Clang＆GCC誤導模板參數

Question

我不擅長C++，所以這可能是新手的錯誤。我正在嘗試製作一個異構鏈接列表類型，其中每個節點的類型和其他列表的類型在每個節點中都已知。

這裏是一個SSSCE：

#include <utility> 

template<typename T, typename... Rest> 
struct hnode { 
    T data; 
    hnode<Rest...>* next; 
}; 

template<typename T> 
struct hnode<T> { 
    T data; 
    std::nullptr_t next; 
}; 

template<typename T, typename... Rest> 
hnode<T> hcons(T&& val, std::nullptr_t) { 
    return { std::forward<T>(val), nullptr }; 
} 

template<typename T, typename... Rest> 
hnode<T, Rest...> hcons(T&& val, hnode<Rest...>& next) { 
    return { std::forward<T>(val), &next }; 
} 

int main() { 
    hnode<int> three = hcons(1, nullptr); 
    auto two = hcons("hi", three); 
} 

然而，GCC給了我這個錯誤代碼：

test.cc: In function ‘int main()’: 
test.cc:28:29: error: no matching function for call to ‘hcons(const char [3], hnode<int>&)’ 
auto two = hcons("hi", three); 
          ^
test.cc:28:29: note: candidates are: 
test.cc:17:10: note: template<class T, class ... Rest> hnode<T> hcons(T&&, std::nullptr_t) 
hnode<T> hcons(T&& val, std::nullptr_t) { 
     ^
test.cc:17:10: note: template argument deduction/substitution failed: 
test.cc:28:29: note: cannot convert ‘three’ (type ‘hnode<int>’) to type ‘std::nullptr_t’ 
auto two = hcons("hi", three); 
          ^
test.cc:22:19: note: hnode<T, Rest ...> hcons(T&&, hnode<Rest ...>&) [with T = const char (&)[3]; Rest = {int, Rest}] 
hnode<T, Rest...> hcons(T&& val, hnode<Rest...>& next) { 
       ^
test.cc:22:19: note: no known conversion for argument 2 from ‘hnode<int>’ to ‘hnode<int, Rest>&’ 

鏘足以讓我來解決它是稍微更簡潔，但仍然沒有幫助：

test.cc:28:12: error: no matching function for call to 'hcons' 
auto two = hcons("hi", three); 
      ^~~~~ 
test.cc:17:10: note: candidate function [with T = char const (&)[3], Rest = <>] not viable: no known conversion from 'hnode<int>' to 'std::nullptr_t' (aka 'nullptr_t') for 2nd argument 
hnode<T> hcons(T&& val, std::nullptr_t) { 
     ^
test.cc:22:19: note: candidate template ignored: substitution failure [with T = char const (&)[3], Rest = <>]: too few template arguments for class template 'hnode' 
hnode<T, Rest...> hcons(T&& val, hnode<Rest...>& next) { 
       ^   ~~~~~ 
1 error generated. 

，因爲它是演繹Rest是似乎不可思議當它應該清楚地是<int>，它說它是<int>上面的線no known conversion from 'hnode<int>' to 'std::nullptr_t'。我犯了什麼錯誤？

Answer 1

不知道爲什麼，但作爲HNODE至少有一個模板參數，這個工程：

#include <utility> 

template<typename T, typename... Rest> 
struct hnode { 
    T data; 
    hnode<Rest...>* next; 
}; 

template<typename T> 
struct hnode<T> { 
    T data; 
    std::nullptr_t next; 
}; 

template<typename T> 
hnode<T> hcons(T&& val, std::nullptr_t) { 
    return { std::forward<T>(val), nullptr }; 
} 

template<typename T, typename S, typename... Rest> 
    hnode<T, S, Rest...> hcons(T&& val, hnode<S, Rest...>& next) { 
    return { std::forward<T>(val), &next }; 
} 

int main() { 
    hnode<int> three = hcons(1, nullptr); 
    auto two = hcons("hi", three); 
    auto one = hcons(5.14f, two); 
} 

在你的版本，一旦未能找到最佳匹配，這是正常的編譯器列表中的所有候選人。真正的問題仍然存在，爲什麼鏗鏘是在模板參數包扣除錯誤...

Visual Studio 2013成功的兩個版本（有和沒有S），仍然失敗的const char（*）[3 ]我用一個int代替測試...

Clang或Visual，誰是對的，我不知道，但在這裏填寫一個bug報告可能是一個好主意。