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, 46 insertions, 0 deletions

diff --git a/src/corelib/itemmodels/qabstractitemmodel_p.h b/src/corelib/itemmodels/qabstractitemmodel_p.h
index e8f00975ed..edc5ad6116 100644
--- a/src/corelib/itemmodels/qabstractitemmodel_p.h
+++ b/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