Add qgis::overload method for new-style Qt connects

Used for new-style Qt connects to overloaded signals,
avoiding the usual horrible connect syntax required in
these circumstances.

Example usage:

connect( mSpinBox, qgis::overload< int >::of( &QSpinBox::valueChanged ),
    this, &MyClass::mySlot );

This is an alternative to qOverload, which was implemented in Qt 5.7.

See https://stackoverflow.com/a/16795664/1861260

src/core/qgis.h

@@ -299,9 +299,10 @@ inline double qgsRound( double number, double places )
 ///@cond PRIVATE
 
 /**
- * Contains "polyfills" for backporting c++ features from standards > c++11.
+ * Contains "polyfills" for backporting c++ features from standards > c++11 and Qt global methods
+ * added later than our minimum version.
  *
- * To be removed when minimum c++ build requirement includes the std implementation
+ * To be removed when minimum c++ or Qt build requirement includes the std implementation
  * for these features.
  *
  * \note not available in Python bindings.
@@ -361,6 +362,27 @@ namespace qgis
   template<class T, class... Args>
   typename _Unique_if<T>::_Known_bound
   make_unique( Args &&... ) = delete;
+
+  /**
+   * Used for new-style Qt connects to overloaded signals, avoiding the usual horrible connect syntax required
+   * in these circumstances.
+   *
+   * Example usage:
+   *
+   * connect( mSpinBox, qgis::overload< int >::of( &QSpinBox::valueChanged ), this, &MyClass::mySlot );
+   *
+   * This is an alternative to qOverload, which was implemented in Qt 5.7.
+   *
+   * See https://stackoverflow.com/a/16795664/1861260
+   */
+  template<typename... Args> struct overload
+  {
+    template<typename C, typename R>
+    static constexpr auto of( R( C::*pmf )( Args... ) ) -> decltype( pmf )
+    {
+      return pmf;
+    }
+  };
 }
 ///@endcond
 #endif

src/gui/symbology/qgssymbollayerwidget.cpp

@@ -1373,8 +1373,8 @@ QgsShapeburstFillSymbolLayerWidget::QgsShapeburstFillSymbolLayerWidget( QgsVecto
   mLayer = nullptr;
 
   setupUi( this );
-  connect( mSpinBlurRadius, static_cast< void ( QSpinBox::* )( int ) >( &QSpinBox::valueChanged ), this, &QgsShapeburstFillSymbolLayerWidget::mSpinBlurRadius_valueChanged );
-  connect( mSpinMaxDistance, static_cast < void ( QDoubleSpinBox::* )( double ) > ( &QDoubleSpinBox::valueChanged ), this, &QgsShapeburstFillSymbolLayerWidget::mSpinMaxDistance_valueChanged );
+  connect( mSpinBlurRadius, qgis::overload< int >::of( &QSpinBox::valueChanged ), this, &QgsShapeburstFillSymbolLayerWidget::mSpinBlurRadius_valueChanged );
+  connect( mSpinMaxDistance, qgis::overload< double >::of( &QDoubleSpinBox::valueChanged ), this, &QgsShapeburstFillSymbolLayerWidget::mSpinMaxDistance_valueChanged );
   connect( mDistanceUnitWidget, &QgsUnitSelectionWidget::changed, this, &QgsShapeburstFillSymbolLayerWidget::mDistanceUnitWidget_changed );
   connect( mRadioUseWholeShape, &QRadioButton::toggled, this, &QgsShapeburstFillSymbolLayerWidget::mRadioUseWholeShape_toggled );
   connect( mOffsetUnitWidget, &QgsUnitSelectionWidget::changed, this, &QgsShapeburstFillSymbolLayerWidget::mOffsetUnitWidget_changed );