// A `Future` allows ONE awaiter — a second concurrent `await` on the same pending
// future would overwrite the single `park` slot, and completion would wake only
// the second, stranding the first forever. Regression (B1.4a review, P1-c): the
// guard aborts loudly instead of silently deadlocking. Now over the unified
// `context.io` async layer (PLAN-IO-UNIFY Phase 5 — the bespoke `Task`/`wait` is
// retired).
//
// aborts (exit 134) after the diagnostic — aarch64-macOS-pinned.
#import "modules/std.sx";
sched :: #import "modules/std/sched.sx";
S :: struct { t: *Future(i64); }
main :: () -> i64 {
    st : S = ---; st.t = null;
    s := sched.Scheduler.init(); ps := @s; pst := @st;
    mkprod :: (ps: *sched.Scheduler, pst: *S) { pst.t = context.io.async(() -> (i64, !) { ps.yield_now(); 42 }); }
    mkw :: (ps: *sched.Scheduler, pst: *S) { ps.spawn(() { x := pst.t.await() or { -1 }; print("got {}\n", x); }); }
    push .{ io = xx s } {
        mkprod(ps, pst); mkw(ps, pst); mkw(ps, pst);   // second waiter → loud abort
        s.run();
    }
    return 0;
}