[GHC] #5951: Panic on Malformed instance A => B => C

[GHC]

#5951: Panic on Malformed instance A => B => C
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  Reporter:  br1                       |          Owner:         
      Type:  bug                       |         Status:  closed 
  Priority:  normal                    |      Milestone:  7.6.1  
 Component:  Compiler                  |        Version:  7.4.2  
Resolution:  fixed                     |       Keywords:         
        Os:  Unknown/Multiple          |   Architecture:  x86    
   Failure:  Compile-time crash        |     Difficulty:  Unknown
  Testcase:  rename/should_fail/T5951  |      Blockedby:         
  Blocking:                            |        Related:         
---------------------------------------+------------------------------------
Changes (by guest):

  * version:  7.4.1 => 7.4.2
  * architecture:  Unknown/Multiple => x86


Comment:

 Hello,

 I get the same error on GHC 7.4.2 with the following (slightly different)
 example:

 instance ExprCoercion a -> b where
     fromExpr (FunctionExpr fn) = fn
     toExpr fn = FunctionExpr $ \expr -> toExpr $ fn (fromExpr expr)


 *** Exception: compiler/rename/RnSource.lhs:430:14-81: Irrefutable pattern
 failed for pattern Data.Maybe.Just (inst_tyvars,
                                                         _,
                                                         SrcLoc.L _ cls,
                                                         _)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5951#comment:9>
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