QDebug: make std::optional stream operator SCARY

Piggy-back on the recently-added, type-erased, std::tuple stream
operator to handle std::optional the same way.

While std::optional doesn't support the Tuple Protocol, and we
therefore can't use putTuple() directly, we can still use
putTupleImplImpl() if we set up its arguments manually.

[ChangeLog][Potentially Source-Incompatible Changes][QDebug] The
std::optional streaming operator is now a member of QDebug, not a free
function. This breaks users that rely on the exact definition of the
operator (e.g. `operator<<(d, opt)`). A backwards-compatible fix is to
call the operator with infix notation (d << opt) only, and to avoid
const QDebug objects.

Pick-to: 6.9
Change-Id: Ib040d65953ca9d3892aee5bdb597d6d30a9694b1
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/io/qdebug.cpp

@@ -1258,10 +1258,9 @@ QDebug &QDebug::putTupleLikeImplImpl(const char *ns, const char *what,
 
 /*!
     \since 6.7
-    \fn template <class T> QDebug operator<<(QDebug debug, const std::optional<T> &opt)
+    \fn template <class T, QDebug::if_streamable<T>> QDebug::operator<<(const std::optional<T> &opt)
-    \relates QDebug
 
-    Writes the contents of \a opt (or \c nullopt if not set) to \a debug.
+    Writes the contents of \a opt (or \c nullopt if not set) to this stream.
     \c T needs to support streaming into QDebug.
 */
 

src/corelib/io/qdebug.h

@@ -299,6 +299,16 @@ public:
         return putTupleLike("std", "tuple", t);
     }
 
+    template <typename T, if_streamable<T> = true>
+    QDebug &operator<<(const std::optional<T> &o)
+    {
+        if (!o)
+            return *this << std::nullopt;
+        StreamTypeErased s = &streamTypeErased<std::remove_cv_t<T>>;
+        const void *d = std::addressof(*o);
+        return putTupleLikeImplImpl("std", "optional", 1, &s, &d);
+    }
+
 private:
     template <typename T>
     using if_ordering_type = std::enable_if_t<QtOrderingPrivate::is_ordering_type_v<T>, bool>;
@@ -490,16 +500,6 @@ inline QDebugIfHasDebugStreamContainer<QMultiHash<Key, T>, Key, T> operator<<(QD
     return QtPrivate::printAssociativeContainer(std::move(debug), "QMultiHash", hash);
 }
 
-template <class T>
-inline QDebugIfHasDebugStream<T> operator<<(QDebug debug, const std::optional<T> &opt)
-{
-    if (!opt)
-        return debug << std::nullopt;
-    const QDebugStateSaver saver(debug);
-    debug.nospace() << "std::optional(" << *opt << ')';
-    return debug;
-}
-
 template <class T1, class T2>
 inline QDebugIfHasDebugStream<T1, T2> operator<<(QDebug debug, const std::pair<T1, T2> &pair)
 {