QException: honor the RO5

QException has an out-of-line destructor in order to pin its vtable.
But like any std::exception subclass it's meant to be (and effectively
is) copy-constructible and copy-assignable (cf. [exception]/2).
Add the missing copy operations, and the default constructor.

One may argue that this class should use some other approach since it
also has a polymorphic clone() -- for instance disable assignments and
make copy constructions protected -- but that ship has sailed.

This prevents a Clang warning about the deprecated copies in the
presence of a user-declared destructor.

Change-Id: I4d8613ade265ea85a5a7e97255b75b7fcf3e5a27
Pick-to: 6.9 6.8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qexception.cpp

@@ -82,10 +82,37 @@ QT_BEGIN_NAMESPACE
     \internal
 */
 
+/*!
+    Destroys this QException object.
+*/
 QException::~QException() noexcept
 {
 }
 
+/*!
+    \fn QException::QException()
+
+    Constructs a QException object.
+*/
+
+/*!
+    \fn QException::QException(const QException &other)
+
+    Creates a copy of \a other.
+
+    \note Be careful when using this function, as you risk slicing.
+
+    \sa clone()
+*/
+
+/*!
+    \fn QException &QException::operator=(const QException &other)
+
+    Copy-assigns \a other over this object.
+
+    \note Be careful when using this function, as you risk slicing.
+*/
+
 void QException::raise() const
 {
     QException e = *this;

src/corelib/thread/qexception.h

@@ -21,7 +21,10 @@ QT_BEGIN_NAMESPACE
 class Q_CORE_EXPORT QException : public std::exception
 {
 public:
+    QException() = default;
     ~QException() noexcept;
+    QException(const QException &) = default;
+    QException &operator=(const QException &) = default;
     virtual void raise() const;
     virtual QException *clone() const;
 };