[Conscat]

Nothing prevents std::vector from having a `constexpr` default-constructor except that it's not considered useful to do if you cannot follow that up with initializing its data in a constant context. For instance, this isn't very useful:

  constinit vector<int> v;

But this would be more so:

  constinit vector<int> v(16, 1); // Fill with 16 1's.

And the reason we can't do this wouldn't be solved by splitting it into multiple functions.

EDIT: Actually, come to think of it, C++20's vector already supports the first example. It's just not used much that way because it's not very helpful.

https://godbolt.org/z/avY4M9oMK