From sender-receiver/issues #280:

The precondition on run_loop::run() is that the state is starting. Given that run_loop::finish() puts the loop in the finishing state, the implication is that one cannot call finish() before calling run().

However, in most cases sync_wait is calling finish() on its run_loop before calling run(), violating this precondition. sync_wait does the following:

run_loop loop;
auto op = connect(sndr, sync_wait_receiver{&loop});
start(op);

loop.run();

If sndr completes synchronously, the sync_wait_receiver will call finish() on the loop before loop.run() is called, violating run's precondition.