Symptom:
When trying to f5 run the VisualStudioDebug project it fails to start with something like:

The reason seems to be that it requires newer version of Roslyn assemblies that Visual Studio 17 currently have.

Solution (or workaround) is to build and deploy newest Roslyn first:

1. clone dotnet/roslyn
2. In Roslyn directory:

Build.cmd -Restore -Configuration Release -deployExtensions -launch

3. Now you can run VisualStudioDebug successfully, double check the versions in experimental instance:

Potential problem is when you have both VS and VS Preview installed. It means two experimental hives with same RoslynDev name and Roslyn will probably install to the non-preview one. If you then start VisualStudioDebug from preview, it won't work.

As an aside: it seems running Build.cmd -c Release no longer deploys the extension.
Which means the command line workflow with devenv /rootsuffix RoslynDev no longer works, this should be reflected in DEVGUIDE.