Nullable and IAsyncEnumerable break syntax highlighting

[jaredpar]

This will reproduce on pretty much every C# project I've tested, even dotnet new console.

Type in the following code to the editor

            string s1 = "hello"; // string is colored
            await foreach (var x in e) { }
            string s2 = "world"; // string is not colored

Notice that the presence of await foreach causes the second usage of string to not get properly syntax highlighted. It will be white instead of blue. The moment you delete await then everything will snap back to normal.

This problem appears to impact a number of C# 8 features. For instance the same effect occurs when using a nullable array.

            string s1 = "hello"; // string is colored
            int[]? example;
            string s2 = "world"; // string is not colored

[jaredpar]

One way to work around this is to add an empty statement after the await foreach

                ; // empty statement to fix vs code syntax bug around await foreach

[filipw]

This is really an issue of C# extension for VS Code not of OmniSharp.
It uses https://github.com/dotnet/csharp-tmLanguage for highlighting, which is very limited and hasn't been updated in quite a while.

OmniSharp exposes semantic highlighting endpoint based on Roslyn's semantic highlighting (I added your two cases as tests here to make sure they are covered (#1789)) but at the moment the C# extension still doesn't use that endpoint - it's a work in progress to incorporate it github.com/dotnet/vscode-csharp/pull/3667

[filipw]

this is resolved via semantic highlighting