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Given multiple in-scope constructors with the same name, say `A`, and a function of type `A -> Int`, say, the compiler reports both a "type `A` is not in scope" and (incorrectly) an ambiguity error. The latter shouldn't be there if `DataKinds` isn't enabled. This issue was recommended to me by @mpickering as a suitable first task, and the fix was also outlined in the original Trac ticket. It involved a simple reordering of the steps taken in `lookup_demoted` in `RnEnv.hs`. The fix is to make the `DataKinds` check happen earlier, ensuring that the ambiguity check doesn't happen at all if we know the constructors couldn't have been promoted. Signed-off-by: Soham Chowdhury <chow.soham@gmail.com> Reviewers: mpickering, austin, bgamari Reviewed By: mpickering, bgamari Subscribers: rwbarton, thomie GHC Trac Issues: #13568 Differential Revision: https://phabricator.haskell.org/D3547 (cherry picked from commit 1381c142)
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