,c)) => b -> ()

but GHC never quantifies over implication constraints at the moment. As a result, type inference is incomplete (although in practice no one notices). I knew about this, but I don't think it's recorded in Trac, hence this ticket.

Following this example through also led me to notice a lurking bug: see "BUG WARNING" around line 715 of TcSimplify.

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown