QObject: treat T* -> bool conversions as narrowing

Following wg21.link/LWG3228, it was found that a proper variant fix
requires that T* -> bool conversions be treated as narrowing
conversions in subclause wg21.link/dcl.init.lst. wg21.link/P1957R2 was
accepted in Prague 2020 as a DR and retroactively applies to older C++
standards.

Since we hard-code the algorithm of [dcl.init.lst], we can and must
add this manually.

[ChangeLog][QtCore][QObject] For the purposes of
QT_NO_NARROWING_CONVERSIONS_IN_CONNECT, pointer
(incl. pointer-to-member) to bool conversions are now considered
narrowing. This matches the resolution of a defect report in C++
itself.

Change-Id: Ifa9a3724c9c8ccd3dd6614928dbbe37477591dc1
Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io>

src/corelib/kernel/qobjectdefs_impl.h

@@ -285,11 +285,15 @@ namespace QtPrivate {
     {
     };
 
+    template <typename T>
+    using is_bool = std::is_same<bool, typename std::decay<T>::type>;
+
     template<typename From, typename To>
     struct AreArgumentsNarrowedBase<From, To, typename std::enable_if<sizeof(From) && sizeof(To)>::type>
         : std::integral_constant<bool,
               (std::is_floating_point<From>::value && std::is_integral<To>::value) ||
               (std::is_floating_point<From>::value && std::is_floating_point<To>::value && sizeof(From) > sizeof(To)) ||
+              ((std::is_pointer<From>::value || std::is_member_pointer<From>::value) && QtPrivate::is_bool<To>::value) ||
               ((std::is_integral<From>::value || std::is_enum<From>::value) && std::is_floating_point<To>::value) ||
               (std::is_integral<From>::value && std::is_integral<To>::value
                && (sizeof(From) > sizeof(To)

tests/auto/corelib/kernel/qobject/tst_qobject.cpp

@@ -7462,6 +7462,12 @@ void tst_QObject::checkArgumentsForNarrowing()
     FITS(bool, const QObject *&);
     FITS(int (*)(bool), void (QObject::*)());
 
+    {
+        // wg21.link/P1957
+        NARROWS(char*, bool);
+        NARROWS(void (QObject::*)(), bool);
+    }
+
 #undef IS_UNSCOPED_ENUM_SIGNED
 
 #undef NARROWS_IF