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Simon Peyton Jones authored
This was a nasty, long-standing bug exposed in Trac #10538. Symptoms were that we had an empty case case (x :: Either a) of {} Core Lint correctly picked this bogus code up. Here is what happened * In SimplUtils.prepareAlts, we call filterAlts then combineIdenticalAlts * We had case x of { Left _ -> e1; Right _ -> e1 } * filterAlts did nothing, but correctly retuned imposs_deflt_cons saying that 'x' cannot be {Left, Right} in the DEFAULT branch, if any (there isn't one.) * combineIdentialAlts correctly combines the identical alts, to give case x of { DEFAULT -> e1 } * BUT combineIdenticalAlts did no adjust imposs_deft_cons * Result: when compiling e1 we did so in the belief that 'x' could not be {Left,Right}. Disaster. Easily fixed. (It is hard to trigger; I can't construct a simple test case.)
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