Conversion Warning with GCC15 when using operator<< within custom failure message and -WConversion

Start by reproducing the BOOST_TEST examples from the linked Boost.Test custom-messages documentation with GCC15 and -Wconversion, using the Compiler Explorer example as a reference. Trace the assertion and streamed-message conversion path; done means the examples retain their behavior without emitting the reported conversion warnings.

Starting with GCC15 ( when using -WConversion ) I get conversion warnings when using stream operator in custom failure messages.

Example code from the docs also produces those warnings:

Boost Test

#define BOOST_TEST_MODULE boost_test_message
#include <boost/test/included/unit_test.hpp>

BOOST_AUTO_TEST_CASE( test_message )
{
  const int a(1), b(2);
  BOOST_TEST(a == b, "a should be equal to b: " << a << "!=" << b);
  BOOST_TEST(a != 10, "value of a=" << a);
}

Compiler Explorer Example

Warning:

<source>: In member function 'void test_message::test_method()':
<source>:8:62: warning: choosing 'boost::test_tools::tt_detail::assertion_evaluate_t<E>::operator boost::test_tools::assertion_result() [with E = boost::test_tools::assertion::binary_expr<boost::test_tools::assertion::value_expr<const int&>, const int&, boost::test_tools::assertion::op::EQ<int, int, void> >]' over 'boost::test_tools::assertion_result::assertion_result(const BoolConvertable&) [with BoolConvertable = boost::test_tools::tt_detail::assertion_evaluate_t<boost::test_tools::assertion::binary_expr<boost::test_tools::assertion::value_expr<const int&>, const int&, boost::test_tools::assertion::op::EQ<int, int, void> > >]' [-Wconversion]
    8 |   BOOST_TEST(a == b, "a should be equal to b: " << a << "!=" << b);
      |                                                              ^
<source>:8:62: warning:   for conversion from 'boost::test_tools::tt_detail::assertion_evaluate_t<boost::test_tools::assertion::binary_expr<boost::test_tools::assertion::value_expr<const int&>, const int&, boost::test_tools::assertion::op::EQ<int, int, void> > >' to 'boost::test_tools::assertion_result' [-Wconversion]
<source>:8:62: note:   because conversion sequence for the argument is better