Not compatible with C++20

[F-I-D-O]

In C++20 std::result_of is removed (Used in App.hpp).

[phlptp]

what compiler are you using that generates an error? and what flags?

[F-I-D-O]

I use Visual Studio 2019 Preview 5.0. But this should be a general issue of all C++20 ready compilers, as std::result of is removed in C++20 (https://en.cppreference.com/w/cpp/types/result_of)

[phlptp]

@F-I-D-O Are you actually getting it to generate an error? I am not able to get an error on the result_of in any compilers I try. I know how to fix it, but I was hoping to see a failing test first.

[F-I-D-O]

Yes:
C:\vcpkg\installed\x64-windows\include\CLI/App.hpp(3055): error C2039: 'result_of': is not a member of 'std'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.26.28801\include\numeric(23): note: see declaration of 'std'
C:\vcpkg\installed\x64-windows\include\CLI/App.hpp(3055): error C2653: 'result_of': is not a class or namespace name
C:\vcpkg\installed\x64-windows\include\CLI/App.hpp(3055): error C3646: 'type': unknown override specifier
C:\vcpkg\installed\x64-windows\include\CLI/App.hpp(3062): error C2039: 'result_of': is not a member of 'std'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.26.28801\include\numeric(23): note: see declaration of 'std'
C:\vcpkg\installed\x64-windows\include\CLI/App.hpp(3062): error C2653: 'result_of': is not a class or namespace name
C:\vcpkg\installed\x64-windows\include\CLI/App.hpp(3062): error C3646: 'type': unknown override specifier

This is a result of compilation of your example code, i.e.:

int main(int argc, char** argv) {
    CLI::App app{"App description"};

    std::string filename = "default";
    app.add_option("-f,--file", filename, "A help string");

    CLI11_PARSE(app, argc, argv);
    return 0;
}

[phlptp]

@F-I-D-O would you mind trying the changed code in #459. Just to make sure? It is enabled for C++14 and higher, but I want to make sure it resolves the error you are seeing.

[F-I-D-O]

I can try it on in a couple of days. Does it mean that I try it with the branch phlptp:cpp20_test? Or something more sophisticated :)

[0vercl0k]

fwiw @phlptp your changes in #459 fixed this issue for me on VS2019 latest.

[henryiii]

Please reopen if this is not working on master (that's merged now)!