Sebastian Graf authored Apr 03, 2019 and Marge Bot committed Apr 08, 2019

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.