[GHC] #3468: GHC panic: boxy_match_s

[GHC]

#3468: GHC panic: boxy_match_s
--------------------------+-------------------------------------------------
 Reporter:  wkahl         |          Owner:         
     Type:  bug           |         Status:  new    
 Priority:  normal        |      Milestone:         
Component:  Compiler      |        Version:  6.10.4 
 Severity:  minor         |     Resolution:         
 Keywords:  boxy_match_s  |       Testcase:         
       Os:  Linux         |   Architecture:  powerpc
--------------------------+-------------------------------------------------
Comment (by wkahl):

 The `boxy_match_s` panic must have come from `TcUnify.boxy_match`, in the
 following case:

 {{{
     go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
         | tc1 == tc2
         , not $ isOpenSynTyCon tc1
         = go_s tys1 tys2
 }}}

 Probably the two arguments came from the unchanged .lhs-boot file and from
 the changed .lhs file, so `tys1` and `tys2` had different lengths. Since
 this is still under the heading "Approximate boxy matching", I'd guess you
 may want to
 either check for the length here where you are closer to the reason of the
 failure, or just return `subst` instead of panic in `boxy_match_s`, and in
 either case let the error be caught by some non-approximate pass.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3468#comment:3>
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