Panic: promoteType with higher-rank datatype

The following code causes the panic in HEAD:

{-# LANGUAGE DataKinds, RankNTypes, TypeFamilies #-}

data HigherRank = HR (forall a. a -> a)

type family F (x :: HigherRank)
type instance F (HR x) = Bool

Here is the panic text:

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.7.20130329 for x86_64-apple-darwin):
	promoteType

This panic does not happen with GHC 7.6.1.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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