Detect invalid uses of parent:: during compilation

We already detect the case where we're entirely outside a class --
now also check whether there actually is a parent.

This is a minor BC break, in that code that was never executed
might have previously contained an invalid parent:: reference without
generating an error.

Author: nikic

UPGRADING

@@ -20,6 +20,11 @@ PHP 7.4 UPGRADE NOTES
 1. Backward Incompatible Changes
 ========================================
 
+- Core:
+  . Referencing parent:: inside a class that does not have a parent will now
+    generate a compile-time error. Previously the error was only emitted at
+    run-time.
+
 - Curl:
   . Attempting to serialize a CURLFile class will now generate an exception.
     Previously the exception was only thrown on unserialization.

Zend/tests/bug75573.phpt

@@ -6,10 +6,6 @@ Bug #75573 (Segmentation fault in 7.1.12 and 7.0.26)
 class A
 {
 	var $_stdObject;
-	function initialize($properties = FALSE) {
-		$this->_stdObject = $properties ? (object) $properties : new stdClass();
-		parent::initialize();
-	}
 	function &__get($property)
 	{
 		if (isset($this->_stdObject->{$property})) {
@@ -31,10 +27,6 @@ class A
 
 class B extends A
 {
-	function initialize($properties = array())
-	{
-		parent::initialize($properties);
-	}
 	function &__get($property)
 	{
 		if (isset($this->settings) && isset($this->settings[$property])) {

Zend/tests/class_name_as_scalar_error_002.phpt

@@ -11,7 +11,4 @@ namespace Foo\Bar {
 }
 ?>
 --EXPECTF--
-Fatal error: Uncaught Error: Cannot use "parent" when current class scope has no parent in %s:%d
-Stack trace:
-#0 {main}
-  thrown in %s on line %d
+Fatal error: Cannot use "parent" when current class scope has no parent in %s on line %d

Zend/zend_compile.c

@@ -1358,10 +1358,16 @@ static uint32_t zend_get_class_fetch_type_ast(zend_ast *name_ast) /* {{{ */
 
 static void zend_ensure_valid_class_fetch_type(uint32_t fetch_type) /* {{{ */
 {
-	if (fetch_type != ZEND_FETCH_CLASS_DEFAULT && !CG(active_class_entry) && zend_is_scope_known()) {
-		zend_error_noreturn(E_COMPILE_ERROR, "Cannot use \"%s\" when no class scope is active",
-			fetch_type == ZEND_FETCH_CLASS_SELF ? "self" :
-			fetch_type == ZEND_FETCH_CLASS_PARENT ? "parent" : "static");
+	if (fetch_type != ZEND_FETCH_CLASS_DEFAULT && zend_is_scope_known()) {
+		zend_class_entry *ce = CG(active_class_entry);
+		if (!ce) {
+			zend_error_noreturn(E_COMPILE_ERROR, "Cannot use \"%s\" when no class scope is active",
+				fetch_type == ZEND_FETCH_CLASS_SELF ? "self" :
+				fetch_type == ZEND_FETCH_CLASS_PARENT ? "parent" : "static");
+		} else if (fetch_type == ZEND_FETCH_CLASS_PARENT && !ce->parent_name) {
+			zend_error_noreturn(E_COMPILE_ERROR,
+				"Cannot use \"parent\" when current class scope has no parent");
+		}
 	}
 }
 /* }}} */