Description:
In /std:c++17 mode, this triggers a deprecation warning in recent versions of MSVC. In /std:c++latest mode, now that microsoft/STL#380 has been merged, this will trigger an error in VS 2019 16.6 Preview 2.

Reproduce steps:

1. git clone --recursive https://github.com/Reactive-Extensions/RxCpp D:\RxCpp\src
2. cd D:\RxCpp\src
3. set CL=/D_SILENCE_CXX17_UNCAUGHT_EXCEPTION_DEPRECATION_WARNING /D_SILENCE_CXX17_ITERATOR_BASE_CLASS_DEPRECATION_WARNING
4. mkdir build_amd64
5. cd D:\RxCpp\src\build_amd64
6. cmake -G "Visual Studio 15 Win64" -DCMAKE_SYSTEM_VERSION=10.0.17134.0 ..\projects\CMake 2>&1
7. msbuild /m /p:Platform=x64 /p:Configuration=Debug RxCpp.sln /t:Rebuild /p:BuildInParallel=true 2>&1

ErrorMessage:
F:\gitP\Reactive-Extensions\RxCpp\Rx\v2\src\rxcpp\rx-util.hpp(49): error C2039: 'result_of': is not a member of 'std' [F:\gitP\Reactive-Extensions\RxCpp\build_amd64\test\rxcpp_test_defer.vcxproj]