Why the combination of decltype and is_same gives wrong result?

Question

I have the following function:

template<typename T> auto print_type_info(const T& t) {  // C++17
    if constexpr (std::is_same<T, int>::value) {
        return t + 1;    // int case
    } else if constexpr (std::is_same<decltype(t), double&>::value) {
        return t + 0.1;    // double case
    } else {
        return t;    // other case
    }
}
void main() {
    assert(print_type_info(1) == 2);      // int test
    assert(print_type_info(0.1) == 0.2);  // double test
    assert(print_type_info("0") == "0");  // char* test
}

Runing the above assert tests in VSCode:

• int test goes into the int case correctly.
• double test goes into the other case wrongly in run-time.
• char* case goes into the double case wrongly in compile-time case.

I have made the following changes for the double case all with the same result:

• double&
• dobule
• const double
• const double&

Why is this?

Answer 1

As pointed out by HolyBlackCat, the correct answer is:

template<typename T> auto print_type_info(const T& t) {  // C++17
    if constexpr (std::is_same<T, int>::value) {
        return t + 1;
    } else if constexpr (std::is_same<decltype(t), const double&>::value) {
        return t + 0.1;
    } else {
        return t;
    }
}