Closure::bindTo warning

index.php

<?php
class Hoge
{
  static function foo()
  {
    return function() {
      echo "test";
    };
  }

  function bar()
  {
    return function() {
      echo "test2";
    };
  }
}

class Caller
{
  function call(callable $fn)
  {
    // http://php.net/manual/en/closure.bindto.php
    $c = $fn->bindTo($this);
    $c();
  }
}

(new Caller())->call(Hoge::foo());  // 5.5 & 5.6 => NG, 7.0 => OK
(new Caller())->call((new Hoge())->bar());  // 5.5 & 5.6 & 7.0 => OK