c++ - 如何访问可变模板参数包成员中存在的内部模板 typedef？

Question

我有一些代码对我来说似乎没有歧义，但 gcc4.7 令人窒息:

#include <iostream>
#include <tuple>

using namespace std;

// Container for mixins
template<template<typename> class... Mixins>
struct Mix : Mixins<Mix<Mixins...>>... {
  typedef tuple<Mixins<Mix<Mixins...>>...> types;
};

// Outer layer extracts the type tuple from the argument
template<typename T>
struct InnerCombiner {
  typedef typename InnerCombiner<typename T::types>::type type;
};

// Typedef type to be a new mix of the inner mixins of the MixedMixins
template<typename... MixedMixins>
struct InnerCombiner<tuple<MixedMixins...>> {
  // This line is the problem. The compiler doesn't seem to be able to make sense
  // of the reference to the inner mixin template template classes
  typedef Mix<MixedMixins::InnerMixin...> type;
};

template<typename Mixed>
struct A {
  template<typename MixedInner>
  struct InnerMixin {
    void foo() { cout << "foo() loves you!" << endl; };
  };
};

template<typename Mixed>
struct B {
  template<typename MixedInner>
  struct InnerMixin {
    void bar() { cout << "bar() loves you!" << endl; };
  };
};

// I'm going to write out the type I expect ic to have. Oh god, it's so nasty:
// Mix<
//   A<Mix<A,B>>::InnerMixin<Mix<A<Mix<A,B>>::InnerMixin,B<Mix<A,B>>::InnerMixin>,
//   B<Mix<A,B>>::InnerMixin<Mix<A<Mix<A,B>>::InnerMixin,B<Mix<A,B>>::InnerMixin>
// >


int main() {
  InnerCombiner<Mix<A,B>>::type ic;

  ic.bar(); // Not working.
}

这样访问 InnerMixins 有问题吗？写的时候觉得挺有道理的:)

Answer 1

我可以通过指定 InnerMixin 模板使其在 clang 3.0 上运行:

typedef Mix<MixedMixins::template InnerMixin...> type;
//                       ^^^^^^^^

但它在 g++ 4.8 上仍然失败，

3.cpp:23:52: error: parameter packs not expanded with ‘...’:3.cpp:23:52: note:         ‘MixedMixins’