Do I need to put constexpr after else-if?

Inspired by this answer, I tried to copy and paste (and add testing in main()) this code:

template<typename T>
std::tuple<int, double> foo(T a) {
if constexpr (std::is_same_v<int, T>)
return {a, 0.0};

else if (std::is_same_v<double, T>)
return {0, a};

else
return {0, 0.0};
}

int main()
{
auto [x, y] = foo("");

std::cout << x << " " << y;
}


This is very straightforward - if T is deduced as int, we want to return a tuple of [a, 0.0]. If T is deduced as double, we want to return a tuple of [0, a]. Otherwise, we want to return [0, 0.0].

As you can see, in the main() function, I am calling foo with const char* argument, which should result in x and y being 0. That is not the case.

While trying to compile it, I encountered a strange error:


error: could not convert '{0, a}' from '<brace-enclosed initializer list>' to 'std::tuple<int, double>'


And I was like what?. Why on earth would I want that... I specifically used std::is_same to enable return {0, a} only when the type of a is deduced as double.

So I quickly ran to cppreference on if-constexpr. At the bottom of the page, above Notes, we can see this snippet of code:

extern int x; // no definition of x required
int f() {
if constexpr (true)
return 0;
else if (x)
return x;
else
return -x;
}


I thought to myself oookay..? I can't really see what's wrong with the original code. They use the same syntax and semantics....

But I was curious. I was curious if maybe something odd (at that time) might fix that issue, so I changed the original code to:

template<typename T>
std::tuple<int, double> foo(T a) {
if constexpr (std::is_same_v<int, T>)
return {a, 0.0};

else if constexpr (std::is_same_v<double, T>) // notice the additional constexpr here
return {0, a};

else
return {0, 0.0};
}

int main()
{
auto [x, y] = foo("");

std::cout << x << " " << y;
}


And voilà! The code compiled and executed as expected. So, my question is - Do we need to put constexpr after every if statement in if-else statement in these kind of situations? Or is it just my compiler? I am using GCC 7.3.


Do we need to put constexpr after every if statement in if-else block in these kind of situations?


Yes. The else-if block1 is a lie :), there are only if blocks1 and else blocks1. This is how your code is seen by the compiler:

if constexpr (std::is_same_v<int, T>)
return {a, 0.0};
else // {
if (std::is_same_v<double, T>)
return {0, a};
else
return {0, 0.0};
// }


else if (/*...*/) is just a formatting convention that everyone uses. As such, you can clearly see that the second constexpr is needed.



1: "block" is not the correct terminology. if is a statement (with optional else part). A block is { /*...*/ }.