If that's the complete program or alternatively if it's the entire scope for the FOO and BAR variables such that the language guarantees no other thread (or process or anything else) may mutate or even access FOO or BAR, then the implementation should throw an error saying as much, the moment the scope is left (e.g. by reaching end-of-input).
However, if one is still free to do the moral equivalent of BAR = 1 or FOO = argc in a third thread, or even at compile/link time, then why should it a mistake to have these two threads blocking on such a (possible) event?
However, if one is still free to do the moral equivalent of BAR = 1 or FOO = argc in a third thread, or even at compile/link time, then why should it a mistake to have these two threads blocking on such a (possible) event?