[GHC] #5147: GHCi ignores missing instance

[GHC]

#5147: GHCi ignores missing instance
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    Reporter:  boris                  |        Owner:                             
        Type:  bug                    |       Status:  new                        
    Priority:  normal                 |    Milestone:  7.4.1                      
   Component:  GHCi                   |      Version:  7.0.2                      
    Keywords:  linker, data families  |     Testcase:                             
   Blockedby:                         |   Difficulty:                             
          Os:  Unknown/Multiple       |     Blocking:                             
Architecture:  Unknown/Multiple       |      Failure:  GHC accepts invalid program
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Comment(by simonpj):

 Thanks for identifying this bug, and boiling it down so nicely.  It turned
 out that the recompilation check was wrong, which is always a subtle one
 to even detect reproducibly, so thank you!

 Well the first commit turns out not to be relevant at all!  (Although it
 is good stuff.)

 The secont patch contains the fix.  Next: add a test case.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5147#comment:4>
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