You need to ensure that you have enlisted and built Roslyn sources with VS2017 tag on your machine. For further guidance, refer to the recipe, Setting up Roslyn enlistment in Chapter 8, Contribute Simple Functionality to Roslyn C# Compiler Open Source Code.
For the steps in the recipe that mention Define ... in the C# language specification, the reader should create a new GitHub issue on (https://github.com/dotnet/roslyn/issues/new) with labels Language-C# and Area-Language Design and get the specification reviewed by the language team. If approved, the reviewers will ensure that it gets added to the C# language specification.
