Given

{-# LANGUAGE PatternSynonyms, ViewPatterns #-}
module Pat (Vec2(Nil,(:>)), pattern (:>)) where

newtype Vec2 a = Vec2 {unvec2 :: [a]}

pattern Nil :: Vec2 a
pattern Nil = Vec2 []

pattern (:>) x xs <- ((\ys -> (head $ unvec2 ys,Vec2 . tail $ unvec2 ys)) -> (x,xs))
  where
    (:>) x xs = Vec2 (x:unvec2 xs)

and

{-# LANGUAGE PatternSynonyms #-}
module Main where

import Pat (Vec2(..),pattern (:>))

I get:

$ ghci Main.hs 
GHCi, version 8.0.0.20160411: http://www.haskell.org/ghc/  :? for help
[1 of 2] Compiling Pat              ( Pat.hs, interpreted )

Pat.hs:2:29: warning: [-Wduplicate-exports]
    ‘:>’ is exported by ‘(:>)’ and ‘Vec2(Nil, type (:>))’
[2 of 2] Compiling Main             ( Main.hs, interpreted )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.0.20160411 for x86_64-unknown-linux):
	filterImports/combine
  (:>, :>, Nothing)
  (:>, Vec2{Vec2, :>, Nil}, Nothing)

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Now, the warning about the duplicate export is of course correct. But the panic shouldn't happen when I try to import both the associated pattern synonym and the normal pattern synonym.