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Ryan Scott authored
Summary: When GHC reports that it cannot solve a constraint in error messages, it often reports what given constraints it has in scope. Unfortunately, sometimes redundant constraints (like `* ~ *`, from #15361) can sneak in. The fix is simple: blast away these redundant constraints using `mkMinimalBySCs`. Test Plan: make test TEST=T15361 Reviewers: simonpj, bgamari Subscribers: rwbarton, thomie, carter GHC Trac Issues: #15361 Differential Revision: https://phabricator.haskell.org/D5002 (cherry picked from commit c552feea)
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