[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:                    |
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Description changed by tejon:

Old description:

> When `DerivingStrategies`, `DeriveAnyClass` and
> `GeneralizedNewtypeDeriving` are enabled together, an instance that could
> be derived with either `newtype` or `anyclass` throws a warning when no
> strategy is specified. It appears to be impossible to suppress this
> warning. Even `-w` doesn't work!

New description:

 When `DeriveAnyClass` and `GeneralizedNewtypeDeriving` are enabled
 together, an instance that could be derived with either one defaults to
 `DeriveAnyClass` and throws a warning to that effect. There is currently
 no flag to suppress that warning (it appears even with `-w`).


 In the presence of `DerivingStrategies`, it seems desirable to be able to
 suppress this.

 Proposed flag to control it: `-Wderiving-defaults`, after the pattern of
 `-Wtype-defaults`.

 This flag should be part of the default warning set, as without
 `DerivingStrategies` it remains a bad idea to have both newtype and
 anyclass active.

 Minimal example (thanks RyanGIScott):
 {{{
 {-# 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.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15839#comment:3>
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