PHP RFC: Allow abstract function override

• Version: 0.1
• Date: 2017-03-01
• Author: @WesNetmo @ Twitter
• Status: Implemented (in PHP 7.2)
• First Published at: https://wiki.php.net/rfc/allow-abstract-function-override

PHP 7 introduced type declarations for return and improved ones for parameters. They currently support a very limited kind of variance (only to no-type):

<?php
class A { function bar(stdClass $x){} }
class B extends A { function bar($x): stdClass{} }

However, this isn't currently matched by the equivalent abstract declarations:

<?php
abstract class A { abstract function bar(stdClass $x); }
abstract class B extends A { abstract function bar($x): stdClass; }
class C extends B { function bar($x): stdClass{} }
// Fatal error: Can't inherit abstract function A::bar()
// (previously declared abstract in B)

This RFC proposes to allow this, even if it has very few uses, and because there is no reason to disallow a compatible redefinition.

Additionally, it comes in handy for documentation:

interface A{
 function doSomething();
}
 
interface B extends A{
 function doSomethingElse();
}
 
abstract class AProxy implements A{
 abstract protected function getOrigin(): A;
 function doSomething(){
 return $this->getOrigin()->doSomething();
 }
}
 
// This phpdoc syntax has quirks in phpdocumentor
// apigen and phpstorm, and it's almost unmanageable
// as phpdoc interpreters have poor multiline support
// (imagine documenting all the parameters, the return
// type, all the throws...):
/** @method B getOrigin() */
abstract class BProxy extends AProxy implements B{
 /** @return B */ // This is much better!
 abstract protected function getOrigin(): A;
 function doSomethingElse(){
 return $this->getOrigin()->doSomethingElse();
 }
}

And, obviously, it will gain more uses when PHP will support a full-featured type variance.

None.

7.2

2/3 majority required.

Pull request to handle the change: https://github.com/php/php-src/pull/2531

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