Allow overriding abstract methods

RFC: https://wiki.php.net/rfc/allow-abstract-function-override

Author: pmmaga

NEWS

@@ -45,6 +45,8 @@ PHP                                                                        NEWS
     loosely-equal value). (pmmaga)
   . Fixed bug #61970 (Restraining __construct() access level in subclass gives
     a fatal error). (pmmaga)
+  . Fixed bug #63384 (Cannot override an abstract method with an abstract
+    method). (pmmaga, wes)
 
 - BCMath:
   . Fixed bug #46564 (bcmod truncates fractionals). (liborm85)

UPGRADING

@@ -101,6 +101,9 @@ PHP 7.2 UPGRADE NOTES
     inherited method. This complies with contravariance of method argument types
     under the Liskov Substitution Principle.
     (https://wiki.php.net/rfc/parameter-no-type-variance)
+  . It is now allowed to override an abstract method with another abstract
+    method in a child class.
+    (https://wiki.php.net/rfc/allow-abstract-function-override)
   . A trailing comma in group use statements is now allowed.
     (https://wiki.php.net/rfc/list-syntax-trailing-commas)
 

Zend/tests/abstract_inheritance_001.phpt

@@ -0,0 +1,12 @@
+--TEST--
+Allow abstract function override
+--FILE--
+<?php
+
+abstract class A           { abstract function bar($x); }
+abstract class B extends A { abstract function bar($x); }
+
+echo "DONE";
+?>
+--EXPECT--	
+DONE

Zend/tests/abstract_inheritance_002.phpt

@@ -0,0 +1,12 @@
+--TEST--
+Allow abstract function override
+--FILE--
+<?php
+
+abstract class A           { abstract function bar($x); }
+abstract class B extends A { abstract function bar($x, $y = 0); }
+
+echo "DONE";
+?>
+--EXPECT--	
+DONE

Zend/tests/abstract_inheritance_003.phpt

@@ -0,0 +1,12 @@
+--TEST--
+Allow abstract function override
+--FILE--
+<?php
+
+abstract class A           { abstract function bar($x, $y = 0); }
+abstract class B extends A { abstract function bar($x); }
+
+echo "DONE";
+?>
+--EXPECTF--	
+Fatal error: Declaration of B::bar($x) must be compatible with A::bar($x, $y = 0) in %s

Zend/zend_inheritance.c

@@ -551,16 +551,6 @@ static void do_inheritance_check_on_method(zend_function *child, zend_function *
 	uint32_t child_flags;
 	uint32_t parent_flags = parent->common.fn_flags;
 
-	if ((parent->common.scope->ce_flags & ZEND_ACC_INTERFACE) == 0
-		&& parent->common.fn_flags & ZEND_ACC_ABSTRACT
-		&& parent->common.scope != (child->common.prototype ? child->common.prototype->common.scope : child->common.scope)
-		&& child->common.fn_flags & (ZEND_ACC_ABSTRACT|ZEND_ACC_IMPLEMENTED_ABSTRACT)) {
-		zend_error_noreturn(E_COMPILE_ERROR, "Can't inherit abstract function %s::%s() (previously declared abstract in %s)",
-			ZSTR_VAL(parent->common.scope->name),
-			ZSTR_VAL(child->common.function_name),
-			child->common.prototype ? ZSTR_VAL(child->common.prototype->common.scope->name) : ZSTR_VAL(child->common.scope->name));
-	}
-
 	if (UNEXPECTED(parent_flags & ZEND_ACC_FINAL)) {
 		zend_error_noreturn(E_COMPILE_ERROR, "Cannot override final method %s::%s()", ZEND_FN_SCOPE_NAME(parent), ZSTR_VAL(child->common.function_name));
 	}