Deprecate implicit dynamic properties

Writing to a proprety that hasn't been declared is deprecated,
unless the class uses the #[AllowDynamicProperties] attribute or
defines __get()/__set().

RFC: https://wiki.php.net/rfc/deprecate_dynamic_properties

Author: nikic

UPGRADING

@@ -48,6 +48,16 @@ PHP 8.2 UPGRADE NOTES
 ========================================
 
 - Core:
+  . Creation of dynamic properties is deprecated, unless the class opts in by
+    using the #[AllowDynamicProperties] attribute. stdClass allows dynamic
+    properties. Usage of __get()/__set() is not affected by this change. A
+    dynamic properties deprecation warning can be addressed by:
+     - Declaring the property (preferred).
+     - Adding the #[AllowDynamicProperties] attribute to the class (which also
+       applies to all child classes).
+     - Using a WeakMap if you wish to associate additional data with an object
+       you do not own.
+
   . Callables that are not accepted by the $callable() syntax (but are accepted
     by call_user_func) are deprecated. In particular:
 

Zend/Optimizer/sccp.c

@@ -1101,7 +1101,9 @@ static void sccp_visit_instr(scdf_ctx *scdf, zend_op *opline, zend_ssa_op *ssa_o
 
 				/* Don't try to propagate assignments to (potentially) typed properties. We would
 				 * need to deal with errors and type conversions first. */
-				if (!var_info->ce || (var_info->ce->ce_flags & ZEND_ACC_HAS_TYPE_HINTS)) {
+				// TODO: Distinguish dynamic and declared property assignments here?
+				if (!var_info->ce || (var_info->ce->ce_flags & ZEND_ACC_HAS_TYPE_HINTS) ||
+						!(var_info->ce->ce_flags & ZEND_ACC_ALLOW_DYNAMIC_PROPERTIES)) {
 					SET_RESULT_BOT(result);
 					SET_RESULT_BOT(op1);
 					return;

Zend/Optimizer/zend_inference.c

@@ -4842,15 +4842,17 @@ ZEND_API bool zend_may_throw_ex(const zend_op *opline, const zend_ssa_op *ssa_op
 					return 1;
 				}
 
-				if (op_array->scope != ce && ce->default_properties_count) {
-					zend_property_info *prop_info =
-						zend_hash_find_ptr(&ce->properties_info, prop_name);
-					if (prop_info && (!(prop_info->flags & ZEND_ACC_PUBLIC)
-								|| ZEND_TYPE_IS_SET(prop_info->type))) {
+				zend_property_info *prop_info =
+					zend_hash_find_ptr(&ce->properties_info, prop_name);
+				if (prop_info) {
+					if (ZEND_TYPE_IS_SET(prop_info->type)) {
 						return 1;
 					}
+					return !(prop_info->flags & ZEND_ACC_PUBLIC)
+						&& prop_info->ce != op_array->scope;
+				} else {
+					return !(ce->ce_flags & ZEND_ACC_ALLOW_DYNAMIC_PROPERTIES);
 				}
-				return 0;
 			}
 			return 1;
 		case ZEND_ROPE_INIT:

Zend/tests/allow_dynamic_properties_on_interface.phpt

@@ -0,0 +1,11 @@
+--TEST--
+#[AllowDynamicProperties] cannot be applied to interface
+--FILE--
+<?php
+
+#[AllowDynamicProperties]
+interface Test {}
+
+?>
+--EXPECTF--
+Fatal error: Cannot apply #[AllowDynamicProperties] to interface in %s on line %d

Zend/tests/allow_dynamic_properties_on_trait.phpt

@@ -0,0 +1,11 @@
+--TEST--
+#[AllowDynamicProperties] cannot be applied to trait
+--FILE--
+<?php
+
+#[AllowDynamicProperties]
+trait Test {}
+
+?>
+--EXPECTF--
+Fatal error: Cannot apply #[AllowDynamicProperties] to trait in %s on line %d

Zend/tests/anon/003.phpt

@@ -4,7 +4,7 @@ reusing anonymous classes
 <?php
 while (@$i++<10) {
     var_dump(new class($i) {
-
+        public $i;
         public function __construct($i) {
             $this->i = $i;
         }

Zend/tests/assign_to_obj_001.phpt

@@ -8,6 +8,7 @@ function &a($i) {
 }
 
 class A {
+    public $a;
     public function test() {
         $this->a = a(1);
         unset($this->a);

Zend/tests/bug27268.phpt

@@ -2,6 +2,7 @@
 Bug #27268 (Bad references accentuated by clone)
 --FILE--
 <?php
+#[AllowDynamicProperties]
 class A
 {
     public function &getA()

Zend/tests/bug30162.phpt

@@ -2,6 +2,7 @@
 Bug #30162 (Catching exception in constructor couses lose of $this)
 --FILE--
 <?php
+#[AllowDynamicProperties]
 class FIIFO {
 
     public function __construct() {
@@ -11,6 +12,7 @@ class FIIFO {
 
 }
 
+#[AllowDynamicProperties]
 class hariCow extends FIIFO {
 
     public function __construct() {

Zend/tests/bug38779_1.phpt

@@ -4,6 +4,7 @@ Bug #38779 (engine crashes when require()'ing file with syntax error through use
 <?php
 
 class Loader {
+    public $context;
     private $position;
     private $data;
     public function stream_open($path, $mode, $options, &$opened_path)  {