ConstraintKinds lets you put the typechecker into a loop

Ed Kmett was playing with them the other day and Dan Doel and I reduced the issue to this:

{-# LANGUAGE ConstraintKinds, FlexibleContexts, FlexibleInstances, KindSignatures #-}

class C (p :: Constraint)
class D (p :: Constraint)

instance C (D p) => C (D (D p))

{-
-- This works
data E a = E
class F a
instance F (E a) => F (E (E a))
-}

Trac metadata

Trac field	Value
Version	7.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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