[GHC] #7777: ghc panic: varargs + sets

[GHC]

#7777: ghc panic: varargs + sets
-------------------------------+--------------------------------------------
Reporter:  litherum            |          Owner:                                                                                     
    Type:  bug                 |         Status:  new                                                                                
Priority:  normal              |      Component:  Compiler                                                                           
 Version:  7.4.1               |       Keywords:  MultiParamTypeClasses FunctionalDependencies FlexibleInstances UndecidableInstances
      Os:  Linux               |   Architecture:  Unknown/Multiple                                                                   
 Failure:  Compile-time crash  |      Blockedby:                                                                                     
Blocking:                      |        Related:                                                                                     
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 This program:

 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE UndecidableInstances #-}

 import qualified Data.Set as S

 class BuildSet a b | b -> a
   where buildset' :: S.Set a -> a -> b

 instance Ord a => BuildSet a (S.Set a)
   where buildset' s i = S.insert i s

 instance Ord a => BuildSet a b => BuildSet a (a -> b)
   where buildset' s i = \ i2 -> buildset' (S.insert i s) i2

 buildset :: BuildSet a b => a -> b
 buildset = buildset' S.empty

 s1 :: S.Set Integer
 s1 = buildset 3

 s2 :: S.Set Integer
 s2 = buildset 1 2 3

 s3 :: S.Set Integer
 s3 = buildset 8 4 2 1 2 4 8 18

 main = do
   putStrLn $ show l1
   putStrLn $ show l2
   putStrLn $ show l3

 produces this output when compiled:

 $ ghc --make test.hs
 [1 of 1] Compiling Main             ( test.hs, test.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.4.1 for x86_64-unknown-linux):
         compiler/rename/RnSource.lhs:429:14-81: Irrefutable pattern failed
 for pattern Data.Maybe.Just (inst_tyvars,
                                                         _,
                                                         SrcLoc.L _ cls,
                                                         _)


 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

 $ ghc --version
 The Glorious Glasgow Haskell Compilation System, version 7.4.1

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7777>
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