为什么SFINAE在此示例中不能按预期工作？

#include <iostream>
#include <type_traits>

using namespace std;

template<typename T>
constexpr auto is_pure_input_iterator(int) ->
conditional_t
<
    is_convertible_v
    <
    iterator_traits<T>::iterator_category,
    input_iterator_tag
    >,
    true_type, false_type
>;

template<typename>
constexpr false_type is_pure_input_iterator(...);

int main()
{
    cout << boolalpha 
        << decltype(is_pure_input_iterator<istream_iterator<int>>(0))::value
        << endl;

    return {};
}
#include <iostream>
#include <type_traits>

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