Panic with incorrect pattern synonym signature

{-# LANGUAGE PatternSynonyms #-}
module Foo () where

data A a = A a

pattern Q :: () => (A ~ f) => a -> f a
pattern Q a = A a

error with HEAD

poly-export2.hs:7:9: error:ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.11.20151030 for x86_64-apple-darwin):
	No skolem info: f_amp[sk]

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
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