Windows + VS2019 + CMake + CLion + Conan：如何启用 C++20？

Question

I'm getting this error:

C:.conan\\7c4aa5\\1\\include\\absl/meta/type_traits.h(622): error C2039: 'result_of': is not a member of 'std'

When trying to compile my project which includes Abseil .

If I click into where the error is coming from, I see:

#if __cplusplus >= 201703L
// std::result_of is deprecated (C++17) or removed (C++20)
template<typename> struct result_of;
template<typename F, typename... Args>
struct result_of<F(Args...)> : std::invoke_result<F, Args...> {};
#else
template<typename F> using result_of = std::result_of<F>;
#endif

And right there at the top is a hint, that std::result_of was removed in C++20.

In my CMakeLists.txt I have

set(CMAKE_CXX_STANDARD 20)

So why isn't __cplusplus >= 201703L true? What sets that variable? How do I tell the compiler I really do want to use C++20?

I don't know if I need to convince Visual Studio 16/2019, or CMake, or CLion, or what. There's too many things in the toolchain, but I shall add some screenshots so you can see what my setup is and hopefully someone can point out what I'm missing.

My full CMakeLists.txt is:

cmake_minimum_required(VERSION 3.21)
set(EXE mario4ever)
project(${EXE})

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

include(${CMAKE_CURRENT_SOURCE_DIR}/conanbuildinfo.cmake)
conan_basic_setup()

add_executable(${EXE} src/bootstrap.cpp)

target_link_libraries(${EXE} ${CONAN_LIBS})

add_custom_command(
        TARGET ${EXE} POST_BUILD
        COMMAND ${CMAKE_COMMAND} -E copy_directory
        ${CMAKE_SOURCE_DIR}/resources $<TARGET_FILE_DIR:${EXE}>/resources
        COMMENT "Copying shaders" VERBATIM
)

Answer 1

I just got it to compile. The magic line was this:

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /std:c++20 /Zc:__cplusplus")

Add that to CMakeLists.txt .