Clash with PHP 8.1 JsonSerializable Interface

[MarkBaker]

Running tests against the latest PHP 8.1 nightly causes fatal errors with a clash between MyClabs Enum and the new return typehints implementation in the interface for jsonSerializable in PHP 8.1

PHPUnit\Framework\Exception: PHP Fatal error:  During inheritance of JsonSerializable: Uncaught Declaration of MyCLabs\Enum\Enum::jsonSerialize() should be compatible with JsonSerializable::jsonSerialize(): mixed

and the stack trace (for reference)

/home/runner/work/PhpSpreadsheet/PhpSpreadsheet/vendor/myclabs/php-enum/src/Enum.php:301
/home/runner/work/PhpSpreadsheet/PhpSpreadsheet/vendor/myclabs/php-enum/src/Enum.php:22

As PHP 8.1 is now in alpha release stage, I thought I'd better raise an issue to ensure that you are aware of the problem.

This is against version 1.8.0 of the Enum library

[MarkBaker]

The issue actually relates to this issue raised in PHP by jrfnl with the interface definition for jsonSerializable having a mixed return type and is highlighted here.

This can easily be resolved by specifying the return type in the implementation. However, the mixed pseudo-datatype was only introduced in PHP 8.0, and simply matching the signature would preclude usage of the library with versions of PHP < 8.0.

There are three possible solutions that I am aware of for this that would still allow usage with PHP < 8.0

• Create your own interface definition for jsonSerializable Example. which doesn't require a mixed return datatype. While simpler to implement, this does feel a very dirty approach, and could create problems for anybody checking that the Enum class implements the native jsonSerializable, and would need to implement the json encoding as well; but this may be the only solution to the mixed datatypes that can be returned.
• Use a Covariant return type, such as array or string - an example here - as suggested by Nikita Popov in the PHP issue linked above; although as the return value could be any datatype, this creates other problems.
• Also suggested by Nikita Popov in the PHP issue linked above, use the #[ReturnTypeWillChange] annotation to suppress the Deprecation notice. Example here. For versions of PHP prior to 8.0 when annotations were introduced, this will simply be treated as a comment inside the docblock, so will not trigger any additional notices

A combination of the first and third solutions might be a good approach, eliminating the deprecation notice, with the annotation highlighting that there is a difference between the implementation signature and the interface signature.

[mnapoli]

Thank you @MarkBaker, this should be fixed in the latest release.