Description:
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If you call a method that does not exist on an IteratorIterator, it delegates that call to the inner iterator. The test script demonstrates this non existent method delegation.

I'm unable to find any reference to this behaviour in the documentation.

It should probably also be documented that implementing __call() will replace the normal delegation.

Test script:
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<?php

$inner = new ArrayIterator([1, 2]);
$outer = new IteratorIterator($inner);

var_dump(method_exists(get_class($outer), 'count'));
var_dump(method_exists($outer, 'count'));
var_dump($outer->count());

// bool(false)
// bool(true)
// int(2)