Determining specific type to use inside "if constexpr"

Question

I am trying to get a feel for using "if constexpr( expr )" and perform actions if a particular type is detected.

I am tinkering with the following bit:

#include <iostream>
#include <type_traits>

struct aT {};
struct bT {};
struct cT {};

constexpr aT atype;
constexpr bT btype;
constexpr cT ctype;

int main()
{
    if constexpr (std::is_same<decltype(atype), aT>::value)
        std::cout << "I am type " << typeid(atype).name() << std::endl;
    else if constexpr (std::is_same<decltype(btype), bT>::value)
        std::cout << "I am type " << typeid(btype).name() << std::endl;
    else if constexpr (std::is_same<decltype(ctype), cT>::value)
        std::cout << "I am type " << typeid(ctype).name() << std::endl;
}

I get no compile errors/warnings. However, on execution, none of the if constexpr tests evaluate to true.

If anyone might shine a bit of light on this I'd appreciate it.

Answer 1

constexpr implies const in the context of variable declaration. Try this:

if constexpr (std::is_same<decltype(atype), const aT>::value)
        std::cout << "I am type " << typeid(atype).name() << std::endl;