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We need the free vars of `t2` to satisfy the substitution invariant. Luckily they are in the in-scope carried around. Test Plan: ./validate Reviewers: bgamari, austin, goldfire, simonpj Reviewed By: simonpj Subscribers: thomie, simonmar Differential Revision: https://phabricator.haskell.org/D2044 GHC Trac Issues: #11371
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