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For a nullary pattern synonym we add an extra void argument to the matcher in order to preserve laziness. The check in buildPatSyn wasn't aware of this special case which was causing the assertion to fail. Reviewers: austin, simonpj, bgamari Reviewed By: simonpj, bgamari Subscribers: simonpj, thomie Differential Revision: https://phabricator.haskell.org/D2624 GHC Trac Issues: #12746
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