Long live QKeyCombination!

C++20 via P1120 is deprecating arithmetic operations between
unrelated enumeration types, and GCC 10 is already complaining.
Hence, these operations might become illegal in C++23 or C++26 at
the latest.

A case of this that affects Qt is in key combinations: a
QKeySequence can be constructed by summing / ORing modifiers and a
key, for instance:

  Qt::CTRL + Qt::Key_A
  Qt::SHIFT | Qt::CTRL | Qt::Key_G (recommended, see below)

The problem is that the modifiers and the key belong to different
enumerations (and there's 2 enumerations for the modifier, and one
for the key).

To solve this: add a dedicated class to represent a combination of
keys, and operators between those enumerations to build instances
of this class.

I would've simply defined operator|, but again docs and pre-existing
code use operator+ as well, so added both to at least tackle simple
cases (modifier + key).

Multiple modifiers create a problem: operator+ between them yields
int, not the corresponding flags type (because operator+ is not
overloaded for this use case):

  Qt::CTRL + Qt::SHIFT + Qt::Key_A
  \__________________/      /
          int              /
           \______________/
                  int

Not only this loses track of the datatypes involved, but it would
also then "add" the key (with NO warnings, now its int + enum, so
it's not mixing enums!) and yielding int again.

I don't want to special-case this; the point of the class is
that int is the wrong datatype. Everything works just fine when
using operator| instead:

  Qt::CTRL | Qt::SHIFT | Qt::Key_A
  \__________________/      /
      Qt::Modifiers        /
           \______________/
            QKeyCombination

So I'm defining operator+ so that the simple cases still work,
but also deprecating it.

Port some code around Qt to the new class. In certain cases,
it's a huge win for clarity. In some others, I've just added
the necessary casts to make it still compile without warnings,
without attempting refactorings.

[ChangeLog][QtCore][QKeyCombination] New class to represent
a combination of a key and zero or more modifiers, to be used
when defining shortcuts or similar.

[ChangeLog][Potentially Source-Incompatible Changes] A keyboard
modifier (such as Qt::CTRL, Qt::AltModifier, etc.) should be
combined with a key (such as Qt::Key_A, Qt::Key_F1, etc.) by using
operator|, not operator+.  The result is now an object of type
QKeyCombination, that stores the key and the modifiers.

Change-Id: I657a3a328232f059023fff69c5031ee31cc91dd6
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

3 files changed, 5 insertions, 5 deletions

diff --git a/src/widgets/widgets/qkeysequenceedit.cpp b/src/widgets/widgets/qkeysequenceedit.cpp
index a38250c195..97ee915037 100644
--- a/src/widgets/widgets/qkeysequenceedit.cpp
+++ b/src/widgets/widgets/qkeysequenceedit.cpp
@@ -63,7 +63,7 @@ void QKeySequenceEditPrivate::init()
     layout->setContentsMargins(0, 0, 0, 0);
     layout->addWidget(lineEdit);
 
-    key[0] = key[1] = key[2] = key[3] = 0;
+    std::fill_n(key, QKeySequencePrivate::MaxKeyCount, QKeyCombination::fromCombined(0));
 
     lineEdit->setFocusProxy(q);
     lineEdit->installEventFilter(q);
@@ -186,7 +186,7 @@ void QKeySequenceEdit::setKeySequence(const QKeySequence &keySequence)
 
     d->keySequence = keySequence;
 
-    d->key[0] = d->key[1] = d->key[2] = d->key[3] = 0;
+    d->key[0] = d->key[1] = d->key[2] = d->key[3] = QKeyCombination::fromCombined(0);
     d->keyNum = keySequence.count();
     for (int i = 0; i < d->keyNum; ++i)
         d->key[i] = keySequence[i];
@@ -286,7 +286,7 @@ void QKeySequenceEdit::keyPressEvent(QKeyEvent *e)
     }
 
 
-    d->key[d->keyNum] = nextKey;
+    d->key[d->keyNum] = QKeyCombination::fromCombined(nextKey);
     d->keyNum++;
 
     QKeySequence key(d->key[0], d->key[1], d->key[2], d->key[3]);
diff --git a/src/widgets/widgets/qkeysequenceedit_p.h b/src/widgets/widgets/qkeysequenceedit_p.h
index 7af034e735..8cd58de68f 100644
--- a/src/widgets/widgets/qkeysequenceedit_p.h
+++ b/src/widgets/widgets/qkeysequenceedit_p.h
@@ -76,7 +76,7 @@ public:
     QLineEdit *lineEdit;
     QKeySequence keySequence;
     int keyNum;
-    int key[QKeySequencePrivate::MaxKeyCount];
+    QKeyCombination key[QKeySequencePrivate::MaxKeyCount];
     int prevKey;
     int releaseTimer;
 };
diff --git a/src/widgets/widgets/qmenu.cpp b/src/widgets/widgets/qmenu.cpp
index c2ef383d28..9007d2af78 100644
--- a/src/widgets/widgets/qmenu.cpp
+++ b/src/widgets/widgets/qmenu.cpp
@@ -3406,7 +3406,7 @@ void QMenu::keyPressEvent(QKeyEvent *e)
                         continue;
                     QAction *act = d->actions.at(i);
                     QKeySequence sequence = QKeySequence::mnemonic(act->text());
-                    int key = sequence[0] & 0xffff;
+                    int key = sequence[0].toCombined() & 0xffff; // suspicious
                     if (key == c.unicode()) {
                         clashCount++;
                         if (!first)