Simplifying an instance context makes a rewrite rule no longer typecheck

This code (taken from the reducers package) compiles:

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}

import Prelude (Applicative(..), Functor(..), (.))

class Semigroup m where
  (<>) :: m -> m -> m

class Semigroup m => Reducer c m where
  snoc :: m -> c -> m

newtype Traversal f = Traversal { getTraversal :: f () }

instance Applicative f => Semigroup (Traversal f) where
  Traversal a <> Traversal b = Traversal (a *> b)

instance Applicative f => Reducer (f a) (Traversal f) where
  Traversal a `snoc` b = Traversal (() <$ (a *> b))

snocTraversal :: Reducer (f ()) (Traversal f) => Traversal f -> f () -> Traversal f
snocTraversal a = (<>) a . Traversal
{-# RULES "snocTraversal" snoc = snocTraversal #-}

But on GHC 8.2.1 and later, it gives this warning:

GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Main             ( Bug.hs, interpreted )

Bug.hs:21:18: warning: [-Wsimplifiable-class-constraints]
    • The constraint ‘Reducer (f ()) (Traversal f)’
        matches an instance declaration
      instance Applicative f => Reducer (f a) (Traversal f)
        -- Defined at Bug.hs:18:10
      This makes type inference for inner bindings fragile;
        either use MonoLocalBinds, or simplify it using the instance
    • In the type signature:
        snocTraversal :: Reducer (f ()) (Traversal f) =>
                         Traversal f -> f () -> Traversal f
   |
21 | snocTraversal :: Reducer (f ()) (Traversal f) => Traversal f -> f () -> Traversal f
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I decided to follow GHC's orders and reduce the Reducer (f ()) (Traversal f) context to just Applicative f:

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}

import Prelude (Applicative(..), Functor(..), (.))

class Semigroup m where
  (<>) :: m -> m -> m

class Semigroup m => Reducer c m where
  snoc :: m -> c -> m

newtype Traversal f = Traversal { getTraversal :: f () }

instance Applicative f => Semigroup (Traversal f) where
  Traversal a <> Traversal b = Traversal (a *> b)

instance Applicative f => Reducer (f a) (Traversal f) where
  Traversal a `snoc` b = Traversal (() <$ (a *> b))

snocTraversal :: Applicative f => Traversal f -> f () -> Traversal f
snocTraversal a = (<>) a . Traversal
{-# RULES "snocTraversal" snoc = snocTraversal #-}

But after doing so, the file no longer typechecks!

GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Main             ( Bug.hs, interpreted )

Bug.hs:23:34: error:
    • Could not deduce (Applicative f)
        arising from a use of ‘snocTraversal’
      from the context: Reducer (f ()) (Traversal f)
        bound by the RULE "snocTraversal" at Bug.hs:23:11-46
      Possible fix:
        add (Applicative f) to the context of the RULE "snocTraversal"
    • In the expression: snocTraversal
      When checking the transformation rule "snocTraversal"
   |
23 | {-# RULES "snocTraversal" snoc = snocTraversal #-}
   |                                  ^^^^^^^^^^^^^

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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