[GHC] #3468: GHC panic: boxy_match_s

[GHC]

#3468: GHC panic: boxy_match_s
-------------------------+--------------------------------------------------
Reporter:  wkahl         |          Owner:          
    Type:  bug           |         Status:  new     
Priority:  normal        |      Component:  Compiler
 Version:  6.10.4        |       Severity:  critical
Keywords:  boxy_match_s  |       Testcase:          
      Os:  Linux         |   Architecture:  powerpc 
-------------------------+--------------------------------------------------
 I get, in two different modules in the same 100+ module project
 (different dependency ordering by ghc --make for different executables):

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 6.10.4 for powerpc-unknown-linux):
         boxy_match_s
 }}}

 I found `boxy_match_s` as auxiliary function of `boxyMatchTypes` in
 `compiler/typecheck/TcUnify.lhs`, but, using

 {{{
 find . -type f | xargs grep -n boxyMatchTypes /dev/null
 }}}

 , could not even find a call site of `boxyMatchTypes`!

 The LANGUAGE pragmas of the two modules:

 {{{

 {-# LANGUAGE DeriveDataTypeable, FlexibleContexts,
     MultiParamTypeClasses, FlexibleInstances
     #-}

 {-# LANGUAGE ScopedTypeVariables,
     MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances
     #-}

 }}}

 (I do not consciously use impredicative types anywhere.)

 This occurred after I added additional type parameters (without fundeps)
 to intermediate type classes (with other fundeps) in larger class
 hierarchies, and performed lots of related changes.

 (`make clean` does not help.)

 Hoping that just the knowledge that this can happen leads to a fix...

 Wolfram

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