Merge branch 'PHP-8.1'

* PHP-8.1:
  Added test
  Fix type inference for assign to string offset with invalid index.

Author: dstogov

Zend/Optimizer/zend_inference.c

@@ -2690,7 +2690,7 @@ static zend_always_inline zend_result _zend_update_type_info(
 			if (ssa_op->result_def >= 0) {
 				tmp = 0;
 				if (t1 & MAY_BE_STRING) {
-					tmp |= MAY_BE_STRING;
+					tmp |= MAY_BE_STRING | MAY_BE_NULL;
 				}
 				if (t1 & (MAY_BE_ARRAY|MAY_BE_FALSE|MAY_BE_NULL|MAY_BE_UNDEF)) {
 					tmp |= (OP1_DATA_INFO() & (MAY_BE_ANY | MAY_BE_ARRAY_KEY_ANY | MAY_BE_ARRAY_OF_ANY | MAY_BE_ARRAY_OF_REF));

ext/opcache/tests/jit/assign_dim_006.phpt

@@ -0,0 +1,29 @@
+--TEST--
+JIT ASSIGN_DIM: 006
+--INI--
+opcache.enable=1
+opcache.enable_cli=1
+opcache.file_update_protection=0
+opcache.jit_buffer_size=1M
+--FILE--
+<?php
+function foo($s) {
+    $s = "123";
+    for($i = 0; $i < 5; $i++) {
+    	$x = $s[-5] = "x";
+    }
+	var_dump($x);
+}
+foo("123");
+?>
+--EXPECTF--
+Warning: Illegal string offset -5 in %sassign_dim_006.php on line 5
+
+Warning: Illegal string offset -5 in %sassign_dim_006.php on line 5
+
+Warning: Illegal string offset -5 in %sassign_dim_006.php on line 5
+
+Warning: Illegal string offset -5 in %sassign_dim_006.php on line 5
+
+Warning: Illegal string offset -5 in %sassign_dim_006.php on line 5
+NULL