Remove reference restrictions from foreach

foreach only allowed variables to be traversed by reference. This never
really made sense because

    a) Expressions like array(&$a, &$b) can be meaningfully iterated by-ref
    b) Function calls can return by-ref (so they can also be meaningfully
       iterated)
    c) Iterators could at least in theory also be iterated by-ref (not
       sure if any iterator makes use of this)

With by-ref generators the restriction makes even less sense, so I removed
it altogether.

Zend/tests/errmsg_043.phpt

@@ -1,12 +0,0 @@
---TEST--
-errmsg: cannot create references to temp array
---FILE--
-<?php
-
-foreach (array(1,2,3) as $k=>&$v) {
-}
-
-echo "Done\n";
-?>
---EXPECTF--	
-Fatal error: Cannot create references to elements of a temporary array expression in %s on line %d

Zend/tests/foreach_temp_array_expr_with_refs.phpt

@@ -0,0 +1,18 @@
+--TEST--
+Temporary array expressions can be iterated by reference
+--FILE--
+<?php
+
+$a = 'a';
+$b = 'b';
+
+foreach ([&$a, &$b] as &$value) {
+    $value .= '-foo';
+}
+
+var_dump($a, $b);
+
+?>
+--EXPECT--
+string(5) "a-foo"
+string(5) "b-foo"

Zend/tests/generators/yield_by_reference.phpt

@@ -9,24 +9,34 @@ function &iter(array &$array) {
     }
 }
 
-$array = [1, 2, 3, 4, 5];
+$array = [1, 2, 3];
 $iter = iter($array);
 foreach ($iter as &$value) {
     $value *= -1;
 }
 var_dump($array);
 
+$array = [1, 2, 3];
+foreach (iter($array) as &$value) {
+    $value *= -1;
+}
+var_dump($array);
+
 ?>
 --EXPECT--
-array(5) {
+array(3) {
+  [0]=>
+  int(-1)
+  [1]=>
+  int(-2)
+  [2]=>
+  &int(-3)
+}
+array(3) {
   [0]=>
   int(-1)
   [1]=>
   int(-2)
   [2]=>
-  int(-3)
-  [3]=>
-  int(-4)
-  [4]=>
-  &int(-5)
+  &int(-3)
 }

Zend/zend_compile.c

@@ -6317,9 +6317,7 @@ void zend_do_foreach_cont(znode *foreach_token, const znode *open_brackets_token
 
 	if (value->EA & ZEND_PARSED_REFERENCE_VARIABLE) {
 		assign_by_ref = 1;
-		if (!(opline-1)->extended_value) {
-			zend_error(E_COMPILE_ERROR, "Cannot create references to elements of a temporary array expression");
-		}
+
 		/* Mark extended_value for assign-by-reference */
 		opline->extended_value |= ZEND_FE_FETCH_BYREF;
 		CG(active_op_array)->opcodes[foreach_token->u.op.opline_num].extended_value |= ZEND_FE_RESET_REFERENCE;

Zend/zend_language_parser.y

@@ -312,7 +312,7 @@ unticked_statement:
 		foreach_statement { zend_do_foreach_end(&$1, &$4 TSRMLS_CC); }
 	|	T_FOREACH '(' expr_without_variable T_AS
 		{ zend_do_foreach_begin(&$1, &$2, &$3, &$4, 0 TSRMLS_CC); }
-		variable foreach_optional_arg ')' { zend_check_writable_variable(&$6); zend_do_foreach_cont(&$1, &$2, &$4, &$6, &$7 TSRMLS_CC); }
+		foreach_variable foreach_optional_arg ')' { zend_do_foreach_cont(&$1, &$2, &$4, &$6, &$7 TSRMLS_CC); }
 		foreach_statement { zend_do_foreach_end(&$1, &$4 TSRMLS_CC); }
 	|	T_DECLARE { $1.u.op.opline_num = get_next_op_number(CG(active_op_array)); zend_do_declare_begin(TSRMLS_C); } '(' declare_list ')' declare_statement { zend_do_declare_end(&$1 TSRMLS_CC); }
 	|	';'		/* empty statement */

tests/lang/foreachLoop.008.phpt

@@ -1,10 +0,0 @@
---TEST--
-Foreach loop tests - error case: reference to constant array, with key.
---FILE--
-<?php
-foreach (array(1,2) as $k=>&$v) {
-  var_dump($v);
-}
-?>
---EXPECTF--
-Fatal error: Cannot create references to elements of a temporary array expression in %s on line 2