Why does BOOST_TEST((Iterator == Iterator)) require extra parentheses?

I am testing my implementation of binary search from binsearch.hpp:

template<typename Iterator, typename T>
Iterator binsearch(Iterator begin, Iterator end, const T &v) {
    if (std::distance(begin, end) == 0) {return end;}

    Iterator save = end;

    while (std::distance(begin, end) > 0) {
        Iterator mid = begin + std::distance(begin, end) / 2;

        if (*mid == v) {
            return mid;
        }

        if (v < *mid) {
            end = mid;
        } else {
            begin = mid + 1;
        }
    }

    return save;
}

with the following boost-driven unit test test_binsearch.cpp:

#define BOOST_TEST_MODULE test_binsearch
#define BOOST_TEST_DYN_LINK
#include <boost/test/unit_test.hpp>

#include "binsearch.hpp"

BOOST_AUTO_TEST_CASE(empty_0) {
    std::vector<int> xs = {};

    const auto result = binsearch(xs.begin(), xs.end(), 42);

    BOOST_TEST((result == xs.end()));
}

Unless I surround the comparison inside the BOOST_TEST with an extra pair of parentheses, I get a very cryptic and long compilation error that repeatedly tries to convert an iterator to a char, an error code and a few other types:

note: cannot convert ‘t’ (type ‘const __gnu_cxx::__normal_iterator std::vector >’) to type ‘char’

    ostr << t;

I got the clue to surround my comparison with extra parentheses from here. Why does it fail to compile without them?