Iterable expansion statements

[katzdm]

Explore the idea of expanding over ranges of constexpr extent which do not require non-transient constexpr allocation.

A motivating example is the std::span returned by define_static_array; i.e., it would be great for this to work:

int fn(int i, int j) {
  template for (constexpr auto p : define_static_array(parameters_of(^^fn)) {
    std::println("{} = {}", identifier_of(p), [:p:]);
  }
  return i + j;
}