We actually already put in a patch for this about 10 days ago, when the discussion came up around U1:

https://ghc.haskell.org/trac/ghc/ticket/11650

-Edward




On Wed, Mar 9, 2016 at 5:49 PM, Fumiaki Kinoshita <fumiexcel@gmail.com> wrote:
This is the only possible definition...so it should exist

instance Alternative Proxy where
  empty = Proxy
  _ <|> _ Proxy

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