[GHC] #8241: GHCi showing its internals with malformed instance

[GHC]

#8241: GHCi showing its internals with malformed instance
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       Reporter:  crntaylor         |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.4.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Using ghci 7.4.2 I observe the following behaviour when I enter a
 malformed instance for a type class:

     Prelude> instance Eq a => Eq b => Eq (a -> b) where (==) = undefined
     *** Exception: compiler\rename\RnSource.lhs:430:14-81: Irrefutable
 pattern failed for pattern Data.Maybe.Just (inst_tyvars,
                                                             _,
                                                             SrcLoc.L _
 cls,
                                                             _)

 I assume that ghci shouldn't be showing its internals like this?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8241>
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