[GHC] #3418: Equality constraint causes ghc panic

[GHC]

#3418: Equality constraint causes ghc panic
------------------------------------------------------+---------------------
Reporter:  blarsen                                    |          Owner:                
    Type:  bug                                        |         Status:  new           
Priority:  normal                                     |      Component:  Compiler      
 Version:  6.10.4                                     |       Severity:  normal        
Keywords:  type families, equality constraint, panic  |       Testcase:                
      Os:  Linux                                      |   Architecture:  x86_64 (amd64)
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 I have the following module:


 {{{
 {-# LANGUAGE TypeFamilies #-}
 module GhcBug where

 newtype (a ~ b) => S a b = S { unS :: a }
 }}}


 Loading this in ghci results in this:


 {{{
 > :l GhcBug
 [1 of 1] Compiling GhcBug           ( GhcBug.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 6.10.4 for x86_64-unknown-linux):
         typePrimRep <pred>a{tv a1MH} [tv] ~ b{tv a1MI} [tv]

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

 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3418>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler