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

[GHC]

#7401: Can't derive instance for Eq when datatype has no constructor, when it's
trivial do do so.
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Reporter:  jpbernardy        |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.6.1             |       Keywords:  deriving        
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
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 On a "phantom datatype" D, one gets the message:

 Can't make a derived instance of `Eq D':
       `D' must have at least one data constructor

 However there is a trivial, correct instance of Eq D:

 instance Eq D where
   (==) = undefined

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