environment
x86-64 gcc 12.2, where you can try at https://godbolt.org/z/eTvTf7n3n and https://godbolt.org/z/4W7hM3Pzs
code
#include <type_traits>
// pass
template<std::enable_if_t<!std::is_same_v<int, int>, bool> = 0>
void f1(){
}
// Compilation failed
template<std::enable_if_t<!std::is_same<int, int>::value, bool> = 0>
void f2(){
}
int main()
{
}
question
Why function f1 and f2 behaves differently? is_same_v is just a constant time wrapper of is_same::value, I don’t understand what makes the difference.
>Solution :
The code is ill-formed NDR1. Both overloads are rejected by Clang and MSVC.
The validity of a template may be checked prior to any instantiation. … The program is ill-formed, no diagnostic required, if:
— no valid specialization can be generated for a template … and the template is not instantiated, …
1 NDR = "no diagnostic required", meaning the program is invalid, but the compilers are not required to catch that (not required to emit an error/warning message, aka "a diagnostic").