I wanted a simpler change than #545, which apparently failed. This seems safer to me

I've never worked with AppVeyor before, but is it reasonably straightforward to get it building with VS2019? Or is RxCpp not claiming to support that version of VS? I can guess what changes are required in appveyor.yml, but I'm not sure the protocol here.

@kirkshoop with C++20 approaching completion on all major compilers, it would be great to get this fix tested and released soon so this library doesn't gate those who want to upgrade (myself included).

@kirkshoop please 🙏 my C++20 project needs this

Update: Here's a quick workaround, before importing rx.hpp have this:

namespace std {
   template <class> struct result_of;
   template <class F, class... TN> struct result_of<F(TN...)>
   {
      using type = std::invoke_result_t<F, TN...>;
   };
}

❤️

Thanks!!