[4.1] Stricter enforcement of the "large space" heuristic #13438
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The Space Engine includes a heuristic that attempted a combinatorics- based check to see if a pattern covered an insufficient amount of cases. In straight-line switches this avoided computing a subspace match that would have been quite expensive computationally. However, when expressive patterns (like tuple patterns) are used, it can fool the check because the covered space is much larger than the actual size of the space due to pattern overlap that counting like this simply can't detect. Instead, just do the right thing and perform a space subtraction after the existing preconditions for the heuristic are satisfied. Resolves SR-6316
This presents a regression in diagnostic quality that is definitely worth it not to lie to SILGen about whether a switch is covered or not. At the same time, disable SIL’s unreachable diagnostic for ‘default’ clauses which would previously cause a warning to be emitted if the default was proven to be unreachable. This analysis is incomplete anyways and can be done by Sema in the future if we desire.
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@swift-ci please test |
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Okay to merge or is there something more you wanted to do here? |
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Does this need a nomination? |
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@swift-ci Please test source compatibility |
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⛵️ I’d appreciate if someone could close out the linked radar. |
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The Space Engine includes a heuristic that attempted a combinatorics-
based check to see if a pattern covered an insufficient amount of cases.
In straight-line switches this avoided computing a subspace match that
would have been quite expensive computationally. However, when
expressive patterns (like tuple patterns) are used, it can fool the
check because the covered space is much larger than the actual size of
the space due to pattern overlap that counting like this simply can't detect.
Instead weaken the heuristic to diagnose large spaces as "uncheckable" and suggest a default clause. This requires disabling SILGen's warning for unreachable default clauses which is itself incomplete.