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Herbert Valerio Riedel authored
This is similiar to the `-fwarn-noncanonical-monad-instances` warning implemented via #11128, but applies to `Semigroup`/`Monoid` instead and the `(<>)`/`mappend` methods (of which `mappend` is planned to move out of `Monoid` at some point in the future being redundant and thus error-prone). This warning is contained in `-Wcompat` but not in `-Wall`. This addresses #11150 Reviewed By: quchen Differential Revision: https://phabricator.haskell.org/D1553
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