問題描述

考慮以下代碼:

template <typename... Types>
struct list
{
    template <typename... Args>
    list(Args...) 
    {
        static_assert(sizeof...(Types) > 0);
    }
};

template <typename... Args>
list(Args...) -> list<Args...>;

int main()
{
    list l{0, 0.1, 'a'};
}

我希望 decltype(l) 是 list.不幸的是，g++ 7.2 和 g++ trunk 未能通過靜態斷言.clang++ 5.0.0 和 clang++ trunk 編譯并按預期工作.

I would expect decltype(l) to be list<int, double, char>. Unfortunately, g++ 7.2 and g++ trunk fail the static assertion. clang++ 5.0.0 and clang++ trunk compile and work as expected.

godbolt.org 一致性視圖

這是一個 g++ 錯誤嗎?或者有什么理由不應該在這里遵循演繹指南?

Is this a g++ bug? Or Is there a reason why the deduction guide should not be followed here?

在構造函數上添加 SFINAE 約束似乎提供了所需的行為:

Adding a SFINAE constraint on the constructor seems to provide the desired behavior:

template <typename... Args, 
          typename = std::enable_if_t<sizeof...(Args) == sizeof...(Types)>>
list(Args...) 
{
    static_assert(sizeof...(Types) > 0);
}

godbolt.org 一致性視圖

推薦答案

這是 gcc 錯誤 80871.問題是，我們最終得到了這組推演的候選對象:

This is gcc bug 80871. The issue is, we end up with this set of candidates for deduction:

template <class... Types, class... Args>
list<Types...> __f(Args... ); // constructor

template <class... Args>
list<Args...>  __f(Args... ); // deduction-guide

兩者都是有效的(Types... 在第一種情況下可以推斷為空)，但是這里的調用應該是模棱兩可的 - 兩者都不比另一個更專業.Types... 不參與這里的排序(類似于 [temp.deduct.partial]/12).所以正確的行為是繼續下一個決勝局，它贊成演繹指南.因此，這應該是一個 list.

Both are valid (Types... can deduce as empty in the first case), but the call here should be ambiguous - neither is more specialized than the other. Types... does not participate in ordering here (similar to the example in [temp.deduct.partial]/12). So the correct behavior is to proceed to the next tiebreaker, which favors deduction-guides. Hence, this should be a list<int, double, char>.

然而，gcc 的行為是有利于構造函數，因此 static_assert 觸發器因為 Types... 在這種情況下確實是空的.

However, gcc's behavior is to favor the constructor, hence the static_assert triggers becuase Types... would indeed be empty in that situation.

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