Bug #81573 [Com]: Invalid fatal error regarding mixed return type compatibility

Edit report at https://bugs.php.net/bug.php?id=81573&edit=1 ID: 81573 Comment by: hanskrentel at yahoo dot de Reported by: azjezz at protonmail dot com Summary: Invalid fatal error regarding mixed return type compatibility Status: Open Type: Bug Package: Compile Failure Operating System: N/A PHP Version: 7.4.25 Block user comment: N Private report: N New Comment: > This bug occurs on PHP 7.4, 8.0, and 8.1-dev. My understanding it that in PHP 7.4 it triggers the autoloader for mixed. > This shouldn't result in any type of error as T is a > sub-type of mixed. Undefined types have never been a sub-type in PHP. Not in PHP 8.1, 8.0 7.4 or any previous version. The introduction of the special type "mixed" in PHP 8.0 has not changed that AFAIK. Compare with a code that demonstrates what establishes a bug on your end with the less special type stdClass of which any type in PHP userspace is a sub-type of (as we can only defined classes): <?php class A { public function foo(): stdClass { exit(0); } } class B extends A { public function foo(): C { exit(0); } } Fatal error: Could not check compatibility between B::foo(): C and A::foo(): stdClass, because class C is not available in /in/vsMsS on line 8 The behaviour on mixed looks pretty well aligned in my eyes to the standard behaviour, therefore it would help if you could elaborate why this should be a) different for mixed and b) even qualify as a bug. Previous Comments: ------------------------------------------------------------------------ [2021-10-30 16:02:33] azjezz at protonmail dot com Description: ------------ This bug occurs on PHP 7.4, 8.0, and 8.1-dev. when the return type of a method is changed in a sub-class from mixed to T and T is undefined, you get a fatal error. This shouldn't result in any type of error as T is a sub-type of mixed. reference: https://github.com/revoltphp/event-loop/pull/15/files Test script: --------------- <?php class A { public function foo(): mixed { exit(0); } } class B extends A { public function foo(): C { exit(0); } } Expected result: ---------------- no errors. Actual result: -------------- Fatal error: Could not check compatibility between B::foo(): C and A::foo(): mixed, because class C is not available in /in/k7Jmq on line 8 ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=81573&edit=1