Bogus "redundant constraint" warning with functional dependencies

Compiling this code with GHC HEAD:

{-# LANGUAGE MultiParamTypeClasses  #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances      #-}
{-# LANGUAGE UndecidableInstances   #-}

module T10100 where

data Zero
data Succ a

class Add a b ab | a b -> ab, a ab -> b
instance Add Zero b b
instance (Add a b ab) => Add (Succ a) b (Succ ab)

Gives a bogus warning about redundant constraint:

Redundant constraint: Add a b ab
In the instance declaration for ‘Add (Succ a) b (Succ ab)’

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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