Before, the functor slot (or lambda expression) had to have the same amount
of arguments as the signal.
This shown to be a big problem to be able to connect to signals that had
a QPrivateSlot.
This implementation use the type of the operator() of the functor to
know how many arguments we have.
As a bonus, we also can check the arguments in a static assert.
The test comes from https://codereview.qt-project.org/#change,38703
But this patch also works without variadic template
If the compiler does not support decltype, we workaround the lack of it
by using another level of indirection.
Change-Id: I9850b43e8caf77356a2ec3f4c0b0ed532d96029e
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
029cc40c2a