> My version absolutely works, Consider: void other_thread(struct large* l) { struct large new_l = { ... }; *l = new_l; } void do_other_thing(

> My version absolutely works, Consider: void other_thread(struct large* l) { struct large new_l = { ... }; *l = new_l; } void do_other_thing() { struct large l = { ... }; thread_t t = start_thread(other_thread,&l); foo1(l); wait_for_thread(t); } Not only can `foo1` never avoid making a copy (because it can't assume it's unknown caller is not this perverse), it can't even make a copy safely unless its caller allocates a new `struct large` for it to copy from. (And the caller does not, in general, know that `start_thread` (or `process_thingy_maybe_concurrently^W^W`) starts a thread with the aliased `&l`, so it would have to make such copies on any substantially escaping alias just to be sure, at which point you're back to the original proposal.) The pass-by-non-mutatably-aliased-reference ABI seems terrible IMO, but it at least allows the callee to decide whether to copy using only information that's actually present in the callee.