Document that QtConcurrent::run doesn't support overloaded functions

After improving QtConcurrent::run() to use parameter packs for the
arguments (see c977e74afd), calling
overloaded functions is ambiguous. Updated the porting guide and the
documentation to mention this and describe possible workarounds.

Task-number: QTBUG-89648
Pick-to: 6.0
Change-Id: I4c1f996ae67bce8c13cc1f99f54240295db6ae1d
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/concurrent/doc/snippets/code/src_concurrent_qtconcurrentrun.cpp

@@ -230,3 +230,21 @@ run<double>(f); // this will select the 2nd overload
 // run(f);      // error, both candidate overloads potentially match
 //! [14]
 
+//! [15]
+void foo(int arg);
+void foo(int arg1, int arg2);
+...
+QFuture<void> future = QtConcurrent::run(foo, 42);
+//! [15]
+
+//! [16]
+QFuture<void> future = QtConcurrent::run([] { foo(42); });
+//! [16]
+
+//! [17]
+QFuture<void> future = QtConcurrent::run(static_cast<void(*)(int)>(foo), 42);
+//! [17]
+
+//! [18]
+QFuture<void> future = QtConcurrent::run(qOverload<int>(foo), 42);
+//! [18]

src/concurrent/doc/src/qt6-changes.qdoc

@@ -71,6 +71,11 @@
     QFuture<void> future = QtConcurrent::run(&QImage::invertPixels, &image, QImage::InvertRgba);
     \endcode
 
+    Another side effect is that \c QtConcurrent::run() will not work with
+    overloaded functions anymore. For example, the code below won't compile:
+
+    \include qtconcurrentrun.cpp run-with-overload-calls
+
     Other methods of QtConcurrent have no behavioral changes and do not introduce
     source compatibility breaks.
 */

src/concurrent/qtconcurrentrun.cpp

@@ -89,6 +89,26 @@
     the function. Changes made to the arguments after calling
     QtConcurrent::run() are \e not visible to the thread.
 
+    Note that QtConcurrent::run does not support calling overloaded functions
+    directly. For example, the code below won't compile:
+
+//! [run-with-overload-calls]
+    \snippet code/src_concurrent_qtconcurrentrun.cpp 15
+
+    The easiest workaround is to call the overloaded function through lambda:
+
+    \snippet code/src_concurrent_qtconcurrentrun.cpp 16
+
+    Or you can tell the compiler which overload to choose by using a
+    \c static_cast:
+
+    \snippet code/src_concurrent_qtconcurrentrun.cpp 17
+
+    Or qOverload:
+
+    \snippet code/src_concurrent_qtconcurrentrun.cpp 18
+//! [run-with-overload-calls]
+
     \section2 Returning Values from the Function
 
     Any return value from the function is available via QFuture: