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The following program is accepted by ghc-7.0 to ghc-7.10, but rejected by ghc-6.12.3 and HEAD (and rightfully so): class Class1 a class Class1 a => Class2 a class Class2 a => Class3 a instance Class3 a => Class2 a The last line is missing a `Class1 a` constraint. Add a regression test for this (typechecker/should_fail/tcfail223). Add similar missing class constraints to T7126 and T5751. I verified that the these changes don't interfer with the intention of the tests (they still result in a loop with ghc-7.4.1). Reviewers: austin, simonpj, bgamari Reviewed By: bgamari Subscribers: rwbarton, thomie Differential Revision: https://phabricator.haskell.org/D1078
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