[GHC] #13154: Standalone-derived anyclass instances aren't as permissive as empty instances

[GHC]

#13154: Standalone-derived anyclass instances aren't as permissive as empty
instances
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        Reporter:  RyanGlScott       |                Owner:  RyanGlScott
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  Generics
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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Changes (by RyanGlScott):

 * milestone:   => 8.4.1


Comment:

 Another thing that standalone `DeriveAnyClass` currently doesn't allow
 (but bare instances do) is the use of nullary classes. That is, this is
 allowed:

 {{{#!hs
 {-# LANGUAGE MultiParamTypeClasses #-}
 module Allowed where

 class C
 instance C
 }}}

 but this is not:

 {{{#!hs
 {-# LANGUAGE DeriveAnyClass #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE StandaloneDeriving #-}
 module Bug where

 class C
 deriving instance C
 }}}

 {{{
 Bug.hs:7:1: error:
     • Cannot derive instances for nullary classes
     • In the stand-alone deriving instance for ‘C’
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13154#comment:2>
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