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#include <iostream>
#include <utility>
 
template<class...>struct voider{using type=void;};
template<class...Ts>using void_t=typename voider<Ts...>::type;
 
template<class T>
void f(std::conditional_t< std::is_same<T,double>::value, T, void >)
{
    std::cout << "hello world!\n";
}
 
int main() {
    f<int>();
}

prog.cpp: In function 'int main()':
prog.cpp:14:12: error: no matching function for call to 'f()'
     f<int>();
            ^
prog.cpp:14:12: note: candidate is:
prog.cpp:8:6: note: template<class T> void f(std::conditional_t<std::is_same<T, double>::value, T, void>)
 void f(std::conditional_t< std::is_same<T,double>::value, T, void >)
      ^
prog.cpp:8:6: note:   template argument deduction/substitution failed:
prog.cpp: In substitution of 'template<class T> void f(std::conditional_t<std::is_same<T, double>::value, T, void>) [with T = int]':
prog.cpp:14:12:   required from here
prog.cpp:8:6: error: invalid parameter type 'std::conditional_t<false, int, void> {aka void}'

Standard output is empty