Sorry, I don't follow?

The .NET implementation detects overflow and reports it back. This doesn't.

This:

    var x = DoubleIt<NumInt, int>(Int32.MaxValue);  // 20

Performs exactly the same as this:

    var a = Int32.MaxValue;
    var b = Int32.MaxValue;
    var c = a + b;

DoubleIt calls NumInt.Add which invokes the same code, why wouldn't it work in exactly the same way?