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Previously, `singleConstructor` didn't handle singleton `COMPLETE` sets of a single pattern synonym, resulting in incomplete pattern warnings in #15753. This is fixed by making `singleConstructor` (now named `singleMatchConstructor`) query `allCompleteMatches`, necessarily making it effectful. As a result, most of this patch is concerned with threading the side-effect through to `singleMatchConstructor`. Unfortunately, this is not enough to completely fix the original reproduction from #15753 and #15884, which are related to function applications in pattern guards being translated too conservatively.
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