I think the MonadPlus instance should probably have a MonadPlus constraint, since MonadPlus makes a sort of statement about the interaction between >>= and mplus, even if it's a bit of an ambiguous one.

On Tue, Apr 2, 2019, 12:30 AM Fumiaki Kinoshita <fumiexcel@gmail.com> wrote:
This is another part of https://gitlab.haskell.org/ghc/ghc/merge_requests/644, which is presumably much less controversial:

Generic (Kleisli m a b)
Functor m => Functor (Kleisli m a)
Applicative m => Applicative (Kleisli m a)
Alternative m => Alternative (Kleisli m a)
Monad m => Monad (Kleisli m a)
(Alternative m, Monad m) => MonadPlus (Kleisli m a)
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