Fixity declaration reported as ambiguous

[simonpj]

Christopher Dove reports: So I have some module, in a work project that I'm portnig from GHC 6.12.3 to GHC 7.4.2,

module Data.Monoid.Operator where

import Data.Monoid

(++) :: Monoid a => a -> a -> a
(++) = mappend
infixr 5 ++

This compiles happily on GHC 6.12.3, but on 7.4.2 says:

src/Data/Monoid/Operator.hs:9:10:
    Ambiguous occurrence `++'
    It could refer to either `Data.Monoid.Operator.++',
                             defined at src/Data/Monoid/Operator.hs:8:1
                          or `Prelude.++',
                             imported from `Prelude' at src/Data/Monoid/Operator.hs:3:8-27
                             (and originally defined in `GHC.Base')