[GHC] #7401: Can't derive instance for Eq when datatype has no constructor, while it is trivial do do so.

[GHC]

#7401: Can't derive instance for Eq when datatype has no constructor, while it is
trivial do do so.
---------------------------------+------------------------------------------
    Reporter:  jpbernardy        |       Owner:  monoidal                 
        Type:  feature request   |      Status:  new                      
    Priority:  normal            |   Milestone:  7.8.1                    
   Component:  Compiler          |     Version:  7.6.1                    
    Keywords:  deriving          |          Os:  Unknown/Multiple         
Architecture:  Unknown/Multiple  |     Failure:  GHC rejects valid program
  Difficulty:  Unknown           |    Testcase:                           
   Blockedby:                    |    Blocking:                           
     Related:                    |  
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Comment(by simonpj):

 Thanks for helping. Concerning your question, I think it would be fine for
 `-XEmptyDataDecls` to imply that we can derive `Eq` etc for them. No need
 for a second flag.

 What would be good would be some documentation in the user manual (in the
 empty data decls section) that says what is derived for the standard
 build-in classes.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7401#comment:10>
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