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

[GHC]

#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
      Resolution:                    |             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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Description changed by chris-martin:

Old description:

> 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.

New description:

 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 and derive-and-class:

 {{{#!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.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13314#comment:2>
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