Add auto-increment keys

When no key is explicitely yielded PHP will used auto-incrementing keys
as a fallback. They behave the same as with arrays, i.e. the key is the
successor of the largest previously used integer key.

Zend/tests/generators/auto_incrementing_keys.phpt

@@ -0,0 +1,22 @@
+--TEST--
+Generator keys are auto-incrementing by default
+--FILE--
+<?php
+
+function *gen() {
+    yield 'foo';
+    yield 'bar';
+    yield 5 => 'rab';
+    yield 'oof';
+}
+
+foreach (gen() as $k => $v) {
+    echo $k, ' => ', $v, "\n";
+}
+
+?>
+--EXPECT--
+0 => foo
+1 => bar
+5 => rab
+6 => oof

Zend/zend_generators.c

@@ -116,6 +116,9 @@ static zend_object_value zend_generator_create(zend_class_entry *class_type TSRM
 	generator = emalloc(sizeof(zend_generator));
 	memset(generator, 0, sizeof(zend_generator));
 
+	/* The key will be incremented on first use, so it'll start at 0 */
+	generator->largest_used_integer_key = -1;
+
 	zend_object_std_init(&generator->std, class_type TSRMLS_CC);
 
 	object.handle = zend_objects_store_put(generator, NULL,

Zend/zend_generators.h

@@ -34,6 +34,8 @@ typedef struct _zend_generator {
 	zval *key;
 	/* Variable to put sent value into */
 	temp_variable *send_target;
+	/* Largest used integer key for auto-incrementing keys */
+	long largest_used_integer_key;
 } zend_generator;
 
 extern ZEND_API zend_class_entry *zend_ce_generator;

Zend/zend_vm_def.h

@@ -5332,11 +5332,19 @@ ZEND_VM_HANDLER(160, ZEND_YIELD, CONST|TMP|VAR|CV|UNUSED, CONST|TMP|VAR|CV|UNUSE
 			generator->key = key;
 		}
 
+		if (Z_TYPE_P(generator->key) == IS_LONG
+		    && Z_LVAL_P(generator->key) > generator->largest_used_integer_key
+		) {
+			generator->largest_used_integer_key = Z_LVAL_P(generator->key);
+		}
+
 		FREE_OP2_IF_VAR();
 	} else {
-		/* Setting the key to NULL signals that the auto-increment key
-		 * generation should be used */
-		generator->key = NULL;
+		/* If no key was specified we use auto-increment keys */
+		generator->largest_used_integer_key++;
+
+		ALLOC_INIT_ZVAL(generator->key);
+		ZVAL_LONG(generator->key, generator->largest_used_integer_key);
 	}
 
 	/* If a value is sent it should go into the result var */