It seems like something that could have been trapped right in k&r.

Was it ever even theoretically under any circumstances for any reason intended to be able to write a stack of functions with no returns that just fall into each other like assembly? I can't believe it.

So it seems like something even the very first compiler could have cought right in an early parser pass or stage.

But I also decline to believe I have a better idea about something than K or R, so there must be a non-triviality I don't see. I mean goto() exists in the language so ?

... I guess simply detecting the end of a function, or detecting that the process reached the end of a function, isn't a good enough definition of the problem. You can have any number of returns or gotos in the middle that you are always supposed to hit, and intentionally no return at the end because instead you have an assert or a goto.

assert you should never get here, goto error, goto not error but just next step, etc. They might or might not be error conditions that the process reached that spot, but it's not an error that the code doesn't end with a return.