[GHC] #11370: Redundant superclass warnings being included in -Wall destroys the "3 Release Policy"

[GHC]

#11370: Redundant superclass warnings being included in -Wall destroys the "3
Release Policy"
-------------------------------------+-------------------------------------
           Reporter:  ekmett         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #11369
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 These redundant constraint warnings being in -Wall make it seemingly
 impossible for us to live up to the '3 Release Policy' in many situations.

 https://prime.haskell.org/wiki/Libraries/3-Release-Policy

 Consider the AMP: `(Monad m, Functor m)` gets used as a replacement for
 `Monad m` in many situations for compatibility with code before GHC 7.8,
 where that wants to call both `fmap` and something from `Monad`, but from
 7.10+ that `Functor` is a redundant constraint. We have no backwards-
 facing 3-release-compatible constraint that someone could use in a type
 signature without warning to get both `fmap` and `return` that won't
 trigger this warning.

 Similarly if we split a class we can't advocate that folks use the
 subclass until the superclass is widely available or they'll get spammed
 with warnings. This affects splitting out `Semigroup` from `Monoid`.

 A similar issue arises with the `MonadFail` proposal as there is a stage
 around 8.4 where the 3-release-compatible advice would be to incur a
 `MonadFail` constraint, even when the active desguaring will be through
 `Monad`.

 I don't actually have a fix from the libraries side for the fact that with
 these showing up in -Wall, the 3 release policy doesn't work.

 Every single proposal the core libraries committee currently has in the
 works (except for adding `log1p`, `expm1`, etc. to Floating) is affected
 by this concern.

 One possible fix is to demote the redundant superclass warnings out of
 `-Wall`.

 This would resolve #11369 as well.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11370>
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