Fix inference if FETCH_DIM_W user optimized away

In this case the user may be a FREE.

Also add the test file that I forgot in
3ce472d.

Author: nikic

Zend/Optimizer/zend_inference.c

@@ -3456,6 +3456,9 @@ static zend_always_inline zend_result _zend_update_type_info(
 								/* This can occur if a DIM_FETCH_FUNC_ARG with UNUSED op2 is left
 								 * behind, because it can't be converted to DIM_FETCH_R. */
 								break;
+							case ZEND_FREE:
+								/* This may happen if the using opcode is DCEd.  */
+								break;
 							EMPTY_SWITCH_DEFAULT_CASE()
 						}
 						j = zend_ssa_next_use(ssa->ops, ssa_op->result_def, j);

Zend/tests/string_offset_as_object.phpt

@@ -0,0 +1,63 @@
+--TEST--
+Using string offset as object
+--FILE--
+<?php
+
+$str = "x";
+try {
+    $str[0]->bar = "xyz";
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    $str[0]->bar[1] = "bang";
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    $str[0]->bar += 1;
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    $str[0]->bar = &$b;
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    ++$str[0]->bar;
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    --$str[0]->bar;
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    $str[0]->bar++;
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    $str[0]->bar--;
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    unset($str[0]->bar);
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+
+?>
+--EXPECT--
+Cannot use string offset as an object
+Cannot use string offset as an object
+Cannot use string offset as an object
+Cannot use string offset as an object
+Cannot use string offset as an object
+Cannot use string offset as an object
+Cannot use string offset as an object
+Cannot use string offset as an object
+Cannot use string offset as an object

Zend/tests/string_offset_optimization.phpt

@@ -0,0 +1,16 @@
+--TEST--
+Invalid string offset use where the actual use may be optimized away
+--FILE--
+<?php
+function test() {
+    $a = "";
+    $c = array(&$a[0]);
+}
+test();
+?>
+--EXPECTF--
+Fatal error: Uncaught Error: Cannot create references to/from string offsets in %s:%d
+Stack trace:
+#0 %s(%d): test()
+#1 {main}
+  thrown in %s on line %d