[Bug c++/101133] New: co_await doesn't accept a valid awaitable object if await_resume()'s return type is not a built-in type.

[liuyaoxin1976 at qq dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101133

            Bug ID: 101133
           Summary: co_await doesn't accept a valid awaitable object if
                    await_resume()'s return type is not a built-in type.
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: liuyaoxin1976 at qq dot com
  Target Milestone: ---

#include <coroutine>
#include <string>

template<typename T>
struct Awaiter
{
    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>{};
    auto a2 = Awaiter<std::string>{};

    [[maybe_unused]] auto v1 = co_await a1; // ok
    [[maybe_unused]] auto v2 = co_await a2; // error
}

The error message is: no suspend point info for ''co_await' not supported by
dump_decl<declaration error>'

See also: https://godbolt.org/z/4xdYreb9f