incorrect redundant-constraints warning

The following code when compiled with GHC 8

{-# LANGUAGE Haskell2010, FunctionalDependencies, KindSignatures,
             ScopedTypeVariables, TypeOperators, UndecidableInstances #-}

import GHC.Generics

data Options
data Value

newtype Tagged s b = Tagged {unTagged :: b}

class GToJSON f where
    gToJSON :: Options -> f a -> Value

class SumToJSON f allNullary where
    sumToJSON :: Options -> f a -> Tagged allNullary Value

class AllNullary (f :: * -> *) allNullary | f -> allNullary

instance (
    AllNullary (a :+: b) allNullary, -- <- removing this line causes a compile error
    SumToJSON  (a :+: b) allNullary )
    => GToJSON (a :+: b) where

    gToJSON opts = (unTagged :: Tagged allNullary Value -> Value) . sumToJSON opts

emits a warning

bug.hs:19:10: warning:
    • Redundant constraint: AllNullary (a :+: b) allNullary
    • In the instance declaration for ‘GToJSON (a :+: b)’

when commenting out the AllNullary constraint, this however results the compile error

bug.hs:19:10: error:
    • Could not deduce (SumToJSON (a :+: b) allNullary0)
      from the context: SumToJSON (a :+: b) allNullary
        bound by an instance declaration:
                   SumToJSON (a :+: b) allNullary => GToJSON (a :+: b)
        at redconstr.hs:(19,10)-(22,24)
      The type variable ‘allNullary0’ is ambiguous
    • In the ambiguity check for an instance declaration
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      In the instance declaration for ‘GToJSON (a :+: b)’

Trac metadata

Trac field	Value
Version	8.0.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
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