c – sfinae上下文中的模板变量

template <typename T,typename = void>
struct is_random_access_iterator : public std::false_type {};

template <typename T>
struct is_random_access_iterator<T,std::enable_if_t<
        std::is_same_v<typename std::iterator_traits<T>::iterator_category,std::random_access_iterator_tag>
    >>
    : public std::true_type {};

template <typename T>
constexpr bool is_random_access_iterator_v = is_random_access_iterator<T>::value;

此代码编译并按预期工作.现在,考虑第二个片段,我将模板变量替换为enable_if条件,而根本不更改其定义：

template <typename T>
constexpr bool has_random_access_iterator_tag = 
    std::is_same_v<typename std::iterator_traits<T>::iterator_category,std::random_access_iterator_tag>;

template <typename T,std::enable_if_t<
        //std::is_same_v<typename std::iterator_traits<T>::iterator_category,std::random_access_iterator_tag>
        has_random_access_iterator_tag<T>
    >>
    : public std::true_type {};

template <typename T>
constexpr bool is_random_access_iterator_v = is_random_access_iterator<T>::value;

SFINAE不再起作用了,编译器(用gcc 8和clang 7测试)抱怨std :: iterator_traits不存在,只要我提供一个不专门的类型.

这是一个有效的例子：

#include <iostream>
#include <vector>
#include <iterator>
#include <type_traits>

template <typename T>
constexpr bool has_random_access_iterator_tag = 
    std::is_same_v<typename std::iterator_traits<T>::iterator_category,std::random_access_iterator_tag>
        has_random_access_iterator_tag<T>
    >>
    : public std::true_type {};

template <typename T>
constexpr bool is_random_access_iterator_v = is_random_access_iterator<T>::value;

int main() {
    std::cout   << std::boolalpha << "Is random access iterator:n"
                << "- int:   " << is_random_access_iterator_v<int> << 'n'
                << "- int*:  " << is_random_access_iterator_v<int*> << 'n'
                << "- v::it: " << is_random_access_iterator_v<std::vector<int>::iterator> << 'n';
}

并输出：

prog.cc:8:54: error: no type named ‘iterator_category’ in ‘std::__1::iterator_traits’
std::is_same_v::iterator_category,std::random_access_iterator_tag>;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~

prog.cc:17:9: note: in instantiation of variable template specialization ‘has_random_access_iterator_tag’ requested here
has_random_access_iterator_tag
^

prog.cc:22:46: note: during template argument deduction for class template partial specialization ‘is_random_access_iterator > >’ [with T = int]
constexpr bool is_random_access_iterator_v = is_random_access_iterator::value;
^

prog.cc:22:46: note: in instantiation of template class ‘is_random_access_iterator’ requested here
prog.cc:26:35: note: in instantiation of variable template specialization ‘is_random_access_iterator_v’ requested here
<< “- int: ” << is_random_access_iterator_v << ‘n’
^
1 error generated.

谁能解释我为什么？我对此感到沮丧,因为使用模板常量使模板编程更加紧凑和易读是非常自然的.