[Bug c++/113773] New: coroutines: promise deconstructed twice if throwing from return object

[Bug c++/113773] New: coroutines: promise deconstructed twice if throwing from return object

[znhihgiasy at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113773

            Bug ID: 113773
           Summary: coroutines: promise deconstructed twice if throwing
                    from return object
           Product: gcc
           Version: 12.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: znhihgiasy at gmail dot com
  Target Milestone: ---

The bug is about C++ coroutines. When I throw exceptions inside the return
object's type conversion operator, the promise (and maybe the coroutine state)
is deconstructed twice. The C++ standard is not clear about when the conversion
operator is called, and what happens when it throws. However clang's behavior
is error-free and intuitive to me: The promise is deconstructed after
continuing from the final suspend point and the exception is propagated to the
call site of resume().

Here is a minimum example:

#include <cassert>
#include <coroutine>
#include <iostream>

struct result {
  operator int() {
    throw 42;
  }
};

class promise {
public:
  result get_return_object() { return {}; }
  std::suspend_never initial_suspend() { std::cout << "initial suspend" <<
std::endl; return {}; }
  void unhandled_exception() { std::cout << "unhandled exception" << std::endl;
}
  std::suspend_never final_suspend() noexcept { std::cout << "final suspend" <<
std::endl; return {}; }
  void return_void() {}
  ~promise() { std::cout << "~promise()" << std::endl; }
};

template <class... Args>
struct std::coroutine_traits<int, Args...> {
  using promise_type = promise;
};

int f() {
  co_return;
}

int main() {
  try {
    f();
  }
  catch (int i) {
    assert(i == 42);
    std::cout << "caught 42" << std::endl;
  }
  return 0;
}

Compiling with `g++ --std=c++20 -g -O0 -Wall -Wextra gccbug.cpp` produced this
output:

initial suspend
final suspend
~promise()
~promise()
free(): double free detected in tcache 2

I am running G++ version 12.3.0 on x86_64 NixOS unstable with kernel 6.6.8.
The behaviour is identical on gcc trunk.