[GHC] #14934: Repeated "impossible" go_axiom_rule error.

[GHC]

#14934: Repeated "impossible" go_axiom_rule error.
-------------------------------------+-------------------------------------
        Reporter:  galen             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.4.1
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * status:  infoneeded => new


Comment:

 I can reproduce the issue now. Take these two files:

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE TypeOperators #-}
 module Foo where

 import GHC.TypeLits

 data Foo :: Nat -> * where
   MkFoo0 :: Foo 0
   MkFoo1 :: Foo 1

 f :: Foo (1 - 0) -> Foo 1
 f x = x
 }}}

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeOperators #-}
 module Bar where

 import Foo
 import GHC.TypeLits

 g :: Foo (1 - 0)
 g = f MkFoo1

 {-
 h :: Foo (1 - 0)
 h = MkFoo1
 -}
 }}}

 And perform the following steps:

 1. Run `/opt/ghc/8.2.2/bin/ghc Bar.hs -O2`.
 2. Uncomment out the definition of `h` in `Bar.hs`.
 3. Re-run `/opt/ghc/8.2.2/bin/ghc Bar.hs -O2`:

 {{{
 $ /opt/ghc/8.2.2/bin/ghc Bar.hs -O2
 [1 of 2] Compiling Foo              ( Foo.hs, Foo.o )
 [2 of 2] Compiling Bar              ( Bar.hs, Bar.o )
 $ vim Bar.hs
 $ /opt/ghc/8.2.2/bin/ghc Bar.hs -O2
 [2 of 2] Compiling Bar              ( Bar.hs, Bar.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.2 for x86_64-unknown-linux):
         go_axiom_rule
   Sub0R
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler/iface/TcIface.hs:1349:15 in
 ghc:TcIface
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14934#comment:3>
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