zero width joiner is ignored in identifiers

ZWJ.cs

using System;

class Program
{
    static void Main()
    {
        var ab = 0;
        a\u200db = 1; // なぜか ab と同じ扱い。\u200d は zero witdh joiner
        Console.WriteLine(ab); // 1

        var s1 = "ab";
        var s2 = "a\u200db";
        Console.WriteLine(s1.Length); // 2
        Console.WriteLine(s2.Length); // 3
        Console.WriteLine(s1 == s2);  // false
    }
}