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This bug, revealed by #18347, is just a missing update to sc_hole_ty in simplCast. I'd missed a code path when I made the recentchanges in commit 6d49d5be Author: Simon Peyton Jones <simonpj@microsoft.com> Date: Thu May 21 12:53:35 2020 +0100 Implement cast worker/wrapper properly The fix is very easy. Two other minor changes * Tidy up in SimpleOpt.simple_opt_expr. In fact I think this is an outright bug, introduced in the fix to #18112: we were simplifying the same coercion twice *with the same substitution*, which is just wrong. It'd be a hard bug to trigger, so I just fixed it; less code too. * Better debug printing of ApplyToVal
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