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Ryan Scott authored
Summary: The code that GND was generating before could crumple over if it derived an instance for a class with an ambiguous type variable in the class head, such as the example in #15637. The solution is straightforward: simply instantiate all variables bound by the class head explicitly using visible type application, which will nip any ambiguity in the bud. Test Plan: make test TEST=T15637 Reviewers: bgamari, simonpj, goldfire Reviewed By: simonpj Subscribers: simonpj, rwbarton, carter GHC Trac Issues: #15637 Differential Revision: https://phabricator.haskell.org/D5148
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