[GHC] #15839: DerivingStrategies defaulting warning has no associated enable/suppress flag

[GHC]

#15839: DerivingStrategies defaulting warning has no associated enable/suppress
flag
-------------------------------------+-------------------------------------
        Reporter:  tejon             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:  deriving,
                                     |  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * keywords:   => deriving, newcomer


Comment:

 To be more precise, this has nothing to do with `DerivingStrategies`, but
 rather the interaction between `DeriveAnyClass` and
 `GeneralizedNewtypeDeriving`. The following code is a minimal example of
 the issue:

 {{{#!hs
 {-# LANGUAGE DeriveAnyClass #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 module Bug where

 class C a
 newtype T a = MkT a deriving C
 }}}
 {{{
 $ /opt/ghc/8.6.1/bin/ghci Bug.hs
 GHCi, version 8.6.1: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:6:30: warning:
     • Both DeriveAnyClass and GeneralizedNewtypeDeriving are enabled
       Defaulting to the DeriveAnyClass strategy for instantiating C
     • In the newtype declaration for ‘T’
   |
 6 | newtype T a = MkT a deriving C
   |                              ^
 Ok, one module loaded.
 }}}

 Indeed, there is no flag associated with this warning, so there is
 currently no way to suppress this. I don't think this would be too
 difficult to fix, luckily. All you'd need to do is:

 1. Think of a name for a flag to control this warning.
 2. Toggle whether this warning gets emitted or not behind said flag.

 Any volunteers?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15839#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler