BOOST_LOG_TRIVIAL and BOOST_LOG behavior seems not to be equivalent to std::cout and other standard iostreams. Here is an example:

main.cpp:

#include <iostream>

#include <boost/log/sources/logger.hpp>
#include <boost/log/trivial.hpp>

namespace test {
struct A {};
} // namespace test

std::ostream &operator<<(std::ostream &os, const test::A &) {
  os << "A";
  return os;
}

int main() {
  test::A a;
  std::clog << a << std::endl;

  namespace src = boost::log::sources;
  src::logger lg;
  BOOST_LOG_TRIVIAL(info) << a;
  BOOST_LOG(lg) << a;
}

meson.build:

project('boost-log-mre', ['cpp'], default_options: ['cpp_std=c++17'])
boost_dep = dependency('boost', modules: ['log'], include_type: 'system')
executable( 'a', ['main.cpp'], dependencies: [boost_dep])

build:

$ mkdir build && cd build
$ meson setup --buildtype debug
$ meson compile

Arch Linux, boost 1.81.0-3, gcc version 12.2.1 20230201, clang version 15.0.7

It fails to compile on GCC and Clang at BOOST_LOG_TRIVIAL and BOOST_LOG lines due to missing overload:

/usr/include/boost/log/utility/formatting_ostream.hpp:929:19: error: no match for ‘operator<<’ (operand types are ‘boost::log::v2_mt_posix::basic_formatting_ostream::ostream_type’ {aka ‘std::basic_ostream’} and ‘test::A’)
929 | strm.stream() << value;

But when I move definition of operator<<() into namespace test, it magically fixes itself. std::clog line works well in both cases. Is this intentional behavior or a bug in boost::log? I've stumbled upon this problem when tried to output Poco::Net::SocketAddress into BOOST_LOG_TRIVIAL because it declares it's overloaded operator<<() in a global scope and not in a scope of SocketAddress.

https://github.com/pocoproject/poco/blob/1211613642269b7d53bea58b02de7fcd25ece3b9/Net/include/Poco/Net/SocketAddress.h#L304