[GHC] #1214: Iface type variable out of scope

[GHC]

#1214: Iface type variable out of scope
----------------------------------------+-----------------------------------
    Reporter:  igloo                    |       Owner:         
        Type:  bug                      |      Status:  new    
    Priority:  normal                   |   Milestone:  6.6.2  
   Component:  Compiler (Type checker)  |     Version:  6.6    
    Severity:  normal                   |    Keywords:         
  Difficulty:  Unknown                  |    Testcase:  gadt23 
Architecture:  Unknown                  |          Os:  Unknown
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Christophe Poucet reports the below bug; the 6.6 branch is broken, but the
 HEAD works.

 ----

 While working some code I stumbled into a bug with GHC. Basically I have a
 GADT that has a phantom-type for all but one of it's data constructors.
 When
 I try to store a record-type in that tag it gives me a problem due to
 interfaces when I recompile something against the interface file
 containing
 the GADT.  Namely, it complains that the type variable is out of scope.

 The code in question (after treeing to find out what the actual problem
 was,
 namely the combination of GADTs with records) is the following:
 {{{
 -- AST.hs
 {-# OPTIONS_GHC -fglasgow-exts #-}
 module AST where

 data Exp_;

 data AST :: * -> * -> * where
   Var   :: String -> AST Exp_ tag
   Tag   :: tag    -> AST a tag -> AST a tag
 }}}
 {{{
 -- Main.hs
 module Main where
 import AST

 data Foo = Foo { bar :: Int }

 convert :: AST a tag -> AST a Foo
 convert t = case t of
   Var v     -> Tag (Foo 42) $ Var v
   Tag t e   -> Tag (Foo 42) $ convert e

 main = return ()
 }}}

 The bug is easily replicated with the following commands:

 {{{
 > ghc --make Main.hs
   -- Works fine
 > touch Main.hs
 > ghc --make Main.hs
   --   The interface for `main:AST'
   --   Declaration for AST
   --   Constructor AST.Tag:
   --     Iface type variable out of scope:  tag
   --   Cannot continue after interface file error
 > rm -f Main.hi Main.o AST.hi AST.o
 > ghc --make Main.hs
   -- Works fine
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1214>
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