std::disjuction not finding type in parameter pack passed to template function-CodePudding

I'm trying to use std::disjuction to test whether a parameter pack contains std::wstring, and nothing I've tried seems to want to make it return the value I'm expecting.

Here's the code as it is currently (https://godbolt.org/z/x99M8avYE):

#include <string>
#include <iostream>
#include <type_traits>

template <typename ...Ts>
using HasWS = std::disjunction<std::is_same<std::wstring,Ts>...>;

template <typename T, typename ...Ts>
using HasWS2 = std::disjunction<std::is_same<T,Ts>...>;

template<typename... Args>
void f(Args&&... args) {
    std::cout << HasWS<Args...>::value << std::endl;
    std::cout << HasWS2<std::wstring, Args...>::value << std::endl;
    std::cout << std::disjunction_v<std::is_same<std::wstring, Args>...> << std::endl;
    (std::cout << typeid(args).name() << std::endl, ...);
}

template <typename T, typename ...Ts>
using areT = std::disjunction<std::is_same<T,Ts>...>;

int main() {
    std::wstring wstr(L"abc");
    std::string str("def");

    static_assert(areT<std::wstring,std::string,std::wstring>::value);    
    static_assert(HasWS<std::wstring,decltype(wstr),decltype(str)>::value);
    static_assert(std::is_same<decltype(wstr), std::wstring>::value);
    
    f(wstr, str);
    f(str, str);
    f(wstr, wstr);

    std::cout << std::is_same_v<decltype(wstr), std::wstring> << std::endl;

    return 0;
}

As you can see, I was experimenting with different ways to call disjuction but none of them would find the wstring and return true. I even stooped as low as printing out the types for each of the elements in the pack and using is_same to make sure that wstr was actually a wstring.

What the heck is going on? Is it something to do with the pack argument to the template that I'm just not aware of?

CodePudding user response：

Note that the parameter type received by f is forwarding reference Args&&, so when you pass in an lvalue wstr, Args will be instantiated as std::wstring& instead of std::wstring, you should remove the reference, e.g.

template<typename... Args>
void f(Args&&... args) {
    std::cout << HasWS<std::remove_reference_t<Args>...>::value << std::endl;
    std::cout << HasWS2<std::wstring, std::remove_reference_t<Args>...>::value << std::endl;
    std::cout << std::disjunction_v<std::is_same<std::wstring, std::remove_reference_t<Args>>...> << std::endl;
}

Demo