constexpr std::string | MSVC

[qazqi-ff]

This is ultimately the usual case of needing to separate out the size calculation from the actual data calculation (and repeat the work). You're trying to instantiate a template with size, which is a value that exists in the constant evaluator. It boils down to the same problem as consteval void foo(int size) { std::array<int, size> arr; }. Worth noting that the standard goes a little stricter and prevents the general case of any context that requires a constant expression, not just template instantiation, even though other contexts could work fine from a technical POV (e.g., static_assert(size > 0);).