Function test

dlib/test/invoke.cpp:544–556

        }

        void test()
        {
            const auto f1 = [](float a, int b, long c) { return a + b + c;};
            const auto f2 = [](int a, float b, double c) { return a + b + c;};
            const auto f3 = [] {};
            int i{0};
            const auto f4 = wrap(f1, i);
            DLIB_TEST(i == 1);
            const auto f5 = wrap(f2, i);
            DLIB_TEST(i == 2);
            const auto f6 = wrap(f3, i);
            DLIB_TEST(i == 3);
        }

        static_assert(!is_callable<int>::value, "bad");
    }