Fix is_object() function for objects inside mixed (#1207)

runtime-common/core/core-types/definition/mixed.inl

@@ -24,7 +24,7 @@ void mixed::init_from(T &&v) {
     if (unlikely(!ptr_to_obj)) {
       php_error("Internal error. Trying to set invalid object to mixed");
     }
-    type_ = type::OBJECT;
+    type_ = v.is_null() ? type::NUL : type::OBJECT;
   } else {
     auto type_and_value_ref = get_type_and_value_ptr(v);
     type_ = type_and_value_ref.first;

runtime-common/core/runtime-core.h

@@ -1185,6 +1185,10 @@ bool f$is_object(const class_instance<T> &v) {
   return !v.is_null();
 }
 
+template<>
+inline bool f$is_object(const mixed &v) {
+  return v.is_object();
+}
 
 template<class T>
 bool f$is_integer(const T &v) {

tests/phpt/mixed/non_primitive/017_is_object.php

@@ -0,0 +1,43 @@
+@ok
+<?php
+require_once 'kphp_tester_include.php';
+
+class A {};
+
+/** @return A */
+function getObj(int $x) {
+    if ($x == 0) {
+        return null;
+    }
+    return new A();
+}
+
+/** @param $x mixed */
+function is_obj_to_str($x) {
+    if (is_object($x)) {
+        return "true";
+    }
+    return "false";
+}
+
+function test() {
+    /** @var mixed */
+    $x;
+    $a = new A();
+
+    $x = to_mixed($a);
+    echo is_obj_to_str($x) . "\n";
+
+    $x = 123;
+    echo is_obj_to_str($x) . "\n";
+
+    #ifndef KPHP
+    echo "false\n"; // PHP emits fatal error
+    return;
+    #endif
+    $a = getObj(0);
+    $x = to_mixed($a);
+    echo is_obj_to_str($x) . "\n";
+}
+
+test();