Why can std::string create a constexpr object only if it's a static variable?

Question

I'm wondering why std::string can't be constexpr if it's a stack variable:

#include <string>
#include <iostream>
#include <cstring>

constexpr std::string str15 {"123456789012345"}; // This compiles!

struct Foo
{
    constexpr Foo() : a(6) {
        for (int i = 0; i < 8; ++i)
            buff[i] = 0;
    }
    int a;
    char buff[8];
};

constexpr void func()
{
    constexpr Foo f{}; // This compiles
    constexpr std::string str15 {"123456789012345"}; // This doesn't compile
}

int main() {
    func();
    return 0;
}

Error:

'std::string{std::__cxx11::basic_string<char>::_Alloc_hider{((char*)(& str15.std::__cxx11::basic_string<char>::<anonymous>.std::__cxx11::basic_string<char>::<unnamed union>::_M_local_buf))}, 15, std::__cxx11::basic_string<char>::<unnamed union>{char [16]{'1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '1', '2', '3', '4', '5', 0}}}' is not a constant expression

I get the stack variable and the static variable are different places in memory, but the compiler will accept arbitrary objects as local variables as constexpr. My Foo class doesn't have a const char* as argument, so that's a difference, but it's not as if const char* constructor can't be used in constexpr, because higher above:

constexpr std::string str15 {"123456789012345"}; // This compiles

https://godbolt.org/z/cffPq6nns

Answer 1

The difference between your class Foo and std::basic_string boils down to libstdc++ implementation of SSO, where the latter has a member of pointer type, which uses the underlying data array as initializer.

This in turn violates possible values a pointer can take in a constant expression (provided the enclosing object doesn't have static storage duration). From cppreference.com:

If the value is of pointer type, it is one of the following values:

• the address of an object with static storage duration
• the address past the end of an object with static storage duration
• the address of a (non-immediate(since C++20)) function
• a null pointer value

If Foo had such a pointer, it would suffer from the same problem:

struct Foo
{
    char buff[8];
    char* ptr = buff;
};

int main() {
    constexpr Foo f{}; // Doesn't compile
    return 0;
}

Answer 2

It's usually best practice for constexpr variables inside functions to also be declared as static. This avoids the possibility that the compiler will decide to extra work each time you call the function. This will also resolve your compile error, while keeping the definitions inside of the function.

#include <string>
#include <iostream>
#include <cstring>

constexpr std::string str15 {"123456789012345"}; // This compiles!

struct Foo
{
    constexpr Foo() : a(6) {
        for (int i = 0; i < 8; ++i)
            buff[i] = 0;
    }
    int a;
    char buff[8];
};

constexpr void func()
{
    static constexpr Foo f{}; // This compiles
    static constexpr std::string str15 {"123456789012345"}; // Complies when static
}

int main() {
    func();
    return 0;
}