boost::test complains about the type missing operator<< where the operator is present.

[bartekbl]

I tried to used boost::test with data driven tests, but one of my input samples is vector<string>. Boost::test complains about missing operator<< even when I define it.

Self contained example:

#define BOOST_TEST_MODULE test module name

#include <string>
#include <vector>
#include <iostream>
#include <sstream>

using namespace std;
using namespace std::string_literals;

ostream& operator<<(ostream& out, const vector<string>& vs)
{
    for (const auto& s:vs) out << s;
    return out;
}

//#include <boost/test/included/unit_test.hpp>
//#include <boost/test/data/test_case.hpp>
#include "boost_1_80_0/boost/test/included/unit_test.hpp"
#include "boost_1_80_0/boost/test/data/test_case.hpp"

namespace bdata = boost::unit_test::data;

BOOST_AUTO_TEST_CASE(test_case_name2)
{
    cout << vector<string>{"a"};
    ostringstream sout;
    sout << vector<string>{{"ab"},{"cd"}};
    BOOST_TEST(sout.str() == "abcd");
}

BOOST_DATA_TEST_CASE(test_case_name, bdata::make({vector<string>{"a"},vector<string>{"a"},vector<string>{"a"}}), value)
{
}

fails to build with the error:

/usr/include/boost/type_traits/detail/has_binary_operator.hpp:54:150: error: no match for ‘operator<<’ (operand types are ‘std::basic_ostream<char>’ and ‘std::vector<std::__cxx11::basic_string<char> >’)

see attachment for full error log error.txt

If you comment-out the BOOST_DATA_TEST_CASE it builds and run correctly.

[bartekbl]

I found a hint on stack overflow that operator<< needs to be defined in the same namespace as the type being printed which in this case means std (because vector comes from std). I think we can close this issue, but this caveat should be mentioned in the docs.