boost build doesn't build cobalt

[douaumont]

When I build boost libraries as described in Getting Started all libraries are built except cobalt. The toolset is GCC 13, OS is Linux Mint 21.3. Here are the steps I attempted:

1. /bootstrap.sh --with-libraries=cobalt
2. ./b2

The last step yield the following output:

Performing configuration checks

    - default address-model    : 64-bit (cached) [1]
    - default architecture     : x86 (cached) [1]

Building the Boost C++ Libraries.

    - cxx20_hdr_concepts       : no  (cached) [2]
    - cxx20_hdr_concepts       : no  (cached) [3]
    - cxx20_hdr_concepts       : no  (cached) [4]
    - cxx20_hdr_concepts       : no  (cached) [5]

[1] gcc-13
[2] gcc-13/release/threading-multi/visibility-hidden
[3] gcc-13/release/build-no/threading-multi/visibility-hidden
[4] gcc-13/release/link-static/threading-multi/visibility-hidden
[5] gcc-13/release/build-no/link-static/threading-multi/visibility-hidden

Component configuration:

    - atomic                   : not building
    - chrono                   : not building
    - cobalt                   : building
    - container                : not building
    - context                  : not building
    - contract                 : not building
    - coroutine                : not building
    - date_time                : not building
    - exception                : not building
    - fiber                    : not building
    - filesystem               : not building
    - graph                    : not building
    - graph_parallel           : not building
    - headers                  : not building
    - iostreams                : not building
    - json                     : not building
    - locale                   : not building
    - log                      : not building
    - math                     : not building
    - mpi                      : not building
    - nowide                   : not building
    - program_options          : not building
    - python                   : not building
    - random                   : not building
    - regex                    : not building
    - serialization            : not building
    - stacktrace               : not building
    - system                   : not building
    - test                     : not building
    - thread                   : not building
    - timer                    : not building
    - type_erasure             : not building
    - url                      : not building
    - wave                     : not building

...found 22 targets...

The Boost C++ Libraries were successfully built!

The following directory should be added to compiler include 

...

The following directory should be added to linker library paths:

...

But after that there is no libboost_cobalt.a nor anything like this. I also tried using cxxflags="-std=c++20 -fcoroutines" but results were the same.

So what am I doing wrong?

[klemens-morgenstern]

Try 'cxxstd=20'.

[douaumont]

Thanks, that worked just fine.

[sehe]

Is this documented anywhere. I just spent an embarrassingly long time figuring this out. In fact, I don't think there's anything in the documentation even hinting at requirements for use? Because, since I couldn't find a CMake module nor a library artefact, I went to see whether the library has a header-only offering, but none of the words "linking", "building" or "header-only" seemed to appear in the docs.