Redesigned GHC Warnings

GHC currently uses a somewhat unsatisfying warning CLI:

-W      (enable normal warnings)
-w      (disable all warnings)
-Wall   (enable almost all warnings)
-Werror (make warnings fatal)
-Wwarn  (make warnings non-fatal)

-Wduplicate-exports
-Widentities
-Wmissing-signatures
-Wunused-binds
...

By reusing the GCC CLI convention for warning-flags, we can make GHC's CLI a bit more intuitive to people used to GCC (& Clang's) CLI.

Changes to be implemented, and timing

Already implemented in GHC 8.0:

• (#11218 (closed)) Keep the current -f(no-)warn-$WARNTYPE flags as hidden flag aliases for newly introduced -W(no-)$WARNTYPE` flags more in line with GCC's conventions, e.g.

  • -Worphans instead of fwarn-orphans
  • -Wno-missing-methods instead of -fno-warn-missing-methods

  This is already done in GHC 8.0.

• (#11429 (closed)) Make unrecognised -W flags a warning (-Wunrecognised-warning-flags) rather than an error.

• (#11370 (closed)) Remove warn-redundant-constraints from the default constraint set and the -Wall constraint set

• (#11451 (closed)) Split off -Wunused-foralls and -Wunused-type-patterns from -Wunused-matches. Make -Wall imply -Wunused-foralls and -Wunused-type-patterns, but not imply -Wunused-type-patterns

Proposedalso implemented for GHC 8.0:

• (phab:D1850) Introduce some new warning sets, e.g.

  • Define set -Wstandard (modulo bikeshed, maybe -Wdefault?) to denote the set of warnings on by default, together with its negation -Wno-standard
  • Define set -Weverything (c.f. clang's -Weverything as precedent) to comprise really *all* warnings (together with its negation -Wno-everything for symmetry, which is a synonym for -w)
  • Define set -Wextra (modulo bikeshed, maybe -Wnormal?) as synonym for -W, together with its negation -Wno-extra
  • (#11000 (closed)) Define set -Wcompat to denote all warnings about future compatility GHC currently knows about (like e.g. -Wcompat-amp, -Wcompat-mfp, -Wcompat-mrp) In addition, have ghc provide a way to dump the current warning-sets (in a format that's parseable by humans and machines)

• (#10752 (closed)) When emitting warnings/errors, show which warning flag was responsible, e.g.

  foo.hs:1:1: Warning:  [-Wmissing-signatures]
    Top-level binding with no type signature: main :: IO ()

  making it easier to silence specific warnings via e.g. -Wno-missing-signatures

Anytime someone is motivated

• (#11219 (closed)) Introduce variant of -Werror (c.f. GCC's -Werror=*) which allows to specify the individual warnings to be promoted to errors, e.g.

  • -Wall -Werror=orphans would only promote -Worphans warnings into errors
  • -Wall -Werror -Wno-error=missing-methods would promote all warnings except -Wmissing-methods into errors

• (this is subsumed by -Weverything now)Introduce -Wpedantic, which turns on warnings that -Wall doesn't turn on (e.g., -Wredundant-constraints and -Wunused-type-patterns)

Intended usage of warnings

Here is the proposed warning framework:

• Intended meanings

  • -Wdefault: the ones you get by default. This indicate things that are almost certainly wrong.
  • -Wall: indicate suspicious code
  • -Wcompat: indicate code that in future releases may trigger errors or -Wdefault warnings. Subset of -Wall.

• With no flags, GHC implements the "default warnings". You can get these by saying -Wdefault (yet to be implemented).

• When GHC implements a new warning it is put into the -Weverything set. In the next release, that warning may move into the -Wall set or the -Wdefault set. So libraray authors may want to future-proof their libraries by compiling with -Weverything.

• A flag should only move into -Wall or -Wdefault if there is a reasonable way for a library author to change their source code to avoid the warning. Experimental, unpredicatable, or compiler-writer-guidance warnings should not be in -Wall.

• Libraries on Hackage should not be distributed with -Werror, so that any warnings do not abort compilation. Authors may want to use -Wall to help encourage other contributors to improve the library to avoid the warning; but -Wdefault is more typical usage.

To -Wcompat ⊂ -Wall, or not? (#11494 (closed))

See Design/Warnings/Wcompat.