Flag to warn when deriving strategy is not explicitly specified

In my code, I aim to always specify deriving strategies. I would like to add a flag -fwarn-unspecified-deriving that would cause a warning to be emitted when someone wrote this:

newtype Foo = Foo Int
  deriving (Show)

The user would be required to instead write:

newtype Foo = Foo Int
  deriving newtype (Show)

Or they could use stock if that was the behavior they wanted. This flag would be off by default.

Trac metadata

Trac field	Value
Version	8.6.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Blocking
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Architecture

changed weight to 5

added Tfeature request Trac import labels

Ryan, if you to see if you had any thoughts on the name of the flag, I am welcome to suggestions.

One other suggestion I've heard is -Wimplicit-deriving-strategies (mirroring the name of the ImplicitPrelude extension).

That does sound better. I'll go with that. And I'm glad to know that someone else has thought about such a flag before.

added newcomer label

I would suggest -Wmissing-deriving-strategies, as there are several -Wmissing-* flags (missing-local-signatures, missing-import-lists, missing-home-modules etc.).

mentioned in commit c121e33f

closed

changed milestone to %8.8.1

Landed in c121e33f9b039acf2ac6939af8bfafe593560039:

Author: chessai <chessai1996@gmail.com>
Date:   Wed Dec 26 12:12:37 2018 -0500

    Add -Wmissing-deriving-strategies
    
    Warn users when -XDerivingStrategies is enabled but not used, at each
    potential use site.
    
    add -Wmissing-deriving-strategies
    
    Reviewers: bgamari, RyanGlScott
    
    Subscribers: andrewthad, rwbarton, carter
    
    GHC Trac Issues: #15798
    
    Differential Revision: https://phabricator.haskell.org/D5451

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	- → rename/should_compile/T15798{a,b,c}

added Pnormal label

mentioned in issue #17014 (closed)

mentioned in issue #17312 (closed)

mentioned in merge request !12817 (closed)