[Bug c++/101244] New: Wrong path in coroutine returning ternary

[victor.burckel at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101244

            Bug ID: 101244
           Summary: Wrong path in coroutine returning ternary
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: victor.burckel at gmail dot com
  Target Milestone: ---

In a coroutine returning a ternary expression with co_await, the co_await part
seems to be evaluated whatever the result of the condition:
See it on godbolt, I added a check of coro.done in the resume operation to
avoid a crash in the clang part as the coroutine is voluntarily resumed one
extra time
https://godbolt.org/z/vzGKoP8d3

#include <fmt/core.h>

#include <coroutine>
#include <functional>

#ifdef __clang__
namespace std::experimental {
using std::coroutine_handle;
using std::coroutine_traits;
}  // namespace std::experimental
#endif

struct task {
  struct promise_type;
  using handle_type = std::coroutine_handle<promise_type>;

  struct promise_type {
    auto initial_suspend() noexcept { return std::suspend_always{}; }
    auto final_suspend() noexcept { return std::suspend_always{}; }
    void return_value(int v) noexcept { value = v; }
    task get_return_object() noexcept {
      return task{handle_type::from_promise(*this)};
    }
    void unhandled_exception() noexcept {}

    int value{};
  };

  task(handle_type h) : h{h} {}
  ~task() {
    if (h) {
      fmt::print("destroying coroutine\n");
      h.destroy();
      fmt::print("coroutine destroyed\n");
    }
  }

  bool await_ready() const noexcept { return false; }
  bool await_suspend(std::coroutine_handle<>) noexcept {
    h.resume();
    return true;
  }
  int await_resume() noexcept { return h.promise().value; }

  void resume() {
    if (!h.done()) h.resume();
  }

  handle_type h;
};

struct S {
  S() { fmt::print("S::S()\n"); }
  S(const S&) { fmt::print("S::S(const S&)\n"); }
  S(S&&) { fmt::print("S::S(S&&)\n"); }
  ~S() { fmt::print("S::~S()\n"); }
};

task g(S) {
  fmt::print("in g()\n");
  co_return 1;
}

task f(bool b) {
  fmt::print("calling g()\n");
  co_return b ? co_await g(S{}) : 2;
}

int main() {
  auto task = f(false);
  fmt::print("resuming f\n");
  task.resume();
  fmt::print("resuming f\n");
  task.resume();
}

Gcc outputs
resuming f
calling g()
S::S()
S::S(S&&)
in g()
resuming f
destroying coroutine
S::~S()
coroutine destroyed
S::~S()
destroying coroutine
coroutine destroyed

While clang outputs
resuming f
calling g()
resuming f
destroying coroutine
coroutine destroyed