Add QDebug operator for Objective-C id type

Not every id is an NSObject, and even those that are are sometimes
passed around as id<SomeProtocol>, in which case we still want them
to have pretty debug descriptions.

Change-Id: I50a02cd8fb9d6a2cbde3c4aef81116033d10b72c
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qdebug.h

@@ -530,6 +530,7 @@ inline QDebug operator<<(QDebug debug, QKeyCombination combination)
 
 QT_END_NAMESPACE
 Q_FORWARD_DECLARE_CF_TYPE(CFString);
+typedef struct objc_object *id;
 Q_FORWARD_DECLARE_OBJC_CLASS(NSObject);
 QT_FOR_EACH_CORE_FOUNDATION_TYPE(QT_FORWARD_DECLARE_CF_TYPE)
 QT_FOR_EACH_MUTABLE_CORE_FOUNDATION_TYPE(QT_FORWARD_DECLARE_MUTABLE_CF_TYPE)
@@ -554,6 +555,7 @@ QT_BEGIN_NAMESPACE
     }
 
 // Defined in qcore_mac_objc.mm
+Q_CORE_EXPORT QDebug operator<<(QDebug, id obj);
 Q_CORE_EXPORT QDebug operator<<(QDebug, const NSObject *);
 Q_CORE_EXPORT QDebug operator<<(QDebug, CFStringRef);
 

src/corelib/kernel/qcore_mac.mm

@@ -196,6 +196,27 @@ os_log_t AppleUnifiedLogger::cachedLog(const QString &subsystem, const QString &
 
 // -------------------------------------------------------------------------
 
+QDebug operator<<(QDebug dbg, id obj)
+{
+    if (!obj) {
+        // Match NSLog
+        dbg << "(null)";
+        return dbg;
+    }
+
+    for (Class cls = object_getClass(obj); cls; cls = class_getSuperclass(cls)) {
+        if (cls == NSObject.class) {
+            dbg << static_cast<NSObject*>(obj);
+            return dbg;
+        }
+    }
+
+    // Match NSObject.debugDescription
+    const QDebugStateSaver saver(dbg);
+    dbg.nospace() << '<' << object_getClassName(obj) << ": " << static_cast<void*>(obj) << '>';
+    return dbg;
+}
+
 QDebug operator<<(QDebug dbg, const NSObject *nsObject)
 {
     return dbg << (nsObject ?