Source generator sample has errors in Visual Studio

[parched]

If I open https://github.com/dotnet/roslyn-sdk/blob/main/samples/CSharp/SourceGenerators/SourceGenerators.sln from a fresh clone in VS I get a lot of errors when I open the demo files.

When I build the solution the errors go away.

Is this the intended behavior or is it a bug in VS or in these samples?