[GHC] #3950: panic: Unify.uTys_s: mismatched type lists!

[GHC]

#3950: panic: Unify.uTys_s: mismatched type lists!
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    Reporter:  ganesh                   |        Owner:  igloo                          
        Type:  merge                    |       Status:  new                            
    Priority:  normal                   |    Milestone:                                 
   Component:  Compiler (Type checker)  |      Version:  6.8.3                          
    Keywords:                           |   Difficulty:                                 
          Os:  Unknown/Multiple         |     Testcase:  typecheck/should_compiler/T3950
Architecture:  Unknown/Multiple         |      Failure:  Compile-time crash             
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Changes (by simonpj):

  * owner:  => igloo
  * testcase:  => typecheck/should_compiler/T3950
  * type:  bug => merge


Comment:

 Fixed in HEAD by
 {{{
 Mon Apr 12 16:18:45 GMT Daylight Time 2010  simonpj at microsoft.com
   * Fix Trac #3950: unifying types of different kinds

   I was assuming that the unifer only unified types of the
   same kind, but now we can "defer" unsolved constraints that
   invariant no longer holds.  Or at least is's more complicated
   to ensure.

   This patch takes the path of not assuming the invariant, which
   is simpler and more robust.  See
   Note [Mismatched type lists and application decomposition]
 }}}
 Try merging to 6.12 branch

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3950#comment:1>
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