[GHC] #12542: Unexpected failure.. (bug?)

[GHC]

#12542: Unexpected failure.. (bug?)
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 With

 {{{#!hs
 {-# Language
 GADTs,LambdaCase,TypeOperators,ViewPatterns,ScopedTypeVariables#-}
 {-# Language TypeApplications #-}

 infixr :->
 infixl 1 :$
 data Full a
 data a :-> b

 data AST dom sig where
   Sym  :: dom sig -> AST dom sig
   (:$) :: AST dom (a :-> sig) -> AST dom (Full a) -> AST dom sig

 class Binding dom where
   viewVar :: dom a         -> Maybe Integer
   viewBnd :: dom (a :-> b) -> Maybe Integer

 freeVars :: forall dom a. Binding dom => AST dom a -> [Integer]
 freeVars = \case
   Sym (a -> Just v) -> [v]
   Sym (b -> Just v) :$ body -> undefined

   where
     (a, b) = (viewVar @dom, undefined)
 }}}

 it also gives the error

 {{{
 /tmp/tPYo.hs:20:8: error:
     • Couldn't match expected type ‘dom (a1 :-> a) -> Maybe t1’
                   with actual type ‘t0’
         because type variable ‘a1’ would escape its scope
       This (rigid, skolem) type variable is bound by
         a pattern with constructor:
           :$ :: forall (dom :: * -> *) sig a.
                 AST dom (a :-> sig) -> AST dom (Full a) -> AST dom sig,
         in a case alternative
         at /tmp/tPYo.hs:20:3-27
     • In the pattern: b -> Just v
       In the pattern: Sym (b -> Just v)
       In the pattern: Sym (b -> Just v) :$ body
     • Relevant bindings include
         a :: dom a -> Maybe Integer (bound at /tmp/tPYo.hs:23:6)
         b :: t0 (bound at /tmp/tPYo.hs:23:9)
         freeVars :: AST dom a -> [Integer] (bound at /tmp/tPYo.hs:18:1)
 Failed, modules loaded: none.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12542#comment:4>
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