[GHC] #13671: Core lint error with PatternSynonyms and undefined

[GHC]

#13671: Core lint error with PatternSynonyms and undefined
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           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms                    |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I've submitted quite a few Core Lint errors lately, I don't have access to
 GHC HEAD so ignore this ticket if this has already been resolved

 {{{#!hs
 {-# Options_GHC -dcore-lint #-}

 {-# Language ViewPatterns, PatternSynonyms #-}

 import Data.Bifunctor.Fix
 import Data.Bifunctor.Tannen

 newtype ZipList a = ZL { getZipList :: Fix (Tannen Maybe (,)) a }

 pattern Nil :: ZipList a
 pattern Nil = ZL (In (Tannen Nothing))

 infixr 5 :::
 pattern (:::) :: a -> ZipList a -> ZipList a
 pattern a:::as <- ZL (In (Tannen (undefined -> (a, as))))
   where a:::as = undefined
 }}}

 `↑` works fine without `-dcore-lint` but fails with it.

 Works fine if `undefined -> (a, as)` is replaced with `let x = x in x ->
 (a, as)`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13671>
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