C++ template template template parameter? - c++

Question

As a contrived example, let's say I want to write a type that can add a new template template parameter to a type that has a variadic template template parameter pack:
namespace
{
template <template <typename...> typename... T>
struct foo {};
template <typename Original, template <typename...> typename T>
class add_to_foo {
template <template <typename...> typename... V>
struct appender {
using type = foo<V..., T>;
explicit appender(foo<V...>) {}
};
template <template <typename...> typename... V>
appender(foo<V...>) -> appender<V...>;
public:
using type = typename decltype(appender(std::declval<Original>()))::type;
};
}
int main() {
using original_foo = foo<std::vector, std::basic_string>;
using new_foo = typename add_to_foo<original_foo, std::deque>::type;
static_assert(std::is_same_v<new_foo, foo<std::vector, std::basic_string, std::deque>>);
return EXIT_SUCCESS;
}
It works, but it's limited in that appender only works with foo rather than a generic template parameter. Let's try to replace foo with a template template parameter:
namespace
{
template <template <typename...> typename... T>
struct foo {};
template <typename Original, template <typename...> typename T>
class add_to_u {
template <template <typename...> typename U, template <typename...> typename... V>
struct appender {
using type = U<V..., T>;
explicit appender(U<V...>) {}
};
template <template <typename...> typename U, template <typename...> typename... V>
appender(U<V...>) -> appender<V...>;
public:
using type = typename decltype(appender(std::declval<Original>()))::type;
};
}
int main() {
using original_foo = foo<std::vector, std::basic_string>;
using new_foo = typename add_to_u<original_foo, std::deque>::type;
static_assert(std::is_same_v<new_foo, foo<std::vector, std::basic_string, std::deque>>);
return EXIT_SUCCESS;
}
This doesn't work, there are lot's of errors, but the main ones are:
<source>:14:25: error: use of template template parameter 'V' requires template arguments
using type = U<V..., T>;
<source>:23:36: error: no viable constructor or deduction guide for deduction of template arguments of 'appender'
using type = typename decltype(appender(std::declval<Original>()))::type;
It looks like the compiler thinks that the template arguments of U need to be complete types rather than template template parameters, this isn't the case when foo was used presumably because the compiler could see that it used template template parameters. So I need to tell the compiler that U is a template template parameter whose parameters are also template template parameters - any idea how?