How do I access an inner template typedef present in the members of a variadic template argument pack?

Question

I have some code which seems unambiguous to me, but gcc4.7 is choking on it:

#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.
}

Is there something wrong with accessing InnerMixins this way? It seemed pretty reasonable when I wrote it :)

Answer 1

I can make it work on clang 3.0 by specifying InnerMixin a template:

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

but it still fails on g++ 4.8, with

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

Answer 2

There's a type/value mismatch, this should at the very least be MixedMixins::template InnerMixin.... However GCC still rejects that and I found no way of cajoling it. Unfortunately I'm having a hard time proving that such a pack expansion is in fact valid. Hopefully someone more versed in the grammar could answer that point.

---

In a more 'lateral' approach, have you considered ditching template template parameters altogether? Not only could that ease the pain of syntax, but you can still 'rebind' the template parameters of a template specialization:

// We accept two types, a template specialization and
// a sequence of would be template parameters.
template<typename Specialization, typename T>
struct rebind;

template<
    template<typename...> class Template
    , typename... Old
    template<typename...> class Sequence
    , typename... T
>
struct rebind<Template<Old...>, Sequence<T...>> {
    using type = Template<T...>;
};

template<typename S, typename... T>
using Rebind = typename rebind<S, T...>::type;

E.g. Rebind<std::vector<int>, std::tuple<double, std::allocator<double>> is std:vector<double>. Combine that with a parameters_of/ParametersOf utility to extract the template parameters of a specialization into e.g. an std::tuple.

As a disclaimer I haven't used those techniques myself for long but I've already appreciated how I can limit the template template parameters pain points to a few, centralized spots of my code.