C++20 constexpr vector and string not working

Your program is actually ill-formed, though the error may be hard to understand. constexpr allocation support in C++20 is limited – you can only have transient allocation. That is, the allocation has to be completely deallocated by the end of constant evaluation.

So you cannot write this:

int main() {
    constexpr std::vector<int> v = {1, 2, 3};
}

Because v‘s allocation persists – it is non-transient. That’s what the error is telling you:

<source>(6): error C2131: expression did not evaluate to a constant
<source>(6): note: (sub-)object points to memory which was heap allocated during constant evaluation

v can’t be constant because it’s still holding on to heap allocation, and it’s not allowed to do so.

But you can write this:

constexpr int f() {
    std::vector<int> v = {1, 2, 3};
    return v.size();
}

static_assert(f() == 3);

Here, v‘s allocation is transient – the memory is deallocated when f() returns. But we can still use a std::vector during constexpr time.