[lwt] Allow to specify custom source location for a check

[Lastique]

Suppose we have a common utility class that performs a check on the passed value:

template< typename T >
class checker
{
    T& m_expected;

public:
    void operator() (T const& res) const noexcept
    {
        BOOST_TEST_EQ(res, m_expected);
    }
};

and then we use this class in multiple tests with different types and expected values.

void check_stuff()
{
    checker<int> checker1(10);
    checker1(20);

    checker<long> checker2(30);
    checker2(40);
}

The problem is if any of these checks fail, the output always shows the location of the BOOST_TEST_EQ line, not the location of the particular use of checker.

Proposed solution 1:

Make test_with_impl API public, so that the user is able to define his own BOOST_TEST_EQ-like macro that will pass a custom source location. This leaves the implementation of such source location tracking mechanism to the user.

Proposed solution 2:

Add a new set of macros that will use a custom source location that was saved previously by a new macro (let's call it BOOST_TEST_CHECKPOINT). The user will use BOOST_TEST_CHECKPOINT to save the location of the original use of the checker. Alternatively (or additionally) we could provide a non-macro API to the same effect. Usage example:

void check_stuff()
{
    checker<int> checker1(10); BOOST_TEST_CHECKPOINT;
    checker1(20);

    checker<long> checker2(30); BOOST_TEST_CHECKPOINT;
    checker2(40);
}

[pdimov]

We can also do (2) without the need for the checkpoint:

template< typename T >
class checker
{
    T& m_expected;

public:
    void operator() (T const& res, boost::source_location const& loc = BOOST_CURRENT_LOCATION) const noexcept
    {
        BOOST_TEST_EQ_SL(res, m_expected, loc);
    }
};

I'm not sure whether we'll be able to overload BOOST_TEST_EQ to avoid the need for separate macros. C++03 will be a problem for sure.

Maybe Glen the Core maintainer can ask Glen the release manager when Boost will move to C++14 as the baseline requirement. :-)

[Lastique]

In my case, I will need a checkpoint-like approach anyway since the checker is not called directly by the test code. But a set of macros that accept a custom location will work. I don't think we need to raise the minimum C++ version for that.