boost::has_left_shift does not work with containers

[davidgumberg]

The following example is a minimal reproduction of this issue:

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

inline std::ostream& operator<<(std::ostream& os, const std::pair<int, int>& _) {
    os << "Pairwise operator called!" << std::endl;
    return os;
}

BOOST_STATIC_ASSERT(boost::has_left_shift<std::ostream&, const std::pair<int, int>&>::value);

// If you comment out the above static assertion, the code compiles and this
// test case prints, as expected "Pairwise operator called!"
BOOST_AUTO_TEST_CASE(test_case1)
{
    std::pair<int, int> pair;
    std::cout << pair;
}

Compiling with gcc 14.2.1 results in:

ex.cpp:10:87: error: static assertion failed: boost::has_left_shift<std::ostream&, const std::pair<int, int>&>::value
   10 | BOOST_STATIC_ASSERT(boost::has_left_shift<std::ostream&, const std::pair<int, int>&>::value);

I have similar results in clang 18.1.6:

ex.cpp:10:21: error: static assertion failed due to requirement 'boost::has_left_shift<std::basic_ostream<char, std::char_traits<char>> &, const std::pair<int, int> &, boost::binary_op_detail::dont_care>::value': boost::has_left_shift<std::ostream&, const std::pair<int, int>&>::value
   10 | BOOST_STATIC_ASSERT(boost::has_left_shift<std::ostream&, const std::pair<int, int>&>::value);
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/static_assert.hpp:71:55: note: expanded from macro 'BOOST_STATIC_ASSERT'
   71 | #     define BOOST_STATIC_ASSERT( ... ) static_assert(__VA_ARGS__, #__VA_ARGS__)
      |   

The same happens if you use a std::vector<int> instead.