C++ Template Template Parameter

// 'TemplateParam' is a template template parameter expecting a template with exactly one type parameter.
template <template <typename U> class TemplateParam>
class HostTemplate {
    // Instantiating the template parameter internally
    TemplateParam<int> int_instance;
    TemplateParam<double> double_instance;
};

// C++17 syntax
template <template <typename U> typename TemplateParam>
class HostTemplate;

#include <memory>

template <template <typename> class T> 
struct Host {};

template <typename U, typename Alloc = std::allocator<U>> 
struct Container {};

// ERROR in C++14: Container has two parameters (even though one has a default), 
// but Host expects a template with exactly one parameter.
Host<Container> h1; 

// VALID in C++17: The compiler recognizes that Container can be 
// instantiated with a single type argument due to the default allocator.
Host<Container> h2; 

// Accepts only variadic templates
template <template <typename...> class VariadicTemplateParam>
class HostTemplate {
    VariadicTemplateParam<int> single_arg;
    VariadicTemplateParam<int, float, char> multi_arg;
};

template <typename... Types> struct VariadicContainer {};
template <typename T> struct SingleContainer {};

HostTemplate<VariadicContainer> valid_host; // OK
// HostTemplate<SingleContainer> invalid_host; // ERROR: SingleContainer is not variadic

#include <cstddef>

// Expects a template that takes a type and a non-type parameter of type std::size_t
template <template <typename T, std::size_t N> class ArrayLike>
class HostTemplate {
    ArrayLike<int, 10> buffer;
};

// 'T' is strictly required here because the non-type parameter 'Value' depends on it.
template <template <typename T, T Value> class DependentParam>
class HostTemplate;