how to use c++ void_t correctly?

I am trying to practice on void_t usage but following code gives this compilation error.

is_fun is typedef in CompS struct so I think Comps::is_fun should be valid.

Is there anything I missed here?

template <typename T, typename Comp, typename = void_t<>>
class my_set
{
    public:
        my_set() : mem(5){}
        T mem;
};

template <typename T, typename Comp, void_t<typename Comp::is_fun> >
class my_set 
{
    public:
        my_set() : mem(10){}
        T mem;
};

struct CompS
{
    typedef int is_fun;
};

int main()
{
    my_set<int, CompS> a;
    std::cout << a.mem << std::endl;



    return 0;
}

voidt.cpp:17:38: error: ‘void’ is not a valid type for a template non-type parameter
 template <typename T, typename Comp, void_t<typename Comp::is_transparent> >
                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
voidt.cpp:9:38: error: template parameter ‘class<template-parameter-1-3>’
 template <typename T, typename Comp, typename = void>
                                  ^~~~~~~~
voidt.cpp:18:7: error: redeclared here as ‘<typeprefixerror><anonymous>’
 class my_set