[GHC] #15012: "Iface type variable out of scope" when compiling with -c

7 Apr 2018

      #15012: "Iface type variable out of scope" when compiling with -c
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.1
           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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 Take these two files:

 {{{#!hs
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE TypeFamilies #-}
 module Foo where

 import GHC.Generics

 type FakeOut a = Int

 data family   TyFamily y z
 data instance TyFamily a b = TyFamily Int (FakeOut b)
   deriving Generic1
 }}}
 {{{#!hs
 module Bar where

 import Foo
 import GHC.Generics

 main :: IO ()
 main = print $ from1 $ TyFamily 1 2
 }}}

 And compile them like so:

 {{{
 $ ghc -c Foo.hs
 $ ghc -c Bar.hs
 ./Foo.hi
 Declaration for Rep1_R:TyFamilyab
 Axiom branches Rep1_R:TyFamilyab:
   Iface type variable out of scope:  b
 Cannot continue after interface file error
 }}}

 I can reproduce this with every version of GHC from 7.8.4 onward, so this
 is quite an old bug!

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15012
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