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>

2 files changed, 5 insertions, 3 deletions

diff --git a/src/gui/doc/snippets/code/src_gui_kernel_qkeysequence.cpp b/src/gui/doc/snippets/code/src_gui_kernel_qkeysequence.cpp
index 2994bb60cf..4be214ad64 100644
--- a/src/gui/doc/snippets/code/src_gui_kernel_qkeysequence.cpp
+++ b/src/gui/doc/snippets/code/src_gui_kernel_qkeysequence.cpp
@@ -62,13 +62,15 @@ struct Wrapper : public QWidget
 QKeySequence(QKeySequence::Print);
 QKeySequence(tr("Ctrl+P"));
 QKeySequence(tr("Ctrl+p"));
-QKeySequence(Qt::CTRL + Qt::Key_P);
+QKeySequence(Qt::CTRL | Qt::Key_P);
+QKeySequence(Qt::CTRL + Qt::Key_P); // deprecated
 //! [0]
 
 
 //! [1]
 QKeySequence(tr("Ctrl+X, Ctrl+C"));
-QKeySequence(Qt::CTRL + Qt::Key_X, Qt::CTRL + Qt::Key_C);
+QKeySequence(Qt::CTRL | Qt::Key_X, Qt::CTRL | Qt::Key_C);
+QKeySequence(Qt::CTRL + Qt::Key_X, Qt::CTRL + Qt::Key_C); // deprecated
 //! [1]
 
 */ // Wrap non-compilable code snippet
diff --git a/src/gui/doc/snippets/code/src_gui_kernel_qshortcut.cpp b/src/gui/doc/snippets/code/src_gui_kernel_qshortcut.cpp
index bac264fcfc..370afd0679 100644
--- a/src/gui/doc/snippets/code/src_gui_kernel_qshortcut.cpp
+++ b/src/gui/doc/snippets/code/src_gui_kernel_qshortcut.cpp
@@ -60,6 +60,6 @@ setKey(QKeySequence());     // no signal emitted
 setKey(0x3b1);              // Greek letter alpha
 setKey(Qt::Key_D);              // 'd', e.g. to delete
 setKey('q');                // 'q', e.g. to quit
-setKey(Qt::CTRL + Qt::Key_P);       // Ctrl+P, e.g. to print document
+setKey(Qt::CTRL | Qt::Key_P);       // Ctrl+P, e.g. to print document
 setKey("Ctrl+P");           // Ctrl+P, e.g. to print document
 //! [1]