Often times, the easiest way to debug complex template code is to intentionally fail the compilation, with a minimal context, so you can read the compiler's output on what the types and values are. Here's a simple example of a function I used to use:

    template <typename T, typename ...Ts>
    void show_types()
    { static_assert((T*)nullptr, "Type log"); }

    int main()
    {
      show_types<int, float, bool>();
    }

When trying to compile this, using something like `g++ show-type.cpp`, you'll get this output:

    show-type.cpp: In instantiation of ‘void show_types() [with T = int; Ts = {float, bool}]’:
    show-type.cpp:7:32:   required from here
    show-type.cpp:3:3: error: static assertion failed: Type log
     { static_assert((T*)nullptr, "Type log"); }

The same can be done for non-type template args, of course.