Using the C# 9 features, Visual Studio Code shows a lot of squiggly lines, although the code compiled and ran. While in Visual Studio 2019 preview everything is okay - no squiggly lines.
dotnet --info output:

VS Code version: 1.48.0- insider (user setup)
C# Extension version: v1.22.1


this will be supported in the next prerelease
try setting "omnisharp.path": "latest" in the meantime to opt into CI build of omnisharp
fixed in 1.23.1
Most helpful comment
this will be supported in the next prerelease
try setting "omnisharp.path": "latest" in the meantime to opt into CI build of omnisharp