For methods like post, and dispatch there is one overload taking only a CompletionToken and another additionally taking an Executor. As far as I know, the overload without Executor works as if the overload with the Executor was called with the CompletionTokens associated executor. However, what is the effect of passing an Executor to post that is not the associated executor of the CompletionToken? What’s the role of each of the executors? What executor will the CompetionToken‘s handler be executed on?

I already found out that the assertion in

boost::asio::io_context ioc;
auto ioc_ex = ioc.get_executor();

auto strand_a = boost::asio::make_strand(ioc_ex);
auto strand_b = boost::asio::make_strand(ioc_ex);

boost::asio::post(strand_a, boost::asio::bind_executor(strand_b, [&]() {
    assert(strand_a.running_in_this_thread()
        && strand_b.running_in_this_thread());
}));
ioc.run();

succeeds. This surprises me since I assumed that a CompetionToken‘s handler would only ever be executed in a single Executor (strand in this case).

In other answers (e.g. here and here) I’ve read the Executor passed to post works as a fallback Executor for execution of the CompletionToken‘s handler, in case the latter does not have an associated executor. However, this cannot be its only use, since otherwise, the code above would simply ignore the “fallback” Executor stand_a, given the CompletionToken is explicitly bound to stand_b.