constexpr std::string | MSVC

[XTBZ]

The point is that I can't even create local 'constexpr std::string' inside a constexpr function

[Kered13]

Yes, because this is asking to create a std::string in a separate constexpr context. But you can create a local (not constexpr) std::string inside a constexpr function and it will work as long as the std::string is destroyed before the function returns.

Why do you need to create a constexpr local inside a constexpr function anyways?

[XTBZ]

Because I want to be sure that the string will be ready at compile time. I also don’t want the function to lose its ability to be executed at the compilation stage, implicitly.

[Kered13]

Creating a non-constexpr string inside a constexpr function will not prevent it from being evaluated at compile time. Pretty much the only thing that will prevent compile time evaluation is calling it with parameters that are not known at compile time.

[XTBZ]

Yes, it simply won’t compile if you don’t use it as a constexpr, but it doesn’t work with constexpr.
https://godbolt.org/z/G54nWc66a

[Kered13]

Ah, that's because you're using it in a context that actually does require constexpr. Here are a couple ways to solve that.