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>

1 files changed, 4 insertions, 3 deletions

diff --git a/src/widgets/itemviews/qheaderview.cpp b/src/widgets/itemviews/qheaderview.cpp
index 27866ecbb0..8fc0df84e2 100644
--- a/src/widgets/itemviews/qheaderview.cpp
+++ b/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,