Description:
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Generators behave differently than Iterators when calling valid on them followed by a loop. A non-rewindable iterator can be asked if it is valid by calling valid on it followed by a loop. The same is not possible with a Generator because it throws an exception stating that an already closed generator cannot be iterated.

This is extremely hard to debug because the problem is not apparent at all to the reader of the code.

Iterator code: https://3v4l.org/v0qAZ
Generator code: https://3v4l.org/2E7EJ

Also, note that HHVM/Hack behaves as expected.

Test script:
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$it = (function () { if (false) { yield 'foo'; }})();

var_dump($it->valid());

foreach ($it as $k => $v) { var_dump($k, $v); }

Expected result:
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bool(false)

Actual result:
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bool(false)

Fatal error: Uncaught Exception: Cannot traverse an already closed generator in /in/2E7EJ:7
Stack trace:
#0 {main}
  thrown in /in/2E7EJ on line 7

Process exited with code 255.