无法解释模棱两可的模板特化答案

【问题标题】：Cannot explain ambiguous template specializations无法解释模棱两可的模板特化
【发布时间】：2015-10-02 21:39:53
【问题描述】：

给定

template <typename...> struct Pack;
using T1 = std::tuple<int, char, double>;
using T2 = std::tuple<bool, double, int, char>;

TupleTree<Pack, T1, T2>应该是

Pack<
    Pack<int, bool>, Pack<int, double>, Pack<int, int>, Pack<int, char>,
    Pack<char, bool>, Pack<char, double>, Pack<char, int>, Pack<char, char>,
    Pack<double, bool>, Pack<double, double>, Pack<double, int>, Pack<double, char>
>

这扩展到任意数量的元组。容易理解定义吗？好的，我的程序运行正常：

http://ideone.com/OumeZK

但现在我想将定义扩展到

`TupleTreeWithRepeats<P, std::index_sequence<Is...>, Tuples...>`

其中Is... 将指示每个元组重复使用的次数，直到移动到下一个元组。请注意<Is...> = <1,1,...,1> 将减少到与TupleTree<P, Tuples...> 相同。我坚持的歧义在于这两个专业：

TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, LoopNumber, P<Ts...>, First, Rest...>
TupleTreeWithRepeatsHelper<P, std::index_sequence<I, Is...>, I, P<Ts...>, First, Rest...>

由于某种原因，P<Ts...> 的存在会导致歧义，因为当我将其替换为单一名称类型时，歧义已被消除。即使我用std::index_sequence<I, Is...> 替换std::index_sequence<Is...>，歧义仍然存在。这是怎么回事？以及如何解决这个问题？下面是代码，和TupleTree的代码差不多：

#include <iostream>
#include <tuple>
#include <type_traits>

template <typename T> struct Identity { using type = T; };

// Merging packs of types.
template <typename...> struct MergePacks;

template <typename Pack>
struct MergePacks<Pack> : Identity<Pack> {};

template <template <typename...> class P, typename... Types1, typename... Types2, typename... Packs>
struct MergePacks<P<Types1...>, P<Types2...>, Packs...> : MergePacks<P<Types1..., Types2...>, Packs...> {};

// Appending a type to a pack.
template <typename Pack, typename T> struct AppendType;

template <template <typename...> class P, typename... Ts, typename T>
struct AppendType <P<Ts...>, T> {
    using type = P<Ts..., T>;
};

// ExpandPackWithTuple takes a pack, and creates N packs that each end with the tuple's elements, N is the size of the tuple.
template <template <typename...> class P, typename Pack, typename Tuple, typename Indices> struct ExpandPackWithTupleHelper;

template <template <typename...> class P, typename Pack, typename Tuple, std::size_t... Is>
struct ExpandPackWithTupleHelper<P, Pack, Tuple, std::index_sequence<Is...>> {
    using type = P<typename AppendType<Pack, typename std::tuple_element<Is, Tuple>::type>::type...>;
};

template <template <typename...> class P, typename Pack, typename Tuple>
using ExpandPackWithTuple = typename ExpandPackWithTupleHelper<P, Pack, Tuple, std::make_index_sequence<std::tuple_size<Tuple>::value>>::type;

// TupleTreeWithRepeats.
template <template <typename...> class P, typename NumRepeats, std::size_t LoopNumber, typename OutputPack, typename... Tuples> struct TupleTreeWithRepeatsHelper;

template <template <typename...> class P, std::size_t I, std::size_t... Is, std::size_t LoopNumber, typename... Ts, typename First, typename... Rest>
struct TupleTreeWithRepeatsHelper<P, std::index_sequence<I, Is...>, LoopNumber, P<Ts...>, First, Rest...> :
    TupleTreeWithRepeatsHelper<P, std::index_sequence<I, Is...>, LoopNumber + 1, typename MergePacks<ExpandPackWithTuple<P, Ts, First>...>::type, First, Rest...> {};

template <template <typename...> class P, std::size_t I, std::size_t... Is, typename... Ts, typename First, typename... Rest>
struct TupleTreeWithRepeatsHelper<P, std::index_sequence<I, Is...>, I, P<Ts...>, First, Rest...> :
    TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, 0, typename MergePacks<ExpandPackWithTuple<P, Ts, First>...>::type, Rest...> {};

template <template <typename...> class P, std::size_t... Is, std::size_t LoopNumber, typename OutputPack>
struct TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, LoopNumber, OutputPack> {
    using type = OutputPack;
};

template <template <typename...> class P, typename NumRepeats, typename... Tuples> struct TupleTreeWithRepeats;

template <template <typename...> class P, std::size_t I, std::size_t... Is, typename... Tuples>
struct TupleTreeWithRepeats<P, std::index_sequence<I, Is...>, Tuples...> : TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, 0, P<P<>>, Tuples...> {};

