Currently we have only the following generic warning "levels": ``` -W enable normal warnings -w disable all warnings -Wall enable almost all warnings (details in ) ``` It would be useful to be able to refer collectively to all currently implemented forward-compatibility warnings (e.g. about upcoming changes in `base`, like `-fwarn-amp`) via a symbolic `-Wcompat` flag. This way, users have more flexibility in opt-in or out of such warnings. So e.g. the following settings are possible in `.cabal` files: > `ghc-options: -Wall` :: enable "almost all warnings" as before. Some compatibility warnings may be part of `-Wall`, some won't (yet). > `ghc-options: -Wall -Wcompat` :: enable "almost all warnings" as well as all currently implemented warnings. E.g. if GHC 8.0 implements `-fwarn-mfp-compat` but doesn't include it in `-Wall`, this gives users a way to opt in to get notified early about upcoming changes. > `ghc-options: -Wall -Wno-compat` :: This enables `-Wall` \*\*minus\*\* any future-compat warnings. This is for users who want to be warned at all about any future compat warnings. > `ghc-options: -Wcompat` :: Of course, `-W(no-)compat` can be used also w/o `-Wall`, and then is equivalent to turning on/off the respective individual `-fwarn-*-compat` flags. > `ghc-options: -Wall -Wcompat -Wno-mfp-compat` :: This would allow to acknowledge the MFP compat warnings, while enabling other future GHC compat warnings.