Mongolian vowel separator

// What's the output of this code, and how confident are you?
// What might affect the result?
class Test
{    
    static void Main()
    {
        string stringx = "a";
        string\u180ex = "b";
        System.Console.WriteLine(stringx);
    }
}

To my eyes it doesn't look like it compiles!

I'm still trying to find a machine that's sufficiently out of date to make it print "a". That should be a bit of a hint :)

(But both outputs are potentially valid. Arguably, only "a" strictly complies with the C# 5 specification, I believe...)

Ah - made more progress. It should either compile with a result of "b", or it should not compile. The Mono compiler (at least the version I'm using) has a bug where even "string\u0020x" works, but that fails to compile with csc.exe.