QML javascript: "this" in class ctor becomes undefined during if(){...} in arrow functions

In QML javascript, when executing a pure-javascript class constructor, "this" becomes momentarily undefined in either branch of a conditional

if (cond) { ... }

but is valid before and after the "if".

This problem (seemingly) occurs only if the code is in the body of an anonymous arrow function. JS arrow functions do not provide "this" or do anything with it, so the symbol "this" should refer to the "this" in the closure scope, i.e. the class constructor;

And it does, except within an 'if' statement!

STEPS TO REPRODUCE: 
  Put the code shown below in a file and run 'qml' on it

RESULTS:

 qml: BEFORE if: this=[object Object], foo=123
 qml: DURING if: this=undefined
 qml: AFTER  if: this=[object Object], foo=123
 qml: At constructor exit: this=[object Object], foo=123

// test.qml   (run 'qml' on this file)
import QtQuick 2.14
Item {
  Component.onCompleted: {
    class Bugtest {
      constructor() {
        this.foo = 123;
        ((arg) => {
          print("BEFORE if: this="+this+", foo="+this.foo);
          if (true) {
            print("DURING if: this="+this); //undefined here only!!
          }
          print("AFTER   if: this="+this+", foo="+this.foo);
        })(42);
        print("At constructor exit: this="+this+", foo="+this.foo);
      }
    };    
    var xyz = new Bugtest();
  }
}

The problem goes away if .bind(this) is used, or if the constructor copies "this" to a temporary variable and the closure refers to that temp instead of the symbol "this".

Previously on stackoverflow at https://stackoverflow.com/questions/69846086/qml-javascript-this-becomes-undefined-in-class-constructor-under-if-if/69858353#69858353