[GHC] #12925: GHC panic: Can't serialise IfaceTcTyVar

[GHC]

#12925: GHC panic: Can't serialise IfaceTcTyVar
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
           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:
-------------------------------------+-------------------------------------
 This bug currently prevents `vector` from being built with GHC HEAD. Here
 is a minimized test case:

 {{{
 module Bug where

 data Foo a x = Foo x

 refoo :: Foo a x -> Foo b x
 refoo (Foo x) = Foo x

 {-# RULES

 "refoo/refoo" forall s.
   refoo (refoo s) = s  #-}
 }}}

 You'll need to compile with optimizations on to trigger this:

 {{{
 $ /opt/ghc/head/bin/ghc -O1 Bug.hs
 [1 of 1] Compiling Bug              (.hs -> .o)

 Bug.hs:10:1: warning: [-Winline-rule-shadowing]
     Rule "refoo/refoo" may never fire
       because ‘refoo’ might inline first
     Probable fix: add an INLINE[n] or NOINLINE[n] pragma for ‘refoo’

 Bug.hs:10:1: warning: [-Winline-rule-shadowing]
     Rule "refoo/refoo" may never fire
       because ‘refoo’ might inline first
     Probable fix: add an INLINE[n] or NOINLINE[n] pragma for ‘refoo’
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.1.20161125 for x86_64-unknown-linux):
         Can't serialise IfaceTcTyVar
   a_ary[sk:1]
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1076:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1080:37 in
 ghc:Outputable
         pprPanic, called at compiler/iface/IfaceType.hs:1331:10 in
 ghc:IfaceType

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 If I had to guess, this commit (
 http://git.haskell.org/ghc.git/commit/5f349fe24066e7b0af85934664e27636d2e84fe5
 ) is responsible. Simon, do you know what's going on here?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12925>
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