Type.getInstanceFields returns extra empty string in HL when inheriting interface

[nspitko]

A try.haxe link: https://try.haxe.org/#38AB9237

And the code for posterity:

interface CDObject {

}

@:keepSub
@:structInit class CUIObject implements CDObject {

	public var name: String = null;

	public function clone( )
	{
		var cls = Type.getClass(this);
		var inst = Type.createEmptyInstance(cls);
		var fields = Type.getInstanceFields(cls);
		for (field in fields)
		{
			// generic copy
			var val:Dynamic = Reflect.field(this,field);
			if ( !Reflect.isFunction(val) )
			{
				Reflect.setField(inst,field,val);
			}
		}
		return inst;
	}
}

class Test {
  static function main() {
    var foo: CUIObject = {};
    foo.clone();
    trace("OK!");
  }
}

This code fails, as the last entry in fields is an empty string (and trying to touch empty string as a field results in an expected error)

This only happens in HL, and only when the object has an interface. If we remove the implements then the code works correctly.

[kLabz]

Removed some noise to extract as test:

interface Bar {}
class Foo implements Bar {}

class Test {
  static function main() {
    var fields = Type.getInstanceFields(Foo); // should be []
    
    // Print extra field(s)
    for (field in fields) trace('field "$field"'); // field ""
  }
}