fun caller(g) {
  g();
  # g should be a function, not nil.
  print(g == nil); # expect: false
}

fun callCaller() {
  var capturedVar = "before";
  var a = "a";

  fun f() {
    # Commenting the next line out prevents the bug!
    capturedVar = "after";

    # Returning anything also fixes it, even nil:
    #return nil;
  }

  caller(f);
}

callCaller();