Particularly in these days of IPv6, just give everything a notionally publicly routable address and then every device can know its real address (of course this doesn't have to mean you actually route public traffic to every address if you don't want to).
In live networks, you don't always know which device presents your last-hop routable address to the public network, so it is not clear who can authoritatively answer the whatismyip question.
You can get an instantaneously-correct answer (i.e. correct in the instant, not necessarily quickly-answered) from an empirical test, but you need to choose an known (and trusted) entity on the public network to query. Querying multiple different entities might very well get you multiple different answers, depending on how your packets are routed and manipulated before they reach the public network.
And in all cases, there is no guarantee that your public address will be stable over time or location.
Also do keep in mind that an OS instance can have multiple Ethernet interfaces, and that each interface can have multiple IPs, and that not all DHCP gives out private addresses(University Wi-Fi gave me 133.xx back in the days ... behind NAT!), and that double NAT exists.
1: https://www.iana.org/assignments/bootp-dhcp-parameters/bootp...
Well, there’s assumption 1 that isn’t true, often, you aren’t allocated a public IP address…at least in IPv4 world
Upnp allows you to do that. But that relies on a lot of assumptions. Your address when connecting to X and Y could be different. It could be different depending on the port. It could be load-balanced and come randomly from a pool. It could vary depending on time of day. And many many other cases.
Once you have been allocated an IP address, there should be a way to fetch said address. That's the whole mechanism behind forging UDP packets. If I didn't know what my source IP address was, it would be OK to send 0.0.0.0 out into the world all the time.
Practically speaking, today, it is, considering most, but not all, ISPs won't let you do this and they will rewrite UDP packets that attempt to forge their source.
Edit: OK, since I'm not being clear, let me be explicit: There should be a mechanism in DHCP which allows for the querying of your public Internet address when receiving a packet from a client in one of the Private-Use Networks as defined in RFC 6890. This query should be exposed as a feature of ifconfig or ip, lest a user be forced to manually write such a packet to receive the data.