Fixed bug #62609: Allow implementing Traversable in abstract class

Master only, as this depends on fixes to calling order of
interface implementation handlers.

Author: nikic

NEWS

@@ -14,6 +14,8 @@ PHP                                                                        NEWS
     checks). (Nikita)
   . Fixed bug #69084 (Unclear error message when not implementing a renamed
     abstract trait function). (Nikita)
+  . Fixed bug #62609 (Allow implementing Traversable on abstract classes).
+    (Nikita)
 
 - CURL:
   . Bumped required libcurl version to 7.29.0. (cmb)

Zend/tests/bug62609.phpt

@@ -0,0 +1,12 @@
+--TEST--
+Bug #62609: Allow implementing Traversable on abstract classes (fail)
+--FILE--
+<?php
+
+abstract class AbstractTraversable implements Traversable {}
+
+class NonAbstractTraversable extends AbstractTraversable {}
+
+?>
+--EXPECT--
+Fatal error: Class NonAbstractTraversable must implement interface Traversable as part of either Iterator or IteratorAggregate in Unknown on line 0

Zend/tests/bug62609_2.phpt

@@ -0,0 +1,22 @@
+--TEST--
+Bug #62609: Allow implementing Traversable on abstract classes (work)
+--FILE--
+<?php
+
+abstract class AbstractTraversable implements Traversable {}
+
+class NonAbstractTraversable extends AbstractTraversable implements IteratorAggregate {
+    public function getIterator() {
+        yield "foo";
+        yield "bar";
+    }
+}
+
+foreach (new NonAbstractTraversable as $value) {
+    echo $value, "\n";
+}
+
+?>
+--EXPECT--
+foo
+bar

Zend/zend_interfaces.c

@@ -293,6 +293,11 @@ static int zend_implement_traversable(zend_class_entry *interface, zend_class_en
 	if (class_type->get_iterator || (class_type->parent && class_type->parent->get_iterator)) {
 		return SUCCESS;
 	}
+	/* Abstract class can implement Traversable only, in which case the extending class must
+	 * implement Iterator or IteratorAggregate. */
+	if (class_type->ce_flags & ZEND_ACC_EXPLICIT_ABSTRACT_CLASS) {
+		return SUCCESS;
+	}
 	if (class_type->num_interfaces) {
 		ZEND_ASSERT(class_type->ce_flags & ZEND_ACC_RESOLVED_INTERFACES);
 		for (i = 0; i < class_type->num_interfaces; i++) {