Use template from dependent type

Question

How to call a template with a template parameter, when the template is inside a template type parameter:

template<template<class> class Template, class T>
struct MakeType
{
    using type = Template<T>;
};

template<class Policy>
struct Blah
{
    using type = MakeType< /*A non-instantiated template member in Policy*/, int>::type;
};

For example, Policy could have a template member ArrayType:

class ExamplePolicy
{
    template<class T>
    using ArrayType = std::vector<T>;
};

How do I call MakeType from Blah with the template Policy::ArrayType as Template.

Calling MakeType directly works well:

static_assert(std::is_same_v<std::vector<int>, MakeType<std::vector, int>::type>);

Would compile

Answer 1

Does this solve your problem?

// -*- compile-command: "g++ SO.cpp"; -*-
#include <vector>

template <template <class> class Template, class T>
struct MakeType
{
  using type = Template<T>;
};

template <class Policy>
struct Blah
{
  using type = typename MakeType<Policy::template ArrayType, int>::type;

  type foo()
  {
    return type();
  }
};

struct ExamplePolicy
{
  template <class T>
  using ArrayType = std::vector<T>;
};

int main()
{
  Blah<ExamplePolicy> blah;

  auto tmp = blah.foo();
}