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Ryan Scott authored
When `GeneralizedNewtypeDeriving` is used with a type class that has no methods, it will generate a redundant context, and as a result, it can trigger warnings when compiled with `-Wredundant-constraints`. This is a simple change in behavior to check beforehand if a class has methods when deriving it with GND, and if it has no methods, avoid inferring the redundant context. Beware that the test for #6088, which used to be expected to fail, now compiles without issue since it doesn't infer a problematic instance context. Thanks to Simon Peyton Jones for doing the necessary refactoring in f05d685a. Fixes #12814. Test Plan: ./validate Reviewers: goldfire, rwbarton, simonpj, austin, bgamari Reviewed By: simonpj Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D2692 GHC Trac Issues: #12814
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