a.md

• https://github.com/scala/scala/blob/v2.13.2/src/library/scala/runtime/IntRef.java#L18
• https://github.com/scala/scala/blob/v2.13.2/src/library/scala/Int.scala#L456-L463
• https://github.com/scala/scala/blob/v2.13.2/src/library/scala/runtime/BoxesRunTime.java#L62-L64

Int.box

$ scala -version
Scala code runner version 2.13.2 -- Copyright 2002-2020, LAMP/EPFL and Lightbend, Inc.
$ scala -Xprint:jvm -e 'if (util.Random.nextInt == 1) 2 else "3"'
[[syntax trees at end of                       jvm]] // scalacmd6816512509873541389.scala
package <empty> {
  object Main extends Object {
    def main(args: Array[String]): Unit = {
      new <$anon: Object>();
      ()
    };
    def <init>(): Main.type = {
      Main.super.<init>();
      ()
    }
  };
  final class anon$1 extends Object {
    def <init>(): <$anon: Object> = {
      anon$1.super.<init>();
      if (scala.util.Random.nextInt().==(1))
        scala.Int.box(2)
      else
        "3";
      ()
    }
  }
}

IntRef

$ scala -Xprint:jvm -e "def foo = { var x = 0 ; Some(1).foreach{ x = _ } ; x }"
[[syntax trees at end of                       jvm]] // scalacmd4840721111041760550.scala
package <empty> {
  object Main extends Object {
    def main(args: Array[String]): Unit = {
      new <$anon: Object>();
      ()
    };
    def <init>(): Main.type = {
      Main.super.<init>();
      ()
    }
  };
  final class anon$1 extends Object {
    private def foo(): Int = {
      var x: runtime.IntRef = scala.runtime.IntRef.create(0);
      new Some(scala.Int.box(1)).foreach({
        $anonfun(x)
      });
      x.elem
    };
    final <static> <artifact> def $anonfun$foo$1(x$2: runtime.IntRef, x$1: Int): Unit = x$2.elem = x$1;
    def <init>(): <$anon: Object> = {
      anon$1.super.<init>();
      ()
    }
  }
}