I've got a class with an async function where the generated C code completely omits the final GError** argument in the corresponding _finish function (so gcc fails with an error).

I'm using other async functions without a problem, and nothing about the vapi, etc. suggest any sort of problems.

================================
Vapi segment
================================

        public class AccountManager : TelepathyGLib.Proxy { ... }

        [CCode (cname = "TpProxy", cheader_filename = "telepathy-glib/telepathy-glib.h")]
        public class Proxy : GLib.Object {
                public weak GLib.Object parent;
                public TelepathyGLib.ProxyPrivate priv;
                ...
                public async void prepare_async ([CCode (array_length = false)] GLib.Quark[]? features);
                public bool prepare_finish (GLib.AsyncResult result) throws GLib.Error;
        }

================================
Vala segment
================================

  private async void setup_account_manager () 
    {
      this.account_manager = AccountManager.dup ();
      
      try 
        {
          yield this.account_manager.prepare_async (null);
        }
      catch (GLib.Error e)
        { 
          warning ("failed to prepare account manager: %s", e.message);
        }
      ...
    }

================================
generated C code
================================

struct _FolksBackendsTpBackendSetupAccountManagerData {
  int _state_;
  GAsyncResult* _res_;
  GSimpleAsyncResult* _async_result;
  FolksBackendsTpBackend* self;
  TpAccountManager* _tmp0_;
  GError * e;
  GList* accounts;
  GList* account_collection;
  GList* account_it;
  TpAccount* account;
  GError * _inner_error_;
};

...

  _state_0:
  {
    data->self->priv->account_manager = (data->_tmp0_ = tp_account_manager_dup (), _g_object_unref0 (data->self->priv->account_manager), data->_tmp0_);
    {
      data->_state_ = 1;
      tp_proxy_prepare_async ((TpProxy*) data->self->priv->account_manager, NULL, folks_backends_tp_backend_setup_account_manager_ready, data);
      return FALSE;
      _state_1:
      tp_proxy_prepare_finish ((TpProxy*) data->self->priv->account_manager, data->_res_);
    }
    goto __finally0;
    __catch0_g_error:
    {
      data->e = data->_inner_error_;
      data->_inner_error_ = NULL;
      {
        g_warning ("tp-backend.vala:57: failed to prepare account manager: %s", data->e->message);
        _g_error_free0 (data->e);
      }
    }
    __finally0:
    if (data->_inner_error_ != NULL) {
      g_critical ("file %s: line %d: uncaught error: %s (%s, %d)", __FILE__, __LINE__, data->_inner_error_->message, g_quark_to_string (data->_inner_error_->domain), data->_inner_error_->code);
      g_clear_error (&data->_inner_error_);
      return FALSE;
    }


=================================

Note the tp_proxy_prepare_finish() line above is missing the GError ** argument entirely (causing a compiler error); yet all the other references to data->_inner_error_ seem as they should be.

=================================

This blocks the Vala bindings for telepathy glib (fdo #27792)