Why does libboost_cobalt.so get built for GCC 16 but not GCC 15?

[jwakely]

I'm packaging Boost 1.90.0 for Fedora and observed that when my new boost-1.90.0 RPM is built using GCC 15 (the current compiler in Fedora Rawhide) there are no libs built for Boost.Cobalt. When I experiment with building it with the GCC 16 package that will arrive in Rawhide some time next month, I get these extra files installed:

   /usr/lib/debug/usr/lib64/libboost_cobalt.so.1.90.0-1.90.0-0.1.fc44.x86_64.debug
   /usr/lib/debug/usr/lib64/libboost_cobalt_io.so.1.90.0-1.90.0-0.1.fc44.x86_64.debug
   /usr/lib64/libboost_cobalt.so
   /usr/lib64/libboost_cobalt.so.1.90.0
   /usr/lib64/libboost_cobalt_io.so
   /usr/lib64/libboost_cobalt_io.so.1.90.0

I assume this is because GCC 16 defaults to -std=gnu++20 so supports C++20 coroutines by default, and GCC 15 only supports them if you use an extra option. But is that really the intended behaviour? Should building Boost automatically add -std=gnu++20 when building Cobalt, so that it actually installs its libraries?

https://www.boost.org/doc/libs/1_90_0/libs/cobalt/doc/html/index.html#requirements says it's been tested with GCC 10, but says nothing about it not being supported out of the box and requiring additional steps to get the library binaries. Am I doing something wrong when building Boost? I'm just using the ./bootstrap.sh and b2 commands as recommended.

[jwakely]

I'm starting to suspect this is a bug, but I don't understand the boost build system well enough to say where the bug is.

You can get binary packages of GCC 16 from https://jwakely.github.io/pkg-gcc-latest/ to experiment with.

[klemens-morgenstern]

You need to set the standard like so:

./b2 cxxstd=20

The cobalt build scripts actually don't check the standard directly, but check if concepts compile - which is why gcc 16 works.

That behaviour is intended, cobalt doesn't automatically change the C++ standard for the build.

[jwakely]

So it's expected that you need to link with -lboost_cobalt when using cobalt in C++20 code? But not when using it in C++17 code?

So if you have a library which uses Boost.Cobalt and sometimes you use that library in C++17 code and sometimes in C++20 code, you need to have a separate installation of Boost libs which is built with C++20? That seems incredibly user hostile. Why doesn't the default boost build+install process (i.e. bootstrap.sh and b2) just build and install all the libs that are needed to use Boost.Cobalt?

[lrineau]

So it's expected that you need to link with -lboost_cobalt when using cobalt in C++20 code? But not when using it in C++17 code?

Boost Cobalt requires C++20. If the current compiler does not implement C++20, then it is not built at all. With GCC 15, Boost Cobalt would require the compilation flag -std=c++20, but GCC 16 switched to C++20 by default.

[lrineau]

So it's expected that you need to link with -lboost_cobalt when using cobalt in C++20 code? But not when using it in C++17 code?

Boost Cobalt requires C++20. If the current compiler does not implement C++20, then it is not built at all. With GCC 15, Boost Cobalt would require the compilation flag -std=c++20, but GCC 16 switched to C++20 by default.

I mean: you cannot use Boost Cobalt in C++17 code.

[jwakely]

So then wouldn't it make sense for the build to add cxxstd=20 automatically (after checking if it's supported by the compiler) so that the default build+install process Just Works?

This is the first boost library I'm aware of that can't be installed out of the box using the standard documented process for installing Boost from source.

[jwakely]

You need to set the standard like so:

./b2 cxxstd=20

How can I add that just for building cobalt?

For a distro still using GCC 15, should I do b2 --without-cobalt ... for all the other libs, and then a second step using b2 --with-cobalt cxxstd=20 ? I want to ensure that libboost_cobalt.so and libboost_cobalt_io.so are built and packaged for users of the distro. If they need to use -std=c++20 to use cobalt, that's fine, but the libs need to be present in the distro's packages for that to work.

Are there side effects for building everything else in Boost with cxxstd=20?

[klemens-morgenstern]

I would hope the are ABI compatible, in which case either works. I would probably just compile with C++20 on GCC 15. The thing was released in 2025, they're just conservative with the default C++ standards.