From e9e16c7464364fd15f69e3f37a9ed3edb15b633b Mon Sep 17 00:00:00 2001
From: Sergio Martins <sergio.martins@kdab.com>
Date: Mon, 3 Dec 2018 17:00:53 +0000
Subject: [PATCH] Warn when setting attributes after QCoreApplication is
 created

It's a recurring bug seen in user code and a warning will help
reduce it.

Warns only for the attributes that have such requirement in the docs,
but maybe we should be more strict and warn for any attribute.

Change-Id: I68148521953221ad0e8be1028181f52a7f22d2cc
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
---
 src/corelib/kernel/qcoreapplication.cpp | 26 +++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/src/corelib/kernel/qcoreapplication.cpp b/src/corelib/kernel/qcoreapplication.cpp
index e5f39a8d35..8652c45634 100644
--- a/src/corelib/kernel/qcoreapplication.cpp
+++ b/src/corelib/kernel/qcoreapplication.cpp
@@ -46,6 +46,7 @@
 #include "qcoreevent.h"
 #include "qeventloop.h"
 #endif
+#include "qmetaobject.h"
 #include "qcorecmdlineargs_p.h"
 #include <qdatastream.h>
 #include <qdebug.h>
@@ -966,6 +967,10 @@ bool QCoreApplication::isSetuidAllowed()
     Sets the attribute \a attribute if \a on is true;
     otherwise clears the attribute.
 
+    \note Some application attributes must be set \b before creating a
+    QCoreApplication instance. Refer to the Qt::ApplicationAttribute
+    documentation for more information.
+
     \sa testAttribute()
 */
 void QCoreApplication::setAttribute(Qt::ApplicationAttribute attribute, bool on)
@@ -974,6 +979,27 @@ void QCoreApplication::setAttribute(Qt::ApplicationAttribute attribute, bool on)
         QCoreApplicationPrivate::attribs |= 1 << attribute;
     else
         QCoreApplicationPrivate::attribs &= ~(1 << attribute);
+    if (Q_UNLIKELY(qApp)) {
+        switch (attribute) {
+            case Qt::AA_EnableHighDpiScaling:
+            case Qt::AA_DisableHighDpiScaling:
+            case Qt::AA_PluginApplication:
+            case Qt::AA_UseDesktopOpenGL:
+            case Qt::AA_UseOpenGLES:
+            case Qt::AA_UseSoftwareOpenGL:
+            case Qt::AA_ShareOpenGLContexts:
+#ifdef QT_BOOTSTRAPPED
+                qWarning("Attribute %d must be set before QCoreApplication is created.",
+                         attribute);
+#else
+                qWarning("Attribute Qt::%s must be set before QCoreApplication is created.",
+                         QMetaEnum::fromType<Qt::ApplicationAttribute>().valueToKey(attribute));
+#endif
+                break;
+            default:
+                break;
+        }
+    }
 }
 
 /*!