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Ryan Scott authored
Summary: According to the GHC users' guide, one cannot derive instances for data types in `.hs-boot` files. However, GHC was not enforcing this in practice, which led to #14365. Fix this by actually throwing an error if a derived instance is detected in an `.hs-boot` file (and recommend how to fix it in the error message.) Test Plan: make test TEST=T14365 Reviewers: ezyang, austin, bgamari, simonpj Reviewed By: simonpj Subscribers: simonpj, rwbarton, thomie GHC Trac Issues: #14365 Differential Revision: https://phabricator.haskell.org/D4102
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