Issue
#include <coroutine>
#include <string>
template<typename T>
struct Awaiter final {
bool await_ready() const { return false; }
void await_suspend(std::coroutine_handle<>) const {}
T await_resume() const { return T{}; }
};
struct ReturnObject {
struct promise_type {
ReturnObject get_return_object() { return {}; }
std::suspend_never initial_suspend()
noexcept { return {}; }
std::suspend_never final_suspend()
noexcept { return {}; }
void return_void() {}
void unhandled_exception() {}
};
};
ReturnObject f()
{
auto a1 = Awaiter<int>{};
[[maybe_unused]] auto v1 = co_await a1; // ok
auto a2 = Awaiter<std::string>{};
[[maybe_unused]] auto v2 = co_await a2; // error
}
int main() { f(); }
See: online demo
The error message:
error: no suspend point info for ''co_await' not supported
by dump_decl<declaration error>'
37 | [[maybe_unused]] auto v2 = co_await a2; // error
| ^~~~~~~~
Why can't co_await return a string?
Solution
This is a compiler bug in the implementation of coroutines in GCC, as there is nothing in the current drafts of the standard prohibiting custom/composite types for await_resume
(as can be seen when replacing string with any user-defined type).
The very same code for example compiles with the latest version of MSVC using the /std:c++latest
flag (which is no surprise, as Gor Nishanov, who prominently develops on the draft, uses Visual Studio to implement a prototype which may be the most tested implementation at the time of writing of this answer).
Answered By - Jodocus Answer Checked By - Katrina (WPSolving Volunteer)