Make Exception::$previous a typed property

Exception::$previous is a private property, so we can add a type:

    private ?Throwable $previous = null;

Author: nikic

Zend/tests/bug70121.phpt

@@ -5,5 +5,9 @@ Bug #70121 (unserialize() could lead to unexpected methods execution / NULL poin
 unserialize('O:12:"DateInterval":1:{s:4:"days";O:9:"Exception":7:{s:10:"'."\0".'*'."\0".'message";s:1:"x";s:17:"'."\0".'Exception'."\0".'string";s:1:"A";s:7:"'."\0".'*'."\0".'code";i:0;s:7:"'."\0".'*'."\0".'file";s:1:"a";s:7:"'."\0".'*'."\0".'line";i:1337;s:16:"'."\0".'Exception'."\0".'trace";a:0:{}s:19:"'."\0".'Exception'."\0".'previous";O:8:"stdClass":0:{}}}');
 ?>
 OK
---EXPECT--
-OK
+--EXPECTF--
+Fatal error: Uncaught TypeError: Cannot assign stdClass to property Exception::$previous of type ?Throwable in %s:%d
+Stack trace:
+#0 %s(%d): unserialize('O:12:"DateInter...')
+#1 {main}
+  thrown in %s on line %d

Zend/zend_exceptions.c

@@ -316,13 +316,7 @@ ZEND_METHOD(Exception, __wakeup)
 	CHECK_EXC_TYPE(ZEND_STR_CODE,     IS_LONG);
 	CHECK_EXC_TYPE(ZEND_STR_FILE,     IS_STRING);
 	CHECK_EXC_TYPE(ZEND_STR_LINE,     IS_LONG);
-	CHECK_EXC_TYPE(ZEND_STR_TRACE,    IS_ARRAY);
-	pvalue = zend_read_property(i_get_exception_base(object), object, "previous", sizeof("previous")-1, 1, &value);
-	if (pvalue && Z_TYPE_P(pvalue) != IS_NULL && (Z_TYPE_P(pvalue) != IS_OBJECT ||
-			!instanceof_function(Z_OBJCE_P(pvalue), zend_ce_throwable) ||
-			pvalue == object)) {
-		zend_unset_property(i_get_exception_base(object), object, "previous", sizeof("previous")-1);
-	}
+	/* The type of $trace and $previous is enforced through typed properties. */
 }
 /* }}} */
 
@@ -751,7 +745,10 @@ static void declare_exception_properties(zend_class_entry *ce)
 		ce, ZSTR_KNOWN(ZEND_STR_TRACE), &val, ZEND_ACC_PRIVATE, NULL,
 		(zend_type) ZEND_TYPE_INIT_MASK(MAY_BE_ARRAY));
 
-	zend_declare_property_null(ce, "previous", sizeof("previous")-1, ZEND_ACC_PRIVATE);
+	ZVAL_NULL(&val);
+	zend_declare_typed_property(
+		ce, ZSTR_KNOWN(ZEND_STR_PREVIOUS), &val, ZEND_ACC_PRIVATE, NULL,
+		(zend_type) ZEND_TYPE_INIT_CE(zend_ce_throwable, /* allow_null */ 1, 0));
 }
 
 void zend_register_default_exception(void) /* {{{ */

ext/standard/tests/serialize/bug69793.phpt

@@ -7,11 +7,8 @@ $e = unserialize('O:9:"Exception":7:{s:17:"'."\0".'Exception'."\0".'string";s:1:
 var_dump($e."");
 ?>
 --EXPECTF--
-Warning: Undefined property: Exception::$file in %s on line %d
-
-Warning: Undefined property: Exception::$previous in %s on line %d
-
-Warning: Undefined property: Exception::$previous in %s on line %d
-string(41) "Exception in :1337
+Fatal error: Uncaught TypeError: Cannot assign int to property Exception::$previous of type ?Throwable in %s:%d
 Stack trace:
-#0 {main}"
+#0 %s(%d): unserialize('O:9:"Exception"...')
+#1 {main}
+  thrown in %s on line %d

sapi/cli/tests/005.phpt

@@ -37,7 +37,7 @@ string(183) "Class [ <internal:Core> class stdClass ] {
 }
 
 "
-string(2166) "Class [ <internal:Core> class Exception implements Throwable, Stringable ] {
+string(2177) "Class [ <internal:Core> class Exception implements Throwable, Stringable ] {
 
   - Constants [0] {
   }
@@ -55,7 +55,7 @@ string(2166) "Class [ <internal:Core> class Exception implements Throwable, Stri
     Property [ protected $file = NULL ]
     Property [ protected $line = NULL ]
     Property [ private array $trace = Array ]
-    Property [ private $previous = NULL ]
+    Property [ private ?Throwable $previous = NULL ]
   }
 
   - Methods [11] {