No Skolem Info with PartialTypeSignatures

The following program triggers a panic:

{-# LANGUAGE DataKinds             #-}
{-# LANGUAGE PartialTypeSignatures #-}
{-# LANGUAGE PolyKinds             #-}
{-# LANGUAGE TypeFamilies          #-}
{-# LANGUAGE TypeOperators         #-}

barWraper :: ('(r,r') ~ Head rngs, Foo rngs) => FooData rngs
barWraper = bar

bar :: (_) => FooData rngs
bar = foo

data FooData rngs

class Foo xs where foo :: (Head xs ~ '(r,r')) => FooData xs

type family Head (xs :: [k]) where Head (x ': xs) = x

> ghci NoSkolem.hs
[1 of 1] Compiling Main             ( NoSkolem.hs, NoSkolem.o )

NoSkolem.hs:8:13: error:ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.1 for x86_64-unknown-linux):
	No skolem info: k_aYV[sk]

I haven't tested with 8.0.2 or head.