[GHC] #9985: GHC panic with ViewPatterns and GADTs in a proc pattern

[GHC]

#9985: GHC panic with ViewPatterns and GADTs in a proc pattern
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              Reporter:  Rafbill     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.8.4
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 The following code causes a ghc panic with versions 7.8.4 and 7.9.

 {{{#!hs
 module A where

 {-# LANGUAGE GADTs, ViewPatterns, PatternSynonyms, DataKinds, Arrows,
 TypeOperators, TypeFamilies, UndecidableInstances #-}

 import Control.Arrow

 data Nat = Z | S Nat
 data Vec n a where
   VNil  :: Vec Z a
   VCons :: a -> Vec n a -> Vec (S n) a
 viewVNil :: Vec Z a -> ()
 viewVNil VNil = ()
 viewVCons :: Vec (S n) a -> (a, Vec n a)
 viewVCons (VCons a as) = (a, as)
 pattern (:>) :: a -> Vec n a -> Vec (S n) a
 pattern a :> as <- (viewVCons -> (a, as))
 pattern VNil' <- (viewVNil -> ())

 type family n + m where
   n + Z   = n
   n + S m = S (n + m)

 type family P2 n where
   P2 Z     = S Z
   P2 (S n) = P2 n + P2 n

 class A n where
   a :: Arrow b => b (Vec (P2 n) a) a
 instance A Z where
   a = proc (a :> VNil) -> returnA -< a

 }}}

 If the pattern (a :> VNil) is changed to (a :> _) or (a :> VNil'), the
 code compiles.

 GADTs pattern are not allowed to appear in proc patterns, but view
 patterns seems to be able to bypass this restriction, #9953, and the check
 on subpatterns.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9985>
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