ROS<T>.LastIndexOf(ROS<T> value) returns wrong index when 'value' is empty

[GrabYourPitchforks]

Console.WriteLine("Hello".LastIndexOf(""));
Console.WriteLine("Hello".LastIndexOf("", StringComparison.Ordinal));
Console.WriteLine("Hello".AsSpan().LastIndexOf(""));
Console.WriteLine("Hello".AsSpan().LastIndexOf("", StringComparison.Ordinal));

// PRINTS:
// 4
// 4
// 0
// 4

The behavior of ROS<T>.LastIndexOf differs based on whether the char-specific overload is called or whether the generic T overload is called, as shown above. We should make the behavior consistent.

(To be fair, all of these APIs should be returning "5", not "4". I'll file a separate bug for that.)

[hannan-sultan]

I'm willing to take this issue. I have cloned the repo and followed the building on Windows instructions everything builds fine. I guess the fix should be in CompareInfo.cs in System.Private.Corelib project, right?
Can you please guide me on how I can test after making changes?

[GrabYourPitchforks]

Hi @hannan-sultan! Check https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/developer-guide.md#testing-with-private-coreclr-bits for complete instructions. But in a nutshell, what this involves is essentially four steps:

1. Clone the coreclr and corefx repos.
2. Make the change in coreclr and build it locally.
3. Build corefx locally using the custom coreclr you've built. (build -build -buildtests /p:coreclroverridepath=c:\whatever)
4. Run the corefx tests locally (build -test /p:coreclroverridepath=c:\whatever)

It's likely that you'll see corefx test failures, as the tests probably test the incorrect behavior. If this occurs make the changes to your local corefx repo, then rebuild and rerun the tests.