[GHC] #7035: `deriving instance Generic (HsExpr Id)` raises panic

[GHC]

#7035: `deriving instance Generic (HsExpr Id)` raises panic
----------------------------------+-----------------------------------------
    Reporter:  Daniel Schuessler  |       Owner:  dreixel           
        Type:  bug                |      Status:  new               
    Priority:  normal             |   Milestone:                    
   Component:  Compiler           |     Version:  7.4.2             
    Keywords:                     |          Os:  Linux             
Architecture:  Unknown/Multiple   |     Failure:  Compile-time crash
  Difficulty:  Unknown            |    Testcase:                    
   Blockedby:                     |    Blocking:                    
     Related:                     |  
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Changes (by simonpj):

  * owner:  => dreixel
  * difficulty:  => Unknown


Comment:

 With HEAD we get
 {{{
 ~/tmp$ ~/5builds/HEAD-1/inplace/bin/ghc-stage2 -c T7035.hs -package ghc

 T7035.hs:8:1:
     Can't make a derived instance of `Generic (HsExpr Id)':
       HsExpr must not be instantiated; try deriving `HsExpr Id' instead
     In the stand-alone deriving instance for `Generic (HsExpr Id)'
 }}}
 Better than a crash, but the error message is messed up. Should say
 something like `try deriving (HsExpr a) instead`, or something.  Currently
 it says "X doesn't work, so try X" which is stupid.

 Pedro, can you look at this?

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7035#comment:1>
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