[GHC] #10997: Pattern synonym causes Iface error.

[GHC]

#10997: Pattern synonym causes Iface error.
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        Reporter:  mpickering        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mpickering):

 Yes, see the attached file and run "stack build" in p2/ is one way.

 Another way to trigger it is to have, two files `Foo.hs` and `Bar.hs` as
 follows. Then run the following commands..

 {{{
 ghc Bar.hs
 vim Bar.hs
 .... make a simple change...
 ghc Bar.hs
 }}}

 {{{
 [2 of 2] Compiling Bar              ( Bar.hs, Bar.o )
 The interface for ‘Foo’
 Declaration for Just'
 Pattern synonym Just':
   Iface type variable out of scope:  k
 Cannot continue after interface file error
 }}}


 {{{
 {-# LANGUAGE PatternSynonyms, ViewPatterns, ConstraintKinds, TypeFamilies,
 PolyKinds, KindSignatures #-}
 module Foo where

 import GHC.Exts

 type family Showable (a :: k) :: Constraint where
   Showable (a :: *) = (Show a)
   Showable a       = ()

 extractJust :: Maybe a -> (Bool, a)
 extractJust (Just a) = (True, a)
 extractJust _        = (False, undefined)

 pattern Just' :: () => (Showable a) => a -> (Maybe a)
 pattern Just' a <- (extractJust -> (True, a)) where
   Just' a = Just a
 }}}

 {{{
 module Bar where

 import Foo

 bar :: (Showable a) => Maybe a -> Maybe a
 bar (Just' a) = Just' a
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10997#comment:3>
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