This long-standing bug in CoreUtils.combineIdenticalAlts was shown up by Trac #11172. The effect was that it returned a correct set of alternatives, but a bogus set of "impossible default constructors". That meant that we subsequently removed all the alternatives from a case, and hence ended up with a bogusly empty case that should not have been empty. See Note [Care with impossible-constructors when combining alternatives] in CoreUtils.
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