Fixed bug #79867

In line with usual rules, give untyped properties a null default
value. Otherwise constructor promotion would give you a property
declaration that cannot be achieved through any other means.

NEWS

@@ -13,6 +13,8 @@ PHP                                                                        NEWS
     (Nikita)
   . Fixed bug #79852 (count(DOMNodeList) doesn't match
     count(IteratorIterator(DOMNodeList))). (Nikita)
+  . Fixed bug #79867 (Promoted untyped properties should get null default
+    value). (Nikita)
 
 09 Jul 2020, PHP 8.0.0alpha2
 

Zend/tests/ctor_promotion_untyped_default.phpt

@@ -0,0 +1,29 @@
+--TEST--
+Bug #79867: Promoted untyped properties should get null default value
+--FILE--
+<?php
+
+class A {
+    public function __construct(
+        public $untyped = 1,
+        public int $typed = 2,
+    ) {}
+}
+
+class B extends A {
+    public function __construct() {
+        // Missing parent::__construct() call,
+        // properties will not be initialized.
+    }
+}
+
+var_dump(new B);
+
+?>
+--EXPECT--
+object(B)#1 (1) {
+  ["untyped"]=>
+  NULL
+  ["typed"]=>
+  uninitialized(int)
+}

Zend/zend_compile.c

@@ -6197,16 +6197,21 @@ void zend_compile_params(zend_ast *ast, zend_ast *return_type_ast, uint32_t fall
 					ZSTR_VAL(scope->name), ZSTR_VAL(name), ZSTR_VAL(str));
 			}
 
-			/* Always use uninitialized as the default. */
-			zval default_value;
-			ZVAL_UNDEF(&default_value);
-
 			/* Recompile the type, as it has different memory management requirements. */
 			zend_type type = ZEND_TYPE_INIT_NONE(0);
 			if (type_ast) {
 				type = zend_compile_typename(type_ast, /* force_allow_null */ 0, /* use_arena */ 1);
 			}
 
+			/* Don't give the property an explicit default value. For typed properties this means
+			 * uninitialized, for untyped properties it means an implicit null default value. */
+			zval default_value;
+			if (ZEND_TYPE_IS_SET(type)) {
+				ZVAL_UNDEF(&default_value);
+			} else {
+				ZVAL_NULL(&default_value);
+			}
+
 			zend_string *doc_comment =
 				doc_comment_ast ? zend_string_copy(zend_ast_get_str(doc_comment_ast)) : NULL;
 			zend_property_info *prop = zend_declare_typed_property(