Reorder execution and expand comment

Zend/zend_inheritance.c

@@ -623,11 +623,11 @@ static inheritance_status zend_perform_covariant_type_check(
 zend_type *single_type;
 bool all_success = true;
 
-/* For intersection types loop over the parent types first as a child
- * can add them */
-if (ZEND_TYPE_IS_INTERSECTION(fe_type)
-|| (ZEND_TYPE_IS_INTERSECTION(proto_type) && !ZEND_TYPE_IS_UNION(fe_type))
-) {
+/* If the child type is an intersection type then we need to loop over
+ * the parents firstFor intersection types loop over the parent types first
+ * as the child can add types, however none of them can be a supertype of
+ * a parent type. */
+if (ZEND_TYPE_IS_INTERSECTION(fe_type)) {
 /* First try to check whether we can succeed without resolving anything */
 ZEND_TYPE_FOREACH(proto_type, single_type) {
 inheritance_status status;
@@ -657,13 +657,12 @@ static inheritance_status zend_perform_covariant_type_check(
 all_success = false;
 }
 } ZEND_TYPE_FOREACH_END();
-} else if (ZEND_TYPE_IS_INTERSECTION(proto_type) && ZEND_TYPE_IS_UNION(fe_type)) {
-/* Here each member of the child union must be a subtype of the intersection */
+} else if (ZEND_TYPE_IS_INTERSECTION(proto_type)) {
+/* Here each member of the child union must be a subtype of the intersection
+ * Note: the union can be a single element */
 
 /* First try to check whether we can succeed without resolving anything */
-zend_type_list *child_union_types = ZEND_TYPE_LIST(fe_type);
-
-ZEND_TYPE_LIST_FOREACH(child_union_types, single_type) {
+ZEND_TYPE_FOREACH(fe_type, single_type) {
 inheritance_status status;
 zend_string *fe_class_name;
 zend_class_entry *fe_ce = NULL;
@@ -689,7 +688,7 @@ static inheritance_status zend_perform_covariant_type_check(
 if (status != INHERITANCE_SUCCESS) {
 all_success = false;
 }
-} ZEND_TYPE_LIST_FOREACH_END();
+} ZEND_TYPE_FOREACH_END();
 }
 /* Only union or single types both in parent and child */
 else {