QSimplex: split QConcreteSimplexVariable from QSimplexVariable

The latter was used both as a concrete type as well as a base class,
driving Coverity nuts because it couldn't prove that we weren't
deleting derived classes (AnchorData) through a QSimplexVariable
pointer.

This is the same issue that Coverity took with QBrushData (CIDs
218724, 11772), and the solution is the same
(cf. 3bbc9e29ef59683351cf35c19a8bd4a030615c64):

Split the Janus-headed class into one that acts only as the base class
(and has a protected dtor) and one that only acts as a concrete class
(and we can mark it final).

The protected dtor in the former now statically ensures we don't
delete a derived class object through a QSimplexVariable pointer.

We don't need to modify AnchorData subclasses, because AnchorData
introduces a virtual destructor.

Coverity-Id: 390828
Pick-to: 6.8 6.5
Change-Id: I981c02e69af44ebacd4ba3aec76792e14eb15836
Reviewed-by: Mate Barany <mate.barany@qt.io>
(cherry picked from commit a405834ae694ef791d0b648b9a6bb65b67904731)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/graphicsview/qgraphicsanchorlayout_p.cpp

@@ -2764,11 +2764,11 @@ enum slackType { Grower = -1, Shrinker = 1 };
 static auto createSlack(QSimplexConstraint *sizeConstraint, qreal interval, slackType type)
 {
     struct R {
-        QSimplexVariable *slack;
+        QConcreteSimplexVariable *slack;
         QSimplexConstraint *limit;
     };
 
-    QSimplexVariable *slack = new QSimplexVariable;
+    auto slack = new QConcreteSimplexVariable;
     sizeConstraint->variables.insert(slack, type);
 
     QSimplexConstraint *limit = new QSimplexConstraint;
@@ -2783,7 +2783,7 @@ bool QGraphicsAnchorLayoutPrivate::solvePreferred(const QList<QSimplexConstraint
                                                   const QList<AnchorData *> &variables)
 {
     QList<QSimplexConstraint *> preferredConstraints;
-    QList<QSimplexVariable *> preferredVariables;
+    QList<QConcreteSimplexVariable *> preferredVariables;
     QSimplexConstraint objective;
 
     // Fill the objective coefficients for this variable. In the

src/widgets/graphicsview/qsimplex_p.cpp

@@ -156,28 +156,28 @@ bool QSimplex::setConstraints(const QList<QSimplexConstraint *> &newConstraints)
     QList <QSimplexVariable *> artificialList;
 
     for (int i = 0; i < constraints.size(); ++i) {
-        QSimplexVariable *slack;
-        QSimplexVariable *surplus;
-        QSimplexVariable *artificial;
+        QConcreteSimplexVariable *slack;
+        QConcreteSimplexVariable *surplus;
+        QConcreteSimplexVariable *artificial;
 
         Q_ASSERT(constraints[i]->helper.first == 0);
         Q_ASSERT(constraints[i]->artificial == nullptr);
 
         switch(constraints[i]->ratio) {
         case QSimplexConstraint::LessOrEqual:
-            slack = new QSimplexVariable;
+            slack = new QConcreteSimplexVariable;
             slack->index = ++variableIndex;
             constraints[i]->helper.first = slack;
             constraints[i]->helper.second = 1.0;
             break;
         case QSimplexConstraint::MoreOrEqual:
-            surplus = new QSimplexVariable;
+            surplus = new QConcreteSimplexVariable;
             surplus->index = ++variableIndex;
             constraints[i]->helper.first = surplus;
             constraints[i]->helper.second = -1.0;
             Q_FALLTHROUGH();
         case QSimplexConstraint::Equal:
-            artificial = new QSimplexVariable;
+            artificial = new QConcreteSimplexVariable;
             constraints[i]->artificial = artificial;
             artificialList += constraints[i]->artificial;
             break;

src/widgets/graphicsview/qsimplex_p.h

@@ -29,8 +29,14 @@ struct QSimplexVariable
 
     qreal result;
     int index;
+protected:
+    QT_DECLARE_RO5_SMF_AS_DEFAULTED(QSimplexVariable)
 };
 
+// "pure" QSimplexVariable without the protected destructor
+struct QConcreteSimplexVariable final : QSimplexVariable
+{
+};
 
 /*!
   \internal
@@ -59,8 +65,8 @@ struct QSimplexConstraint final
     qreal constant;
     Ratio ratio;
 
-    std::pair<QSimplexVariable *, qreal> helper;
-    QSimplexVariable * artificial;
+    std::pair<QConcreteSimplexVariable *, qreal> helper;
+    QConcreteSimplexVariable *artificial;
 
     void invert();