[Bug c++/95111] New: coroutines use-after-free with lambdas

[avi@cloudius-systems.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95111

            Bug ID: 95111
           Summary: coroutines use-after-free with lambdas
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: avi@cloudius-systems.com
  Target Milestone: ---

coroutines copy their input to the coroutine frame, so a coroutine like

    future<T> f(T x) {
        co_await something();
        co_return x;
    }


will copy x back from the coroutine frame. However, lambdas are passed by
pointer, so something like


    [x] () -> future<T> {
        co_await something();
        co_return x;
    }

will fail, it is translated as something like


    struct lambda {
        T x;
    }

    future<T> lambda_operator_parens(const lambda* l) {
        co_await something();
        co_return l->x;
    }

Since l is captured by value, *l is dangling and is leaked.


I think the following translation would be more useful:


    future<T> lambda_operator_parens_rref(const lambda l) {
        co_await something();
        co_return l.x;
    }

l would be copied by value and would survive copying/moving into the coroutine
frame.

I don't know if the current behavior is mandated by the standard, but if it is,
it seems a serious defect.