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Ryan Scott authored
Before, GHC was extremely permissive about the form a default type signature could take on in a class declaration. Notably, it would accept garbage like this: class Monad m => MonadSupply m where fresh :: m Integer default fresh :: MonadTrans t => t m Integer fresh = lift fresh And then give an extremely confusing error message when you actually tried to declare an empty instance of MonadSupply. We now do extra validity checking of default type signatures to ensure that they align with their non-default type signature counterparts. That is, a default type signature is allowed to differ from the non-default one only in its context - they must otherwise be alpha-equivalent. Fixes #12918. Test Plan: ./validate Reviewers: goldfire, simonpj, austin, bgamari Reviewed By: bgamari Subscribers: mpickering, dfeuer, thomie Differential Revision: https://phabricator.haskell.org/D2983 GHC Trac Issues: #12918
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