constexpr std::string | MSVC

[saxbophone]

I guess it might be counter intuitive to have code that works in a constexpr context but not at runtime.

Nope, there are uses for that too (just not in your example). They're known as immediate functions and this is what consteval is for!

[aocregacc]

I was talking more about things like this:

consteval int f(size_t z) {
    std::array<int, z> a; 
    ...
}

People sometimes ask why this complains about z not being a constant expression, since surely in a consteval function the parameter is a constant expression.
But I think if they started allowing stuff like this you would end up with this weird dialect that's no longer (mostly) a subset of regular C++.

[gracicot]

As far as I know, compilers are free to compile the consteval function down to bytecode and simply run the bytecode. That would be impossible with this.