[GHC] #13314: StandaloneDeriving and DeriveAnyClass don't work together

21 Feb 2017

      #13314: StandaloneDeriving and DeriveAnyClass don't work together
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           Reporter:  chris-martin   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I'm starting with the `SPretty` example from the `DeriveAnyClass` section
 in the GHC users guide:

 {{{#!hs
 #!/usr/bin/env stack
 -- stack --resolver lts-8.0
 {-# LANGUAGE DefaultSignatures, DeriveAnyClass, StandaloneDeriving #-}

 import Prelude
 import Numeric.Natural (Natural)

 class SPretty a where
     sPpr :: a -> String
     default sPpr :: Show a => a -> String
     sPpr = show
 }}}

 I can write an empty instance for `Natural`:

 {{{#!hs
 instance SPretty Natural where
 }}}

 So then I would expect to be able to write an equivalent definition using
 standalone deriving:

 {{{#!hs
 deriving instance SPretty Natural
 }}}

 But instead it fails with this error:

 {{{
 error:
     • Can't make a derived instance of ‘SPretty Natural’:
         The data constructors of ‘Natural’ are not all in scope
           so you cannot derive an instance for it
     • In the stand-alone deriving instance for ‘SPretty Natural’
 }}}

 It seems like this ought to work; I'm not sure why the constructors should
 need to be in scope, given that the instance can be derived trivially
 without defining any methods.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13314
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