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Trac #16376 showed the danger of failing to report an error that exists only in the unsolved constraints, if an exception is raised (via failM). Well, the commit 5c1f268e (Fail fast in solveLocalEqualities) did just that -- i.e. it found errors in the constraints, and called failM to avoid a misleading cascade. So we need to be sure to call captureTopConstraints to report those insolubles. This was wrong in TcRnDriver.tcRnExpr and in TcRnDriver.tcRnType. As a result the error messages from test T13466 improved slightly, a happy outcome.
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