Tune usage of resultConforms to prefer the chosen alternative
#22730
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`resultConforms` should only make us change our mind when we're sure that the chosen alternative does not fit the result type but another alternative would. The problem is that due to approximations and adaptation, we can guess wrong on both counts. In i22713 in particular, we guessed that the chosen alternative does not work (but it does because of Unit insertion) and that another alternative would, but it doesn't because `resultConforms` prefers false positives to false negatives (the result type is under-approximated to `Nothing` which is indeed a subtype of `Unit`). We only really need to prefer false positives to false negatives when considering whether to discard the currently chosen alternative (because there's probably a good reason we chose it in the first place), so this commit introduces a parameter to `resultConforms` to control this. Fixes scala#22713.
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resultConformsshould only make us change our mind when we're sure that the chosen alternative does not fit the result type but another alternative would. The problem is that due to approximations and adaptation, we can guess wrong on both counts.In i22713 in particular, we guessed that the chosen alternative does not work (but it does because of Unit insertion) and that another alternative would, but it doesn't because
resultConformsprefers false positives to false negatives (the result type is under-approximated toNothingwhich is indeed a subtype ofUnit).We only really need to prefer false positives to false negatives when considering whether to discard the currently chosen alternative (because there's probably a good reason we chose it in the first place), so this commit introduces a parameter to
resultConformsto control this.Fixes #22713.