Itemviews: start fixing mixups of int/enum for Qt's item roles

A model is supposed to return a Qt::CheckState for a CheckStateRole,
and a Qt::Alignment for a Qt::TextAlignmentRole. This is what the
documentation says (and what makes sense), but unfortunately
Qt's default delegate expected a plain `int` instead.
This sometimes worked (via QVariant conversions, e.g. when using a plain
enum) and sometimes didn't (e.g. when using a flag type).

This is confusing for end-users (and type unsafe, killing the whole
point of using enums and flags in the first place).

Adding some automatic flags<->int conversions through QVariant is
frowned upon, so I don't want to go there. Instead, add some private
convenience functions that extract either the right type from a variant,
or try to extract an `int` and convert it to the expected type.

Use these from within itemviews code.

Change-Id: I44bee98c4a26a1ef6c3b2fa1b8de2edfee7aef32
Pick-to: 6.2 6.3
Fixes: QTBUG-75172
Task-number: QTBUG-74639
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/itemmodels/qabstractitemmodel_p.h

@@ -157,6 +157,52 @@ public:
 };
 Q_DECLARE_TYPEINFO(QAbstractItemModelPrivate::Change, Q_RELOCATABLE_TYPE);
 
+namespace QtPrivate {
+
+/*!
+    \internal
+    This is a workaround for QTBUG-75172.
+
+    Some predefined model roles are supposed to use certain enum/flag
+    types (e.g. fetching Qt::TextAlignmentRole is supposed to return a
+    variant containing a Qt::Alignment object).
+
+    For historical reasons, a plain `int` was used sometimes. This is
+    surprising to end-users and also sloppy on Qt's part; users were
+    forced to use `int` rather than the correct datatype.
+
+    This function tries both the "right" type and plain `int`, for a
+    given QVariant. This fixes the problem (using the correct datatype)
+    but also keeps compatibility with existing code using `int`.
+
+    ### Qt 7: get rid of this. Always use the correct datatype.
+*/
+template <typename T>
+T legacyEnumValueFromModelData(const QVariant &data)
+{
+    static_assert(std::is_enum_v<T>);
+    if (data.userType() == qMetaTypeId<T>())
+        return data.value<T>();
+    else if (data.userType() == qMetaTypeId<int>())
+        return T(data.toInt());
+
+    return T();
+}
+
+template <typename T>
+T legacyFlagValueFromModelData(const QVariant &data)
+{
+    if (data.userType() == qMetaTypeId<T>())
+        return data.value<T>();
+    else if (data.userType() == qMetaTypeId<int>())
+        return T::fromInt(data.toInt());
+
+    return T();
+}
+
+} // namespace QtPrivate
+
+
 QT_END_NAMESPACE
 
 #endif // QABSTRACTITEMMODEL_P_H

src/widgets/itemviews/qabstractitemdelegate_p.h

@@ -55,6 +55,9 @@
 #include "qabstractitemdelegate.h"
 #include <private/qobject_p.h>
 
+#include <qvariant.h>
+#include <qmetatype.h>
+
 QT_REQUIRE_CONFIG(itemviews);
 
 QT_BEGIN_NAMESPACE

src/widgets/itemviews/qheaderview.cpp

@@ -60,6 +60,7 @@
 #endif
 #include <private/qheaderview_p.h>
 #include <private/qabstractitemmodel_p.h>
+#include <private/qabstractitemdelegate_p.h>
 
 #ifndef QT_NO_DATASTREAM
 #include <qdatastream.h>
@@ -2936,9 +2937,9 @@ void QHeaderView::initStyleOptionForIndex(QStyleOptionHeader *option, int logica
                                                   Qt::TextAlignmentRole);
     opt.section = logicalIndex;
     opt.state |= state;
-    opt.textAlignment = Qt::Alignment(textAlignment.isValid()
-                                      ? Qt::Alignment(textAlignment.toInt())
-                                      : d->defaultAlignment);
+    opt.textAlignment = textAlignment.isValid()
+                        ? QtPrivate::legacyFlagValueFromModelData<Qt::Alignment>(textAlignment)
+                        : d->defaultAlignment;
 
