In N4169 the author dropped the invoke<R> support by claiming that it's an unnecessary cruft in TR1, obsoleted by C++11 type inference. But now we have some new business went to *INVOKE*(f, t1, t2, ..., tN, R), that is to discard the return type when R is void. This form is very useful, or possible even more useful than the basic form when implementing a call wrapper. Also note that the optional R support is already in std::is_callable and std::is_nothrow_callable.