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Accroding to https://git.haskell.org/ghc.git/commitdiff/49672659113371c3bee691e6d913d f8e6f60a1d8, `-Wredundant-constraints` is no longer turn on by default. Reviewers: bgamari Reviewed By: bgamari Subscribers: rwbarton, thomie, carter Differential Revision: https://phabricator.haskell.org/D4528
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