Add a unit test for #5705 (#5726)

* Add a unit test for #5705

* Apparently another issue already uses "BaseClass"...

* ..

tests/unit/src/unit/issues/Issue5705.hx

@@ -0,0 +1,17 @@
+package unit.issues;
+
+class Issue5705 extends Test {
+	static function foo(baseClass:BaseClass_):Int {
+		return cast(baseClass, IInterface).z;
+	}
+}
+
+class BaseClass_ {}
+
+interface IInterface {
+	public var z:Int;
+}
+
+class ImplementingClass extends BaseClass_ implements IInterface {
+	public var z:Int;
+}