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GHC will panic when given either of the following modules. The error messages are somewhat different for the two different rules.

module GHCBug1 where

foo :: (forall a . a -> f a) -> f Int
foo f = f undefined

{-# RULES
    "bug1"
    forall (g :: a -> f a) .
    head (foo g) = undefined
 #-}

module GHCBug2 where

foo :: (forall a . a -> f a) -> f Int
foo f = f undefined

{-# RULES
    "bug2"
    forall (g :: forall a . a -> f a) .
    head (foo g) = undefined
 #-}

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
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Operating system
Architecture