Improved expect_exception

ImprovedExpectException

template<typename ExecLambda, typename OnThrowLambda, typename NoThrowLambda>
auto expect_exception( ExecLambda exec_lambda, bool expect_to_throw,
                       OnThrowLambda exc_lambda, NoThrowLambda no_exc_lambda)
{
    bool threw;
    decltype( exec_lambda() ) x;

    try {
        x = exec_lambda();
        threw = false;
    }
    catch (...) {
        threw = true;
    }

    if ( expect_to_throw != threw ) {
        if (expect_to_throw)
            throw "failed to catch expected exception";
        else
            throw "caught unexpect exception";
    } else {
        if (expect_to_throw)
            return exc_lambda();
        else
            return no_exc_lambda(x);
    }
}