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.