Compilation error: circular includes for static_thread_pool

[dkolsen-pgi]

Compiling a file on Linux where the first stdexec header included is <exec/static_thread_pool.hpp> and the macro STDEXEC_SYSTEM_CONTEXT_HEADER_ONLY is defined set up a circular include and leads to a compilation error about exec::static_thread_pool not being defined.

$ cat poolonly.cpp
#include <exec/static_thread_pool.hpp>
int main() { }
$ g++ -std=c++23 -DSTDEXEC_SYSTEM_CONTEXT_HEADER_ONLY -I/proj/cuda/stdexec/main/include poolonly.cpp -ltbb
In file included from /proj/cuda/stdexec/main/include/exec/../stdexec/__detail/__system_context_default_impl_entry.hpp:30,
                 from /proj/cuda/stdexec/main/include/exec/../stdexec/__detail/__parallel_scheduler.hpp:770,
                 from /proj/cuda/stdexec/main/include/exec/../stdexec/execution.hpp:44,
                 from /proj/cuda/stdexec/main/include/exec/static_thread_pool.hpp:25,
                 from poolonly.cpp:1:
/proj/cuda/stdexec/main/include/exec/../stdexec/__detail/__system_context_default_impl.hpp:393:66: error: ‘static_thread_pool’ is not a member of ‘exec’
  393 | ng __parallel_scheduler_backend_impl = __generic_impl<exec::static_thread_pool>;
      |                                                             ^~~~~~~~~~~~~~~~~~

The sequence of includes is:

exec/static_thread_pool.hpp
  stdexec/execution.hpp
    stdexec/__detail/__parallel_scheduler.hpp
      stdexec/__detail/__system_context_default_impl_entry.hpp
        stdexec/__detail/__system_context_default_impl.hpp
          exec/static_thread_pool.hpp

This has an easy enough workaround; just include <stdexec/execution.hpp> before <exec/static_thread_pool.h>. So I am not blocked. But this should be fixed (though I am not sure how best to do that).