[php-src] PR #24030: [Draft RFC] Automated delegation using implements by

Pull Request: https://github.com/php/php-src/pull/24030 Author: knegyesi ====== PHP RFC: Automated delegation using implements by ====== * Version: 0.9 * Date: 2026-09-30 * Author: Karoly Negyesi <karoly@negyesi.net> * Status: Draft * Implementation: right here * Discussion thread: https://news-web.php.net/php.internals/… * Voting thread: tbd ===== Introduction ===== Composition is a frequently used design pattern. For example, ```php class MetadataBubblingUrlGenerator implements UrlGeneratorInterface { public function __construct( private UrlGeneratorInterface $url_generator, private RendererInterface $renderer ) {} public function setContext(SymfonyRequestContext $context): void { $this->urlGenerator->setContext($context); } public function getContext(): SymfonyRequestContext { return $this->urlGenerator->getContext(); } ``` The disadvantages are obvious: unnecessary boilerplate and maintenance burden as the interface gets more methods over time. ===== Proposal ===== Automate the pattern above using a new syntax implements by. When a class uses implements T by $t then generate all the proxy methods for methods of T if the class doesn't have them. Explicitly defined methods -- whether they are on the class, a parent class or a trait -- always win. Generated methods win over __call. ==== Examples ==== ```php interface T1 { function T1method(); } class ExampleImplementation implements T1 { function T1method() { print "Example"; } } class C implements T1 by $t1 { public function __construct( private T1 $t1, ) {} } new C(new ExampleImplementation)->T1method(); // Example ``` ===== Backward Incompatible Changes ===== None. The scanner rule ensures only the "by" keyword followed by a variable gets parsed into T_BY. (I copied the rule from enum with appropriate adjustements.) ===== Proposed PHP Version(s) ===== next PHP 8.x ===== RFC Impact ===== ==== To the Ecosystem ==== Less boilerplate. ==== To Existing Extensions ==== No. ==== To SAPIs ==== None. ===== Open Issues ===== 1. Does opcode need an update? 2. Does reflection need something? ===== Future Scope ===== When two interfaces have a method of the same name the first one wins naturally because it gets a generated method and the second interface will see it as an existing method. Using novel call time syntax to resolve the ambiguity is planned but it is out of scope. ===== References ===== - Kotlin, Delegation: https://kotlinlang.org/docs/delegation.html - Go, Embedding: https://go.dev/doc/effective_go#embedding ===== Rejected Features ===== ===== Changelog =====