Incompatability between desktop and coreclr for Generic Array Enumeration

[KevinRansom]

Enumerator fails to throw when calling current after MoveNext returns false

repro:
This code throws on the NETFramework and the coreclr doesn't

namespace ConsoleApp23
{
    class Program
    {
        static void Main(string[] args)
        {
            var keyValuePairs =
                new KeyValuePair<int, int>[3]
                {
                    new KeyValuePair<int, int>(1,1),
                    new KeyValuePair<int, int>(2,2),
                    new KeyValuePair<int, int>(3,3),
                };

                IEnumerable<KeyValuePair<int, int>> collection = keyValuePairs;

            var e = collection.GetEnumerator();

            var step2 = e.MoveNext();
            var step3 = e.Current;
            Console.WriteLine("{0} {1}", step2, step3);

            var step4 = e.MoveNext();
            var step5 = e.Current;
            Console.WriteLine("{0} {1}", step4, step5);

            var step6 = e.MoveNext();
            var step7 = e.Current;
            Console.WriteLine("{0} {1}", step6, step7);

            var step8 = e.MoveNext();
            var step9 = e.Current;
            Console.WriteLine("{0} {1}", step8, step9);
       }
    }
}

Produces this output on NET Framework: (Note the exception is thrown)

C:\Users\kevinr\source\repos\ConsoleApp23\ConsoleApp23\bin\Debug>net472\ConsoleApp23.exe
True [1, 1]
True [2, 2]
True [3, 3]

Unhandled Exception: System.InvalidOperationException: Enumeration already finished.
   at System.SZArrayHelper.SZGenericArrayEnumerator`1.get_Current()
   at ConsoleApp23.Program.Main(String[] args) in C:\Users\kevinr\source\repos\ConsoleApp23\ConsoleApp23\Program.cs:line 35

C:\Users\kevinr\source\repos\ConsoleApp23\ConsoleApp23\bin\Debug>

and this output on the Coreclr: Note it produces [3,3] when MoveNext returns false

True [1, 1]
True [2, 2]
True [3, 3]
False [3, 3]

[KevinRansom]

cc/ @jkotas

[jkotas]

Regression from dotnet/coreclr#13157

[jkotas]

The behavior of IEnumerator<T>.Current in this situation is documented as undefined:

https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.ienumerator-1.current?view=netcore-2.1#remarks

The behavior of enumerators throughout the framework is not consistent in this situation, exactly as the documentation says. Some throw exception, some do not.

While we may still want to consider fixing this for array enumerator, good code should not be depend on undefined behavior.