Fix SCCP evaluation of isset()/empty() on dimension of non-escaping object

`ct_eval_isset_dim()` treated partial objects like scalars and folded the
result to false/true. But using an object as an array either calls
`ArrayAccess::offsetExists()` or throws an Error.

Author: ndossche

Zend/Optimizer/sccp.c

@@ -436,6 +436,9 @@ static inline zend_result ct_eval_isset_dim(zval *result, uint32_t extended_valu
 	} else if (Z_TYPE_P(op1) == IS_STRING) {
 		// TODO
 		return FAILURE;
+	} else if (IS_PARTIAL_OBJECT(op1)) {
+		/* Objects may implement ArrayAccess or throw. */
+		return FAILURE;
 	} else {
 		ZVAL_BOOL(result, (extended_value & ZEND_ISEMPTY));
 		return SUCCESS;

ext/opcache/tests/opt/sccp_isset_dim_partial_object.phpt

@@ -0,0 +1,36 @@
+--TEST--
+SCCP: isset()/empty() on dimension of non-escaping object must not be evaluated
+--INI--
+opcache.enable=1
+opcache.enable_cli=1
+opcache.optimization_level=-1
+--EXTENSIONS--
+opcache
+--FILE--
+<?php
+class P {}
+
+function f() {
+    $o = new P;
+    return isset($o[1]);
+}
+
+function g() {
+    $o = new P;
+    return empty($o[1]);
+}
+
+try {
+    var_dump(f());
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    var_dump(g());
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+?>
+--EXPECT--
+Cannot use object of type P as array
+Cannot use object of type P as array