// Testing
template <typename...> struct Pack;
using T1 = std::tuple<int, char, double>;
using T2 = std::tuple<bool, double, int, char>;
using T3 = std::tuple<double, int>;

int main() {
    std::cout << std::is_same<
        TupleTreeWithRepeats<Pack, std::index_sequence<1,1,1>, T1, T2, T3>::type,
        Pack<
            Pack<int, bool, double>, Pack<int, bool, int>, Pack<int, double, double>, Pack<int, double, int>, Pack<int, int, double>, Pack<int, int, int>, Pack<int, char, double>, Pack<int, char, int>,
            Pack<char, bool, double>, Pack<char, bool, int>, Pack<char, double, double>, Pack<char, double, int>, Pack<char, int, double>, Pack<char, int, int>, Pack<char, char, double>, Pack<char, char, int>,
            Pack<double, bool, double>, Pack<double, bool, int>, Pack<double, double, double>, Pack<double, double, int>, Pack<double, int, double>, Pack<double, int, int>, Pack<double, char, double>, Pack<double, char, int>
        >
    >::value << '\n';  // ambiguous
}

【问题讨论】：

有趣的是，clang 并不模棱两可：coliru.stacked-crooked.com/a/4272e7d68f252d6e
我正在使用 GCC 5.1 进行编译。顺便说一下，我刚刚稍微编辑了我的代码，因为我在第二个专业化中发现了一个逻辑错误。但模糊性仍然是主要问题。但在我微调专业化和测试输出之前，首先需要消除歧义。
@MarcoA。我认为编译只是在那个上超时。查看深度较少的输出，clang 似乎在进行一些无限递归。
@T.C.我倾向于认为你是对的，尽管我本来希望得到反馈

标签： c++ templates c++11 variadic

【解决方案1】：

据我所知，这两个不应该是模棱两可的。

无论如何，一个简单的解决方法是仅根据“无重复”来实现“有重复”：

// TupleTreeWithRepeats.
template <template <typename...> class P, typename Tuples>
struct TupleTreeWithRepeatsHelper;

template <template <typename...> class P, typename... Tuples>
struct TupleTreeWithRepeatsHelper<P, Pack<Tuples...>> :
    Identity<TupleTree<P, Tuples...>> {};

template <template <typename...> class P, typename NumRepeats, typename... Tuples> 
struct TupleTreeWithRepeats;

template <template <typename...> class P, std::size_t... Is, typename... Tuples>
struct TupleTreeWithRepeats<P, std::index_sequence<Is...>, Tuples...> :
    TupleTreeWithRepeatsHelper<P, typename MergePacks<repeat<Tuples, Is>...>::type> {};

repeat 在哪里

template<class T, std::size_t> using id = T;

template<class T, std::size_t... Is>
Pack<id<T, Is>...> do_repeat(std::index_sequence<Is...>);

template<class T, std::size_t I>
using repeat = decltype(do_repeat<T>(std::make_index_sequence<I>()));

Demo。有人可能会争辩说，这也是一种更好的设计。

最小化为：

template<class> class Purr { };

template <template <class> class, class>
struct Meow;

template <template <class> class P>
struct Meow<P, P<int>> { };

template <template <class> class P, class T>
struct Meow<P, P<T>> { };

Meow<Purr, Purr<int>> c;

GCC 报告了一个模棱两可的问题，对我来说这绝对是一个错误。 Clang 可以正确处理这个问题。

【讨论】：

感谢您的解决方法，T.C.但是既然理解 C++ 语言比我的程序更重要，那么第一个实现是否真的模棱两可？我知道你表达了你的意见。但是，如果它确实是模棱两可的，那么像您上面的解决方案这样的解决方法会一直存在吗？

【解决方案2】：

虽然 T.C. 的解决方法肯定更优雅，但我发现的这种解决方法可能是遇到此 GCC 错误时应该始终有效的一般解决方法。我没有避免 GCC 错误，而是直面它。只需将 P<Ts...> 替换为单个类型名并定义一个使用它的单独元函数：

template <template <typename...> class P, typename OutputPack, typename Tuple> struct AddToOutput;

template <template <typename...> class P, typename... Ts, typename Tuple>
struct AddToOutput<P, P<Ts...>, Tuple> : MergePacks<ExpandPackWithTuple<P, Ts, Tuple>...> {};

