Panic "lookupVarEnv_NF" when using a functional dependency with a kind variable

The following code causes a panic:

{-# LANGUAGE PolyKinds, DataKinds, MultiParamTypeClasses,
             FunctionalDependencies
 #-}

class Foo a (b :: k) | a -> k
instance Foo Bool False

The error is

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.7.20120807 for x86_64-apple-darwin):
	lookupVarEnv_NF: Nothing

In previous versions of GHC 7.5.x, a functional dependency of this nature worked just fine and had the expected behavior of fixing the value of k once a was known. It's unclear to me whether this should compile or not, but it shouldn't cause a panic. I believe that my code that used the functional dependency could use a -> b in place of a -> k with no ill effect, so turning this into an error is not a problem.

Trac metadata

Trac field	Value
Version	7.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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