[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:  7.4.1           
Resolution:  fixed                     |       Keywords:                  
        Os:  Unknown/Multiple          |   Architecture:  Unknown/Multiple
   Failure:  Compile-time crash        |     Difficulty:  Unknown         
  Testcase:  rename/should_fail/T5951  |      Blockedby:                  
  Blocking:                            |        Related:                  
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Comment(by dag):

 I couldn't reproduce the issue without equality constraints, so those may
 be necessary:

 {{{
 {-# LANGUAGE FlexibleInstances, TypeFamilies, MultiParamTypeClasses #-}

 import qualified Data.Set as Set

 class IsList a b c where
     fromList :: [b] -> a c

 -- Note the missing => before IsList here
 instance (Ord b, b ~ c) IsList Set.Set b c where
     fromList = Set.fromList
 }}}

 {{{
 *** Exception: compiler/rename/RnSource.lhs:429:14-81: Irrefutable pattern
 failed for pattern Data.Maybe.Just (inst_tyvars,
                                                         _,
                                                         SrcLoc.L _ cls,
                                                         _)

 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5951#comment:6>
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