Disable operator+ and operator- for QFlags

... and the associated enumeration.

Using them is almost always a certain mistake.

1) op+ between two enumerators of the same enumeration yields int,
not QFlags, so removing the type safety that QFlags is supposed to
give and breaking the semantics of bitwise operations.

2) op+ between two enumerators of different enumerations is
deprecated in C++20 (already flagged by GCC10), and again yields
int. This is a code smell. Dedicated classes (holding a combination
of unrelated enums) should be used instead.

3) op+ between an enumerator and its QFlags loses the semantic
meaning of bitwise operations. If the real meaning was to use
operator|, then use that instead; operator+ hides the intent,
and can introduce bugs by creating a result not expressible via
OR combinations of enumerators:

  enum E { A = 0x01, B = 0x02 };
  QFlags<E> f = E::A;

  f + E::A; // ???
  f + E::B; // ???

Identical reasoning applies for operator-. Technically the
other arithmetic operators could be disabled as well, but I
really don't expect any real-world usage for them.

This has spotted bugs in Qt.

[ChangeLog][Potentially Source-Incompatible Changes][QFlags] Using
operator+ or operator- with a QFlags object or with an enumeration
that has a corresponding QFlags object will now result in a
compile-time error, because it's a generally unsafe operation.
Use the proper bitwise operations instead (|, &, ~); or cast
the enumeration to a integral type before attempting arithmetic
manipulations on it.

Change-Id: I5eabc5195dec3d3082bc9da10dbc8cf5dff3e1eb
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

1 files changed, 3 insertions, 3 deletions

diff --git a/src/widgets/kernel/qwhatsthis.cpp b/src/widgets/kernel/qwhatsthis.cpp
index b483655a71..d264dfc4b2 100644
--- a/src/widgets/kernel/qwhatsthis.cpp
+++ b/src/widgets/kernel/qwhatsthis.cpp
@@ -218,8 +218,8 @@ QWhatsThat::QWhatsThat(const QString& txt, QWidget* parent, QWidget *showTextFor
             sw = 300;
 
         r = fontMetrics().boundingRect(0, 0, sw, 1000,
-                                        Qt::AlignLeft + Qt::AlignTop
-                                        + Qt::TextWordWrap + Qt::TextExpandTabs,
+                                        Qt::AlignLeft | Qt::AlignTop
+                                        | Qt::TextWordWrap | Qt::TextExpandTabs,
                                         text);
     }
     shadowWidth = dropShadow() ? 0 : 6;
@@ -333,7 +333,7 @@ void QWhatsThat::paintEvent(QPaintEvent*)
     }
     else
     {
-        p.drawText(r, Qt::AlignLeft + Qt::AlignTop + Qt::TextWordWrap + Qt::TextExpandTabs, text);
+        p.drawText(r, Qt::AlignLeft | Qt::AlignTop | Qt::TextWordWrap | Qt::TextExpandTabs, text);
     }
 }