Panic in tcSimplifyRuleLhs

[rl]

With this module, both 6.8.3 and the HEAD panic in tcSimplifyRuleLhs:

{-# LANGUAGE Rank2Types, ScopedTypeVariables #-}
module Foo where

foo :: (forall m. Monad m => Maybe (m a) -> Maybe (m a)) -> Maybe a -> Maybe a
foo _ x = x

{-# RULES

"foo/foo"
  forall (f :: forall m. Monad m => Maybe (m a) -> Maybe (m a))
         (g :: forall m. Monad m => Maybe (m b) -> Maybe (m b)) x.
  foo f (foo g x) = foo (f . g) x
 #-}

Note that the rule is incorrectly typed; with the correct types, no panic happens.

Incidentially, I'm not sure if ScopedTypeVariables should be required here.

[simonpj]

Good point. I have a patch for this. Simon

[simonpj]

Fixed by

Mon Aug 11 03:44:38 PDT 2008  simonpj@microsoft.com
  * Fix Trac #2494: tcSimplifyRuleLhs

Simon