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Ryan Scott authored
Summary: Because we weren't checking for duplicate variables on the right-hand sides of pattern synonyms, bogus definitions like this one passed the renamer: ```lang=haskell pattern Foo a <- (a,a) ``` Luckily, the fix is simple. Test Plan: make test TEST=T14114 Reviewers: mpickering, austin, bgamari, simonpj Reviewed By: simonpj Subscribers: simonpj, rwbarton, thomie GHC Trac Issues: #14114 Differential Revision: https://phabricator.haskell.org/D3866
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