Support logical operators in pattern matching

[patrickt]

C# 9.0 added and, or and not statements in pattern matching: dotnet/csharplang#2850

This feature is present in dotnet/roslyn:

            if (obj is not WorkspaceKindMetadata metadata)
                return false;

[damieng]

Do we want to use the "bug" tag for C# 9 support? It was ratified only recently. If we do any analysis in the future it might be nicer to break out what we thought was in but broken vs what C# added and we were playing catch-up?

[patrickt]

@damieng I don’t have strong feelings re. that taxonomy—maybe @maxbrunsfeld does—but IMO this grammar should be able to parse as much of dotnet/roslyn as is possible, so departures from that are bugs.

[damieng]

So just as a heads up C# 9.0 isn't actually released until November with the release of .NET 5.0.

I agree we should add support - just wanted to clarify it's not a bug in the traditional sense - it's supporting new cutting-edge syntax.

[damieng]

One further point to make here - donet/roslyn is probably unique in that it is the only significantly-sized repo that is using C# 9.0 at this point in time.