Support negative layout spacings (do not assert)

This tests
* default spacing
* explicit positive spacing
* negative spacings

Negative spacings that causes the layout to shrink down to 0 or
a negative spacing are not supported.
(But make sure they don't crash the whole engine)

Task-number: QTBUG-35741

Change-Id: I69521dffb03c35a63db37ab51a2e99152d88272b
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>

2 files changed, 41 insertions, 1 deletions

diff --git a/src/layouts/qgridlayoutengine.cpp b/src/layouts/qgridlayoutengine.cpp
index 910e34a0..35aa04f1 100644
--- a/src/layouts/qgridlayoutengine.cpp
+++ b/src/layouts/qgridlayoutengine.cpp
@@ -813,7 +813,6 @@ int QGridLayoutEngine::effectiveLastRow(Qt::Orientation orientation) const
 
 void QGridLayoutEngine::setSpacing(qreal spacing, Qt::Orientations orientations)
 {
-    Q_ASSERT(spacing >= 0.0);
     if (orientations & Qt::Horizontal)
         q_defaultSpacings[Hor].setUserValue(spacing);
     if (orientations & Qt::Vertical)
diff --git a/tests/auto/controls/data/tst_gridlayout.qml b/tests/auto/controls/data/tst_gridlayout.qml
index 7e6240fd..22784aa7 100644
--- a/tests/auto/controls/data/tst_gridlayout.qml
+++ b/tests/auto/controls/data/tst_gridlayout.qml
@@ -786,5 +786,46 @@ Item {
 
             layout.destroy();
         }
+
+        Component {
+            id: layout_spacings_Component
+            GridLayout {
+                id: layout
+                Repeater {
+                    model: 2
+                    Rectangle {
+                        implicitWidth: 10
+                        implicitHeight: 10
+                    }
+                }
+            }
+        }
+
+        function test_spacings()
+        {
+            var layout = layout_spacings_Component.createObject(container);
+            waitForRendering(layout)
+
+            // breaks down below -19. This is acceptable, since it means that the implicit size of the layout is negative
+            var testSpacings = [Number.NaN, 0, 10, -5, -19]
+            layout.rowSpacing = 0
+            for (var i = 0; i < testSpacings.length; ++i) {
+                var sp = testSpacings[i]
+                if (isNaN(sp)) {
+                    sp = 5  // Test defaults
+                } else {
+                    layout.columnSpacing = sp
+                }
+                compare(itemRect(layout.children[0]), [ 0,  0,  10,  10])
+                compare(itemRect(layout.children[1]), [10 + sp,  0,  10,  10])
+                compare(layout.implicitWidth, 20 + sp)
+            }
+
+            // do not crash
+            layout.columnSpacing = -100
+            waitForRendering(layout)
+            verify(isFinite(layout.implicitWidth))
+            layout.destroy();
+        }
     }
 }