Req #72208 [Wfx]: Covariant parameter types should be supported

Edit report at https://bugs.php.net/bug.php?id=72208&edit=1 ID: 72208 Updated by: bwoebi@php.net Reported by: mbrowne83 at gmail dot com Summary: Covariant parameter types should be supported Status: Wont fix Type: Feature/Change Request Package: Class/Object related Operating System: Ubuntu 14.04.4 LTS PHP Version: 7.0.6 Block user comment: N Private report: N New Comment: Contravariant parameter types are something to be considered and already are … but they have their own problems (regarding binding-time verification of the type … as classes are currently not autoloaded when just a typehint is encountered, so you cannot check whether there's a relation between two differently named types or not.) Previous Comments: ------------------------------------------------------------------------ [2016-05-12 14:49:20] mbrowne83 at gmail dot com Sorry, I was confused. I noticed that contravariant parameter types are also not supported, but I imagine that was a design decision rather than an oversight. Thank you. ------------------------------------------------------------------------ [2016-05-12 14:30:05] requinix@php.net Oh, and the entry goes on to mention that generics/templating can help to address the use case. That feature is being considered: https://wiki.php.net/rfc/generics ------------------------------------------------------------------------ [2016-05-12 14:27:13] requinix@php.net How much of that Wikipedia entry did you read? 1. As it says, Eiffel is "[unique] among mainstream languages" for its support for covariant parameters. 2. The code does not obey LSP because of exactly what the entry points out: CatShelter needs to support everything that AnimalShelter does. By restricting the parameter to only Cat objects, it *breaks* LSP "which states that objects of subclasses should always be less restricted than objects of their superclass". function PutDogInShelter(AnimalShelter $shelter) { $dog = new Dog(); // class Dog extends Animal $shelter->putAnimal($dog); } The above code needs to work with all AnimalShelter-type objects, however it will not work with CatShelters. ------------------------------------------------------------------------ [2016-05-12 13:50:28] mbrowne83 at gmail dot com Description: ------------ It seems that PHP does not support covariant parameter types. It should be possible to override a base class method and make its parameter type more specific. In the code example, the warning message is erroneous (or at least arbitrary) because the code does in fact conform to the Liskov Substitution Principle and is valid OOP. Test script: --------------- //example use case from https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)#Covariant_method_return_type class Animal {} class Cat extends Animal {} class AnimalShelter { function putAnimal(Animal $a) {} } class CatShelter extends AnimalShelter { function putAnimal(Cat $c) {} } Expected result: ---------------- No output Actual result: -------------- Warning: Declaration of CatShelter::putAnimal(Cat $c) should be compatible with AnimalShelter::putAnimal(Animal $a) in /vagrant/test.php on line 14 ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=72208&edit=1