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Simon Peyton Jones authored
Gracious me. Ever since this patch commit 37445780 Author: Jan Stolarek <jan.stolarek@p.lodz.pl> Date: Fri Jul 11 13:54:45 2014 +0200 Injective type families TcRnMonad.askNoErrs has been wrong. It looked like this askNoErrs :: TcRn a -> TcRn (a, Bool) askNoErrs m = do { errs_var <- newTcRef emptyMessages ; res <- setErrsVar errs_var m ; (warns, errs) <- readTcRef errs_var ; addMessages (warns, errs) ; return (res, isEmptyBag errs) } The trouble comes if 'm' throws an exception in the TcRn monad. Then 'errs_var is never read, so any errors are simply lost. This mistake was then propgated into DsMonad.dsWhenNoErrs, where it gave rise to Trac #13642. Thank to Ryan for narrowing it down so sharply. I did some refactoring, as usual.
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