It would be nice if there was a way to tell compilers not to drop the lock stack and thus issue warnings when accessing a symbol that needs a lock inside `Assume`/`Assert`.

Hit by (at least): @ajtowns , @jnewbery , me.
{
    BOOST_CHECK_EQUAL(IsDigit('0'), true);
    BOOST_CHECK_EQUAL(IsDigit('1'), true);
    BOOST_CHECK_EQUAL(IsDigit('8'), true);
    BOOST_CHECK_EQUAL(IsDigit('9'), true);
    BOOST_CHECK_EQUAL(IsDigit('0' - 1), false);
    BOOST_CHECK_EQUAL(IsDigit('9' + 1), false);
    BOOST_CHECK_EQUAL(IsDigit(0), false);
    BOOST_CHECK_EQUAL(IsDigit(1), false);
    BOOST_CHECK_EQUAL(IsDigit(8), false);
    BOOST_CHECK_EQUAL(IsDigit(9), false);
}
template <typename T>
static void TestAddMatrixO()