constexpr recursive function using if constexpr or not

Question

Using gcc (HEAD 7.0.0 201612) I was surprised to discover that this works:

constexpr long value(const char *definition)
{
    if (definition && *definition)
    {
        return *definition + value(definition + 1);
    }

    return *definition;
}

int main()
{
    long l{};
    std::cin >> l;

    switch (l)
    {
        case value("AAAA"): f1(); break;
        case value("BBBB"): f2(); break;
        default: error();         break;
    }

    return 0;
}

The literal strings "AAAA" and "BBBB" were treated as compile-time values and the calls to value function yields the values 260 and 264 used directly in the switch context; I should admit that I was expecting the compiler complain about "strings are no constant expressions". So I moved to the next step and tried to add a if constexpr to the algorithm:

constexpr long value(const char *definition)
{
    if constexpr (definition && *definition)
    {
        return *definition + value(definition + 1);
    }

    return *definition;
}

But by adding the if constexpr the code no longer compiles:

In function 'constexpr long int value(const char*)':
 error: 'definition' is not a constant expression
     if constexpr (definition && *definition)
                                            ^

So it seems that the definition pointer itself is not evaluable at compile time in the if constexpr context but using traditional if the whole function is evaluable at compile time.

Why is this happening?

Answer 1

It's happening for the same reason you can't do this:

constexpr auto func(const int foo)
{
  return std::array<int, foo>{};
}

foo may or may not be a constant expression, depending on how you call func. But what you put into a template parameter must be a constant expression. Therefore, this code fails to compile.

The same goes for your value function. What you put into if constexpr must be a constant expression, just like with template arguments. So you cannot put something there which may or may not be a constant expression, like parameters to constexpr functions.