panic: Unify.uTys_s: mismatched type lists!

This erroneous program provokes a panic instead of a type error, with GHC 6.8.3, 6.10.4 and 6.12.1. I don't have any recent snapshots to hand to try.

{-# LANGUAGE GADTs #-}

data Id p x y = Id (p x y)

data Sealed p where
   Sealed :: p x -> Sealed p

rp :: Bool -> Maybe (w (Id p))
rp _ = Just rp'
     where rp' :: Sealed (Id p x)
           rp' = undefined

ghc: panic! (the 'impossible' happened)
  (GHC version 6.12.1 for x86_64-unknown-linux):
        Unify.uTys_s: mismatched type lists!

Trac metadata

Trac field	Value
Version	6.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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