// My workaround, using struct AddToOutput.
template <template <typename...> class P, std::size_t... Is, std::size_t LoopNumber, typename OutputPack, typename First, typename... Rest>
struct TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, LoopNumber, OutputPack, First, Rest...> :
    TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, LoopNumber + 1, typename AddToOutput<P, OutputPack, First>::type, First, Rest...> {};

template <template <typename...> class P, std::size_t I, std::size_t... Is, typename OutputPack, typename First, typename... Rest>
struct TupleTreeWithRepeatsHelper<P, std::index_sequence<I, Is...>, I, OutputPack, First, Rest...> :
    TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, 1, typename AddToOutput<P, OutputPack, First>::type, Rest...> {};

整个代码：

#include <iostream>
#include <tuple>

template <typename T> struct Identity { using type = T; };

// Merging packs of types.
template <typename...> struct MergePacks;

template <typename Pack>
struct MergePacks<Pack> : Identity<Pack> {};

template <template <typename...> class P, typename... Types1, typename... Types2, typename... Packs>
struct MergePacks<P<Types1...>, P<Types2...>, Packs...> : MergePacks<P<Types1..., Types2...>, Packs...> {};

// Appending a type to a pack.
template <typename Pack, typename T> struct AppendType;

template <template <typename...> class P, typename... Ts, typename T>
struct AppendType <P<Ts...>, T> {
    using type = P<Ts..., T>;
};

// ExpandPackWithTuple takes a pack, and creates N packs that each end with the tuple's elements, where N is the size of the tuple.
template <template <typename...> class P, typename Pack, typename Tuple, typename Indices> struct ExpandPackWithTupleHelper;

template <template <typename...> class P, typename Pack, typename Tuple, std::size_t... Is>
struct ExpandPackWithTupleHelper<P, Pack, Tuple, std::index_sequence<Is...>> {
    using type = P<typename AppendType<Pack, typename std::tuple_element<Is, Tuple>::type>::type...>;
};

template <template <typename...> class P, typename Pack, typename Tuple>
using ExpandPackWithTuple = typename ExpandPackWithTupleHelper<P, Pack, Tuple, std::make_index_sequence<std::tuple_size<Tuple>::value>>::type;

// Now, for TupleTree itself.
template <template <typename...> class P, typename OutputPack, typename... Tuples> struct TupleTreeHelper;

template <template <typename...> class P, typename... Ts, typename First, typename... Rest>
struct TupleTreeHelper<P, P<Ts...>, First, Rest...> : TupleTreeHelper<P, typename MergePacks<ExpandPackWithTuple<P, Ts, First>...>::type, Rest...> {};

template <template <typename...> class P, typename OutputPack>
struct TupleTreeHelper<P, OutputPack> {
    using type = OutputPack;
};

template <template <typename...> class P, typename... Tuples>
using TupleTree = typename TupleTreeHelper<P, P<P<>>, Tuples...>::type;

// TupleTreeWithRepeats.
template <template <typename...> class P, typename NumRepeats, std::size_t LoopNumber, typename OutputPack, typename... Tuples> struct TupleTreeWithRepeatsHelper;

template <template <typename...> class P, typename OutputPack, typename Tuple> struct AddToOutput;  // This is only needed for a workaround against the GCC 5.1 bug explained below.

template <template <typename...> class P, typename... Ts, typename Tuple>
struct AddToOutput<P, P<Ts...>, Tuple> : MergePacks<ExpandPackWithTuple<P, Ts, Tuple>...> {};

// The following two specializations run into ambiguity with GCC 5.1, though Clang runs it fine.  It is GCC that is bugged.
//template <template <typename...> class P, std::size_t... Is, std::size_t LoopNumber, typename... Ts, typename First, typename... Rest>
//struct TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, LoopNumber, P<Ts...>, First, Rest...> :
//  TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, LoopNumber + 1, typename MergePacks<ExpandPackWithTuple<P, Ts, First>...>::type, First, Rest...> {};
//
//template <template <typename...> class P, std::size_t I, std::size_t... Is, typename... Ts, typename First, typename... Rest>
//struct TupleTreeWithRepeatsHelper<P, std::index_sequence<I, Is...>, I, P<Ts...>, First, Rest...> :
//  TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, 1, typename MergePacks<ExpandPackWithTuple<P, Ts, First>...>::type, Rest...> {};

