Related: Bug #71436

I hit this bug too today, still valid in PHP7.4. See https://3v4l.org/R4AH3. For others who hit this bug: use a closure and yield from as alternative.

function createEmptyGen1() { if(false) { yield 1; } }
function createEmptyGen2() { if(false) { yield 2; } }

$ai = (function () {
    yield from createEmptyGen1();
    yield from createEmptyGen2();
})();

foreach ($ai as $_);

What's even worse is that this bug is quite hidden - if the empty generator is NOT the first iterator in the AppendIterator then everything works fine. It only breaks if the empty generator is first.

----

Test script:

<?php

function createGenerator(array $array): Generator
{
    yield from $array;
}

// Example 1 (works)

$iterator = new AppendIterator();
$iterator->append(createGenerator([1]));
$iterator->append(createGenerator([2]));
$iterator->append(createGenerator([]));
$iterator->append(createGenerator([3]));

foreach ($iterator as $value) {
    echo $value;
}

// Example 1 (fails)

$iterator = new AppendIterator();
$iterator->append(createGenerator([])); // <-- this line is new
$iterator->append(createGenerator([1]));
$iterator->append(createGenerator([2]));
$iterator->append(createGenerator([]));
$iterator->append(createGenerator([3]));

foreach ($iterator as $value) {
    echo $value;
}

----

Expected result:

123456

----

Actual result:

123
Fatal error: Uncaught Exception: Cannot traverse an already closed generator in /in/jRMa7:29
Stack trace:
#0 /in/jRMa7(29): AppendIterator->rewind()
#1 {main}
  thrown in /in/jRMa7 on line 29

Process exited with code 255.