Local variables have backwards scope in AIR 23

Hi!

Upon updating to latest AIR to rebuild my iOS app with iOS 10 support, I have run into an incompatibility, and narrowed it down to a change in scope behaviour of locally declared variables.

Formerly, a locally declared variable was only accessible starting from wherever it was declared and retaining the scope until the end of a function. Currently, a locally defined variable is scoped backwards and can be accessed from the beginning of the function, even before its declaration, even if it is declared in a deeper scope. This poses issues when local variables are named exactly the same as class properties.

Below is a reproduction snipped.

package {

     import flash.display.Sprite;

     public class Main extends Sprite {

          public var a:Number = 4;

          public function Main() {

               trace(a); // NaN

               if (true) { var a:Number = 0; }

               trace(a); // 0

          }

     }

}

I would like to inquire whether this is a bug, and undocumented language specification change, or undefined behaviour.

Cheers!

Sos

EDIT:

Apache Flex, and previous versions of Adobe AS3 compilers would trace 4 and 0, respectively, as expected.