Note - This issue is split off from JDK-8300691 which discusses two possible language changes; this issue represents one of the changes described in that issue ("change #2").

Allow this code, which doesn't currently compile, to compile:

  for (int i = 1; i <= 3; i++) {
    Runnable r = () -> System.out.println(i); // error: "i" is not effectively final
  }

just like this code, which does compile:

  for (int i : new int[] { 1, 2, 3 }) {
    Runnable r = () -> System.out.println(i); // ok
  }

In other words, for the purposes of whether to allow a lambda, nested class, or switch case guard to capture a basic for() loop variable, treat a loop that looks like this:

    for (int i = 1; i <= 3; i++) {
        // statements involving i...
    }

as if it were written like this:

    for (int i = 1; i <= 3; i++) {
        int i$ = i;
        // statements involving i$...
    }

Amber-dev discussion: https://mail.openjdk.org/pipermail/amber-dev/2024-September/008921.html