PHP's support to contravariance and covariance

Hi internals, During my regular open source work, I realized that PHP yet do not support contravariance and covariance for user classes. Some examples that I found that could be really easy to implement and useful to end user are highlighted here: - contravariance.php - https://3v4l.org/I3v0u - covariance.php - https://3v4l.org/i79O5 For all those lazy people that don't want to open 2 new tabs to understand what I mean, here's a more elaborate example covering both scenarios: <?php class A {} class B extends A {} class Foo { public function bar(B $b) : A {} } class Woo extends Foo { public function bar(A $b) : B {} } $w = new Woo(); Basically, here are the highlights: - method arguments are ok to reference wider parameter types (and we just approved the PR for Parameter Type Widening) - this is what we call contravariant, as contra inheritance direction - return types are ok to enforce strictness for subtypes - this is what we call covariant, as Woo is a subtype of Foo I've walked through PHP source and it looks like the only needed place to modify is https://github.com/php/php-src/blob/master/Zend/zend_inheritance.c#L184, which would require to be decoupled in 2 functions to check which direction is should support (covariance or contravariance). Is there anything else that I am missing? I'm happy to write an RFC for that... =) Cheers, -- Guilherme Blanco Senior Technical Architect at Huge Inc.