Ignore inheritance rules on private methods

Closes GH-5401

UPGRADING

@@ -194,6 +194,9 @@ PHP 8.0 UPGRADE NOTES
     RFC: https://wiki.php.net/rfc/locale_independent_float_to_string
   . Removed support for deprecated curly braces for offset access
     RFC: https://wiki.php.net/rfc/deprecate_curly_braces_array_access
+  . Applying the final modifier on a private method will now produce a warning
+    unless that method is the constructor.
+    RFC: https://wiki.php.net/rfc/inheritance_private_methods
 
 - COM:
   . Removed the ability to import case-insensitive constants from type
@@ -584,6 +587,10 @@ PHP 8.0 UPGRADE NOTES
     RFC: https://wiki.php.net/rfc/constructor_promotion
   . Added support for `match` expression.
     RFC: https://wiki.php.net/rfc/match_expression_v2
+  . Private methods declared on a parent class no longer enforce any
+    inheritance rules on the methods of a child class. (with the exception of
+    final private constructors)
+    RFC: https://wiki.php.net/rfc/inheritance_private_methods
 
 - Date:
   . Added DateTime::createFromInterface() and

Zend/tests/method_argument_binding.phpt

@@ -24,7 +24,7 @@ class C extends B {
 (new C)->test();
 
 class D {
-    private final function method(&$x) {
+    private function method(&$x) {
         ++$x;
     }
 }

Zend/zend_compile.c

@@ -6486,6 +6486,10 @@ zend_string *zend_begin_method_decl(zend_op_array *op_array, zend_string *name,
 
 	zend_string *lcname;
 
+	if ((fn_flags & ZEND_ACC_PRIVATE) && (fn_flags & ZEND_ACC_FINAL) && !zend_is_constructor(name)) {
+		zend_error(E_COMPILE_WARNING, "Private methods cannot be final as they are never overridden by other classes");
+	}
+
 	if (in_interface) {
 		if (!(fn_flags & ZEND_ACC_PUBLIC) || (fn_flags & (ZEND_ACC_FINAL|ZEND_ACC_ABSTRACT))) {
 			zend_error_noreturn(E_COMPILE_ERROR, "Access type for interface method "

Zend/zend_inheritance.c

@@ -828,6 +828,14 @@ static zend_always_inline inheritance_status do_inheritance_check_on_method_ex(
 	uint32_t parent_flags = parent->common.fn_flags;
 	zend_function *proto;
 
+	if (UNEXPECTED((parent_flags & ZEND_ACC_PRIVATE) && !(parent_flags & ZEND_ACC_ABSTRACT) && !(parent_flags & ZEND_ACC_CTOR))) {
+		if (!check_only) {
+			child->common.fn_flags |= ZEND_ACC_CHANGED;
+		}
+		/* The parent method is private and not an abstract so we don't need to check any inheritance rules */
+		return INHERITANCE_SUCCESS;
+	}
+
 	if (!checked && UNEXPECTED(parent_flags & ZEND_ACC_FINAL)) {
 		if (check_only) {
 			return INHERITANCE_ERROR;
@@ -869,10 +877,6 @@ static zend_always_inline inheritance_status do_inheritance_check_on_method_ex(
 		child->common.fn_flags |= ZEND_ACC_CHANGED;
 	}
 
-	if ((parent_flags & ZEND_ACC_PRIVATE) && !(parent_flags & ZEND_ACC_ABSTRACT)) {
-		return INHERITANCE_SUCCESS;
-	}
-
 	proto = parent->common.prototype ?
 		parent->common.prototype : parent;
 

tests/classes/clone_005.phpt

@@ -6,7 +6,7 @@ abstract class base {
   public $a = 'base';
 
   // disallow cloning once forever
-  final private function __clone() {}
+  final protected function __clone() {}
 }
 
 class test extends base {

tests/classes/final_private_ctor.phpt

@@ -0,0 +1,21 @@
+--TEST--
+Final private constructors cannot be overridden
+--FILE--
+<?php
+
+class Base
+{
+    private final function __construct()
+    {
+    }
+}
+class Extended extends Base
+{
+    public function __construct()
+    {
+    }
+}
+
+?>
+--EXPECTF--
+Fatal error: Cannot override final method Base::__construct() in %s on line %d

tests/classes/inheritance_007.phpt

@@ -0,0 +1,51 @@
+--TEST--
+Ensure private methods with the same name are not checked for inheritance rules - final
+--FILE--
+<?php
+class A {
+    function callYourPrivates() {
+        $this->normalPrivate();
+        $this->finalPrivate();
+    }
+    function notOverridden_callYourPrivates() {
+        $this->normalPrivate();
+        $this->finalPrivate();
+    }
+    private function normalPrivate() {
+        echo __METHOD__ . PHP_EOL;
+    }
+    final private function finalPrivate() {
+        echo __METHOD__ . PHP_EOL;
+    }
+}
+class B extends A {
+    function callYourPrivates() {
+        $this->normalPrivate();
+        $this->finalPrivate();
+    }
+    private function normalPrivate() {
+        echo __METHOD__ . PHP_EOL;
+    }
+    final private function finalPrivate() {
+        echo __METHOD__ . PHP_EOL;
+    }
+}
+$a = new A();
+$a->callYourPrivates();
+$a->notOverridden_callYourPrivates();
+$b = new B();
+$b->callYourPrivates();
+$b->notOverridden_callYourPrivates();
+?>
+--EXPECTF--
+Warning: Private methods cannot be final as they are never overridden by other classes %s
+
+Warning: Private methods cannot be final as they are never overridden by other classes %s
+A::normalPrivate
+A::finalPrivate
+A::normalPrivate
+A::finalPrivate
+B::normalPrivate
+B::finalPrivate
+A::normalPrivate
+A::finalPrivate

tests/classes/inheritance_008.phpt

@@ -0,0 +1,16 @@
+--TEST--
+Ensure private methods with the same name are not checked for inheritance rules - static
+--FILE--
+<?php
+class A {
+    static private function foo() { }
+    private function bar() {}
+}
+class B extends A {
+    private function foo() {}
+    static private function bar() {}
+}
+echo 'OK';
+?>
+--EXPECT--
+OK

tests/classes/inheritance_009.phpt

@@ -0,0 +1,14 @@
+--TEST--
+Ensure private methods with the same name are not checked for inheritance rules - abstract
+--FILE--
+<?php
+class A {
+    private function test() {}
+}
+abstract class B extends A {
+    abstract function test();
+}
+echo 'OK';
+?>
+--EXPECT--
+OK