Redundant superclass warnings being included in -Wall destroys the "3 Release Policy"

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.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related	#11369
Blocking
CC	ekmett
Operating system
Architecture