// My workaround, using struct AddToOutput.
template <template <typename...> class P, std::size_t... Is, std::size_t LoopNumber, typename OutputPack, typename First, typename... Rest>
struct TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, LoopNumber, OutputPack, First, Rest...> :
    TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, LoopNumber + 1, typename AddToOutput<P, OutputPack, First>::type, First, Rest...> {};

template <template <typename...> class P, std::size_t I, std::size_t... Is, typename OutputPack, typename First, typename... Rest>
struct TupleTreeWithRepeatsHelper<P, std::index_sequence<I, Is...>, I, OutputPack, First, Rest...> :
    TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, 1, typename AddToOutput<P, OutputPack, First>::type, Rest...> {};

template <template <typename...> class P, typename OutputPack>
struct TupleTreeWithRepeatsHelper<P, std::index_sequence<>, 1, OutputPack> {
    using type = OutputPack;
};

template <template <typename...> class P, typename NumRepeats, typename... Tuples> struct TupleTreeWithRepeats;

template <template <typename...> class P, std::size_t... Is, typename... Tuples>
struct TupleTreeWithRepeats<P, std::index_sequence<Is...>, Tuples...> : TupleTreeWithRepeatsHelper<P, std::index_sequence<Is...>, 1, P<P<>>, Tuples...> {};

// Testing
#include <type_traits>

template <typename...> struct Pack;
using T1 = std::tuple<int, char, double>;
using T2 = std::tuple<bool, double, int, char>;
using T3 = std::tuple<double, int>;

int main() {
    std::cout << std::boolalpha;
    std::cout << std::is_same<
        ExpandPackWithTuple<Pack, Pack<int, bool>, std::tuple<int, char, double>>,
        Pack<Pack<int, bool, int>, Pack<int, bool, char>, Pack<int, bool, double>>
    >::value << '\n';  // true

    std::cout << std::is_same<
        MergePacks<ExpandPackWithTuple<Pack, Pack<>, T1>>::type,
        Pack<Pack<int>, Pack<char>, Pack<double>>
    >::value << '\n';  // true

    std::cout << std::is_same<
        MergePacks<ExpandPackWithTuple<Pack, Pack<int>, T2>, ExpandPackWithTuple<Pack, Pack<char>, T2>, ExpandPackWithTuple<Pack, Pack<double>, T2>>::type,
        Pack<
            Pack<int, bool>, Pack<int, double>, Pack<int, int>, Pack<int, char>,
            Pack<char, bool>, Pack<char, double>, Pack<char, int>, Pack<char, char>,
            Pack<double, bool>, Pack<double, double>, Pack<double, int>, Pack<double, char>
        >
    >::value << '\n';  // true

    std::cout << std::is_same<
        TupleTree<Pack, T1, T2>,
        Pack<
            Pack<int, bool>, Pack<int, double>, Pack<int, int>, Pack<int, char>,
            Pack<char, bool>, Pack<char, double>, Pack<char, int>, Pack<char, char>,
            Pack<double, bool>, Pack<double, double>, Pack<double, int>, Pack<double, char>
        >
    >::value << '\n';  // true

    std::cout << std::is_same<
        TupleTree<Pack, T1, T2, T3>,
        Pack<
            Pack<int, bool, double>, Pack<int, bool, int>, Pack<int, double, double>, Pack<int, double, int>, Pack<int, int, double>, Pack<int, int, int>, Pack<int, char, double>, Pack<int, char, int>,
            Pack<char, bool, double>, Pack<char, bool, int>, Pack<char, double, double>, Pack<char, double, int>, Pack<char, int, double>, Pack<char, int, int>, Pack<char, char, double>, Pack<char, char, int>,
            Pack<double, bool, double>, Pack<double, bool, int>, Pack<double, double, double>, Pack<double, double, int>, Pack<double, int, double>, Pack<double, int, int>, Pack<double, char, double>, Pack<double, char, int>
        >
    >::value << '\n';  // true

    std::cout << std::is_same<
        TupleTreeWithRepeats<Pack, std::index_sequence<1,1>, T1, T2>::type,
        Pack<
            Pack<int, bool>, Pack<int, double>, Pack<int, int>, Pack<int, char>,
            Pack<char, bool>, Pack<char, double>, Pack<char, int>, Pack<char, char>,
            Pack<double, bool>, Pack<double, double>, Pack<double, int>, Pack<double, char>
        >
    >::value << '\n';  // true

    std::cout << std::is_same<
        TupleTreeWithRepeats<Pack, std::index_sequence<1,1,1>, T1, T2, T3>::type,
        Pack<
            Pack<int, bool, double>, Pack<int, bool, int>, Pack<int, double, double>, Pack<int, double, int>, Pack<int, int, double>, Pack<int, int, int>, Pack<int, char, double>, Pack<int, char, int>,
            Pack<char, bool, double>, Pack<char, bool, int>, Pack<char, double, double>, Pack<char, double, int>, Pack<char, int, double>, Pack<char, int, int>, Pack<char, char, double>, Pack<char, char, int>,
            Pack<double, bool, double>, Pack<double, bool, int>, Pack<double, double, double>, Pack<double, double, int>, Pack<double, int, double>, Pack<double, int, int>, Pack<double, char, double>, Pack<double, char, int>
        >
    >::value << '\n';  // true
}

【讨论】：