     opt.iconAlignment = Qt::AlignVCenter;
     opt.text = d->model->headerData(logicalIndex, d->orientation,

src/widgets/itemviews/qitemdelegate.cpp

@@ -58,6 +58,7 @@
 #include <qmetaobject.h>
 #include <qtextlayout.h>
 #include <private/qabstractitemdelegate_p.h>
+#include <private/qabstractitemmodel_p.h>
 #include <private/qtextengine_p.h>
 #include <qdebug.h>
 #include <qlocale.h>
@@ -432,7 +433,7 @@ void QItemDelegate::paint(QPainter *painter,
     Qt::CheckState checkState = Qt::Unchecked;
     value = index.data(Qt::CheckStateRole);
     if (value.isValid()) {
-        checkState = static_cast<Qt::CheckState>(value.toInt());
+        checkState = QtPrivate::legacyEnumValueFromModelData<Qt::CheckState>(value);
         checkRect = doCheck(opt, opt.rect, value);
     }
 
@@ -1182,7 +1183,7 @@ bool QItemDelegate::editorEvent(QEvent *event,
         return false;
     }
 
-    Qt::CheckState state = static_cast<Qt::CheckState>(value.toInt());
+    Qt::CheckState state = QtPrivate::legacyEnumValueFromModelData<Qt::CheckState>(value);
     if (flags & Qt::ItemIsUserTristate)
         state = ((Qt::CheckState)((state + 1) % 3));
     else
@@ -1209,7 +1210,7 @@ QStyleOptionViewItem QItemDelegate::setOptions(const QModelIndex &index,
     // set text alignment
     value = index.data(Qt::TextAlignmentRole);
     if (value.isValid())
-        opt.displayAlignment = Qt::Alignment(value.toInt());
+        opt.displayAlignment = QtPrivate::legacyFlagValueFromModelData<Qt::Alignment>(value);
 
     // set foreground brush
     value = index.data(Qt::ForegroundRole);

src/widgets/itemviews/qstyleditemdelegate.cpp

@@ -66,6 +66,7 @@
 #include <qmetaobject.h>
 #include <qtextlayout.h>
 #include <private/qabstractitemdelegate_p.h>
+#include <private/qabstractitemmodel_p.h>
 #include <private/qtextengine_p.h>
 #include <private/qlayoutengine_p.h>
 #include <qdebug.h>
@@ -302,7 +303,7 @@ void QStyledItemDelegate::initStyleOption(QStyleOptionViewItem *option,
 
     value = modelRoleDataSpan.dataForRole(Qt::TextAlignmentRole);
     if (value->isValid() && !value->isNull())
-        option->displayAlignment = Qt::Alignment(value->toInt());
+        option->displayAlignment = QtPrivate::legacyFlagValueFromModelData<Qt::Alignment>(*value);
 
     value = modelRoleDataSpan.dataForRole(Qt::ForegroundRole);
     if (value->canConvert<QBrush>())
@@ -311,7 +312,7 @@ void QStyledItemDelegate::initStyleOption(QStyleOptionViewItem *option,
     value = modelRoleDataSpan.dataForRole(Qt::CheckStateRole);
     if (value->isValid() && !value->isNull()) {
         option->features |= QStyleOptionViewItem::HasCheckIndicator;
-        option->checkState = static_cast<Qt::CheckState>(value->toInt());
+        option->checkState = QtPrivate::legacyEnumValueFromModelData<Qt::CheckState>(*value);
     }
 
     value = modelRoleDataSpan.dataForRole(Qt::DecorationRole);
@@ -633,7 +634,7 @@ bool QStyledItemDelegate::editorEvent(QEvent *event,
         return false;
     }
 
-    Qt::CheckState state = static_cast<Qt::CheckState>(value.toInt());
+    Qt::CheckState state = QtPrivate::legacyEnumValueFromModelData<Qt::CheckState>(value);
     if (flags & Qt::ItemIsUserTristate)
         state = ((Qt::CheckState)((state + 1) % 3));
     else