[Bug c++/97452] New: [coroutines] incorrect sequencing of await_resume() when multiple co_await expressions occur in a single statement

["lewissbaker.opensource at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97452

            Bug ID: 97452
           Summary: [coroutines] incorrect sequencing of await_resume()
                    when multiple co_await expressions occur in a single
                    statement
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lewissbaker.opensource at gmail dot com
  Target Milestone: ---

See https://godbolt.org/z/9Kj9o8

Compile the following program with GCC trunk and flags: -std=c++20

----
#include <coroutine>
#include <exception>
#include <utility>
#include <cassert>
#include <cstdio>

struct task {
    struct promise_type {
        task get_return_object() noexcept {
            return
task{std::coroutine_handle<promise_type>::from_promise(*this)};
        }
        std::suspend_always initial_suspend() noexcept {
            std::puts("task initial_suspend");
            return {};
        }

        struct yield_awaiter {
            bool await_ready() noexcept { return false; }
            auto await_suspend(std::coroutine_handle<promise_type> h) noexcept
{
                std::puts("task yielding/returning value");
                return std::exchange(h.promise().continuation, {});
            }
            void await_resume() noexcept {
                std::puts("task resumed");
            }
        };

        yield_awaiter yield_value(int x) noexcept {
            value = x;
            return {};
        }

        void return_value(int x) noexcept {
            value = x;
        }

        yield_awaiter final_suspend() noexcept {
            return {};
        }

        [[noreturn]] void unhandled_exception() noexcept {
            std::terminate();
        }

        int value;
        std::coroutine_handle<> continuation;
    };

    explicit task(std::coroutine_handle<promise_type> coro) noexcept
    : coro(coro)
    {}

    task(task&& t) noexcept
    : coro(std::exchange(t.coro, {}))
    {}

    ~task() {
        if (coro) coro.destroy();
    }

    __attribute__((noinline)) bool await_ready() {
        std::puts("task::await_ready");
        return false;
    }

    __attribute__((noinline)) auto await_suspend(std::coroutine_handle<> h)
noexcept {
        std::puts("task::await_suspend");
        coro.promise().continuation = h;
        return coro;
    }

    __attribute__((noinline)) int await_resume() {
        std::puts("task::await_resume");
        return coro.promise().value;
    }

    std::coroutine_handle<promise_type> coro;
};

task two_values() {
    co_yield 1;
    co_return 2;
}

task example() {
    task t = two_values();
    int result = co_await t + co_await t;
    std::printf("result = %i (should be 3)\n", result);
    std::fflush(stdout);
    co_return result;
}

int main() {
    task t = example();
    t.coro.resume();
    assert(t.coro.done());
}
----

Expected output:
----
task initial_suspend
task initial_suspend
task::await_ready
task::await_suspend
task yielding/returning value
task::await_resume
task::await_ready
task::await_suspend
task resumed
task yielding/returning value
task::await_resume
result = 3 (should be 3)

Actual output:
----
task initial_suspend
task initial_suspend
task::await_ready
task::await_suspend
task yielding/returning value
task::await_ready
task::await_suspend
task resumed
task yielding/returning value
task::await_resume
task::await_resume
result = 4 (should be 3)


Note that the output indicates the the compiler is only calling await_resume()
on both awaiters immediately before evaluating the plus-expression rather than
evaluating await_resume() on the first awaiter immediately upon resuming from
the first suspend-point and before evaluating await_ready() for the second
awaiter.