Variadic alias templates

Everything that can be templatized can also be made variadic. An alias template is an alias (another name) for a family of types. A variadic alias template is a name for a family of types with a variable number of template parameters. With the knowledge accumulated so far, it should be fairly trivial to write alias templates. Let's see an example:

template <typename T, typename... Args>
struct foo 
{
};
template <typename... Args>
using int_foo = foo<int, Args...>;

The class template foo is variadic and takes at least one type template argument. int_foo, on the other hand, is only a different name for a family of types instantiated from the foo type with int as the first type template arguments. These could be used as follows:

foo<double, char, int> f1;
foo<int, char, double> f2;
int_foo<char, double> f3;
static_assert(std::is_same_v<decltype(f2), decltype(f3)>);

In this snippet, f1 on one hand and...