GADT + impredicative polymorphism => stack overflow

[guest]

The following makes the GHC 6.6 type checker loop/overflow:

{-# OPTIONS_GHC -fglasgow-exts #-}
data Foo a b where F :: a -> Foo () a

bar :: Foo () (forall a.a) -> ()
bar (F _) = ()

ghc yields stack overflow, as does ghci. Reloading in GHCi leads to an infinite loop.

[simonpj]

Great bug. I'd written a loop in the type-refinement code, which only shows up when forall types are involved. Now fixed.

Alas no decent workaround for 6.6. The fix will be in 6.6.1 though. And the HEAD is fine.

Simon