process.hpp fails to compile with BOOST_PROCESS_V2_STANDALONE defined

[jwhitworth-vicon]

When compiling code including the boost\process\v2\process.hpp header with the defines BOOST_PROCESS_STANDALONE, BOOST_PROCESS_V2_STANDALONE and BOOST_PROCESS_USE_STD_FS, there are compiler errors with Visual Studio 2022 17.14:

1>e:\thirdparty\Boost\boost-1.88.0-windows-vc143-x64\include\boost-1_88\boost\process\v2\process.hpp(167,57): error C2653: 'is_convertible': is not a class or namespace name
1>(compiling source file 'UtilsSystem.pch.cpp')
1>    e:\thirdparty\Boost\boost-1.88.0-windows-vc143-x64\include\boost-1_88\boost\process\v2\process.hpp(167,57):
1>    the template instantiation context (the oldest one first) is
1>        e:\thirdparty\Boost\boost-1.88.0-windows-vc143-x64\include\boost-1_88\boost\process\v2\process.hpp(38,8):
1>        while compiling class template 'process_v2::basic_process'
1>e:\thirdparty\Boost\boost-1.88.0-windows-vc143-x64\include\boost-1_88\boost\process\v2\process.hpp(167,59): error C2065: 'value': undeclared identifier
1>(compiling source file 'UtilsSystem.pch.cpp')
1>e:\thirdparty\Boost\boost-1.88.0-windows-vc143-x64\include\boost-1_88\boost\process\v2\process.hpp(167,59): error C2975: '_Test': invalid template argument for 'std::enable_if', expected compile-time constant expression
1>(compiling source file 'UtilsSystem.pch.cpp')
1>    C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.44.35207\include\xtr1common(47,28):
1>    see declaration of '_Test'
1>e:\thirdparty\Boost\boost-1.88.0-windows-vc143-x64\include\boost-1_88\boost\process\v2\process.hpp(167,75): error C2039: 'type': is not a member of 'std::enable_if<false,void *>'
1>(compiling source file 'UtilsSystem.pch.cpp')
1>    C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.44.35207\include\xtr1common(48,8):
1>    see declaration of 'std::enable_if<false,void *>'

This is because one of the basic_process constructors uses is_convertible without fully qualifying it with the std:: namespace like the other earlier constructors do.

process/include/boost/process/v2/process.hpp

Line 168 in 2a41d0a

is_convertible<ExecutionContext&,

I am not sure if it is one of the defines or the combination of those defines. It is also not clear which defines I actually need if I want to use standalone asio and filesystem from the standard library.

[klemens-morgenstern]

Thanks for reporting. The standalone mode has been a bit neglected by me, I'll fix it up.

How are you getting the library into your project?

[jwhitworth-vicon]

If you fix that there are then further compilation errors:

1. https://github.com/boostorg/process/blob/develop/include/boost/process/v2/windows/default_launcher.hpp#L253 has a spurious v2:: prefix that needs removing.
2. The BOOST_PROCESS_V2_ASSIGN_LAST_ERROR macro defined https://github.com/boostorg/process/blob/develop/include/boost/process/v2/detail/config.hpp#L104 doesn't provide the correct number of arguments to std::error_code::assign.

I can submit a pull request if that is easier?

@klemens-morgenstern, we're including it via boost but we're using standalone asio (so some other libraries don't have a boost dependency) so I was trying to use standalone asio with boost::process and discovered these defines when looking through the headers. I guess It's a bit of a non-standard configuration.

[klemens-morgenstern]

I'd really appreciate a PR. That will probably be much faster than me fixing this.

The v2 started out as standalone and got ported into boost. I kept the flag, but as you can tell, it's neither documented not tested by CI. Mainly because I didn't think there'd be any interest.

For your use-case, would the option of a process standalone with boost.filesystem make sense?

[jwhitworth-vicon]

No, we're using std::filesystem as we're on C++17.

I think the set of things that we would like to toggle would be:

1. std::filesystem instead of boost::filesystem.
2. asio instead of boost::asio.
3. std::error_* instead of boost::error_*.

It doesn't matter to us if process is in its own namespace or in the boost namespace. I appreciate that is more work though.

[jwhitworth-vicon]

Submitted a pull request. I haven't had a chance to try replacing all our usages of boost::process so there's a chance there may be additional errors but I'm about to go on holiday for the Christmas break so won't be able to do any more until the new year.