From a7da8e0dab8f6290b2af9bc12c0cd391af30bfb7 Mon Sep 17 00:00:00 2001
From: Marc Mutz <marc.mutz@kdab.com>
Date: Thu, 22 Jul 2021 19:13:27 +0200
Subject: QWidget: cope with QObject::connect()'s incomplete
 SFINAE-friendliness

The plan for QObject::connect() (perfect) forwarders, such as
QWidget::addAction(), was to just use a variant of the Detection Idiom
to see whether QObject::connect() with the arguments as given would
compile and SFINAE out the forwarder otherwise.

It turns out that the "functor" overload of QObject::connect(), in
particular, is severly underconstrained and accepts e.g. QKeySequence
as a function object, only erroring out via a static_assert() in the
body of the function, and thus at instantiation time and not, as
needed, at overload resolution time.

At the same time, we don't really want QObject::connect() to SFINAE
out on argument mismatches between signal and slot, because the
resulting error messages would be ... unkind to users of the API. We
would like to keep the static_assert()s for easier error reporting.

Reconciling these two contradicting requirements has so far eluded
this author, so for now, to unblock progress, we explicitly black-
and, in one case, white-list possible arguments. Because QKeySequence,
in particular, is implicitly constructible from int(!), and therefore
any enum type(!), incl. Qt::ConnectionType, we need to do way too much
coding in the addAction() constraints. Hopefully, we'll be able to fix
the issue at the root cause, in QObject, before Qt 6.3 is out, but
until then, this is an ok-ish stop-gap measure.

Add thorough overload set checks (positive ones only, for now) to
tst_qwidget and tst_qmenu.

Change-Id: Ia05233df818bc82ecc924fc44c1b349af41cbbf1
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
---
 src/widgets/kernel/qwidget.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

(limited to 'src')

diff --git a/src/widgets/kernel/qwidget.h b/src/widgets/kernel/qwidget.h
index ee6e7d5fe6..6c30e018df 100644
--- a/src/widgets/kernel/qwidget.h
+++ b/src/widgets/kernel/qwidget.h
@@ -211,11 +211,28 @@ class Q_WIDGETS_EXPORT QWidget : public QObject, public QPaintDevice
     Q_PROPERTY(QString windowFilePath READ windowFilePath WRITE setWindowFilePath)
     Q_PROPERTY(Qt::InputMethodHints inputMethodHints READ inputMethodHints WRITE setInputMethodHints)
 
+#if 0
+    // ### TODO: make this work (requires SFINAE-friendly connect())
     template <typename...Args>
     using compatible_action_slot_args = std::void_t<
         decltype(QObject::connect(std::declval<QAction*>(), &QAction::triggered,
                                   std::declval<Args>()...))
     >;
+#else
+    // good-enough compromise for now
+    template <typename...Args>
+    using compatible_action_slot_args = std::enable_if_t<std::conjunction_v<
+#if QT_CONFIG(shortcut)
+            std::disjunction<
+                std::is_same<Args, Qt::ConnectionType>,
+                std::negation<std::is_convertible<Args, QKeySequence>>
+            >...,
+#endif
+            std::negation<std::is_convertible<Args, QIcon>>...,
+            std::negation<std::is_convertible<Args, const char*>>...,
+            std::negation<std::is_convertible<Args, QString>>...
+        >>;
+#endif
 public:
     enum RenderFlag {
         DrawWindowBackground = 0x1,
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