How to match only variadic templates with a template template parameter?

We can determine whether a class template that can be instantiated with 0 template arguments is genuinely variadic or (merely) has defaults for all its non-variadic template arguments, by counting the arguments to an 0-argument instantiation:

template<class> constexpr unsigned argc_v;
template<template<class...> class C, class... A> constexpr unsigned argc_v<C<A...>> = sizeof...(A);
template<template<class...> class, class = void> constexpr bool is_variadic_v = false;
template<template<class...> class C> constexpr bool is_variadic_v<C, std::void_t<C<>>> = argc_v<C<>> == 0;

Then we can use this to build a set of specializations that respectively accept only variadic, 1-argument (with possible default) and 2-argument (with possible default/s) class templates:

template<template<class...> class, class = std::true_type>
struct foo;

template<template<class...> class C>
struct foo<C, std::bool_constant<is_variadic_v<C>>> {
    foo() { std::cout << "variable case\n"; }
};

template<template<class> class C>
struct foo<C, std::bool_constant<!is_variadic_v<C> && argc_v<C<void>> == 1>> {
    foo() { std::cout << "single param case\n";}
};

template<template<class, class> class C>
struct foo<C, std::bool_constant<!is_variadic_v<C> && argc_v<C<void, void>> == 2>> {
    foo() { std::cout << "two param case\n";}
};

I'm a bit disappointed that the latter argc_v tests are necessary (in C++20 mode); I think it's something to do with P0522 / CWG150.

Demo.