Req #73435 [Opn->Dup]: Return type-hinting polymorphism

Edit report at https://bugs.php.net/bug.php?id=73435&edit=1 ID: 73435 Updated by: requinix@php.net Reported by: php at haravikk dot me Summary: Return type-hinting polymorphism -Status: Open +Status: Duplicate Type: Feature/Change Request -Package: PHP Language Specification +Package: Scripting Engine problem PHP Version: 7.0.12 Block user comment: N Private report: N New Comment: Duplicate of request #72442. Previous Comments: ------------------------------------------------------------------------ [2016-11-01 17:31:16] php at haravikk dot me Description: ------------ I'd like to propose that when implementing/extending that type-hints for return values can be fulfilled by any sub-class of the same type. In other words, sub-classes, when implementing/extending a method, should be able to provide a stricter type-hint for their return value, so long as it is a sub-class of the parent's type-hint. Please see the test script for an example; in it I would expect to be able to provide a return type-hint of "Bar", as it is a sub-type of "Foo", therefore meeting the original requirements while clarifying that the sub-class is more specific in what specific flavour of "Foo" it will actually return. There is a similar issue (#60107) in which the Liskov Substitution Principle is referenced, however, while allowing this behaviour on method arguments might breach LSP, I believe doing so only for return types should be okay. Test script: --------------- class Foo {} class Bar extends Foo {} abstract class MyType { abstract function example(): Foo; } class MyTypeImpl extends MyType { function example(): Bar { /* Do something here */ } } Expected result: ---------------- The implementation of the example method by "MyTypeImpl" should be accepted as a valid implementation of the method in "MyType". Actual result: -------------- Fatal error: Declaration of MyTypeImpl::example(Bar $value) must be compatible with MyType::example(Foo $value) in test.php on line 12 ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=73435&edit=1