std::result_of is deprecated #19
Description
According to https://en.cppreference.com/w/cpp/types/result_of std::result_of<> is
deprecated in C++17 and will be removed in C++20. It's used in sigc++/tuple-utils/tuple_transform_each.h.
It looks like it's replaced by std::invoke_result<>.
I tried to replace result_of by invoke_result in tuple_transform_each.h, but it's
not as easy as that. make check resulted in several compile-time errors.
The first error is
/home/kjell/checkout/gnome/libsigcplusplus/sigc++/tuple-utils/tuple_transform_each.h:49:81: error: no type named ‘type’ in ‘struct std::invoke_result<int& (*(sigc::bound_argument<int>&))(sigc::bound_argument<int>&)>’
&T_transformer<from_element_type>::transform)(from_element_type&)>::type;
^
/home/kjell/checkout/gnome/libsigcplusplus/sigc++/tuple-utils/tuple_transform_each.h:51:14: error: no type named ‘type’ in ‘struct std::invoke_result<int& (*(sigc::bound_argument<int>&))(sigc::bound_argument<int>&)>’
std::tuple<to_element_type>(T_transformer<from_element_type>::transform(std::get<index>(t_original)));
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~