This Vala code:

async void go_async() throws Error {
    xyzzy.notify["b"].disconnect(on_b);
}

results in invalid C:

test.vala: In function ‘go_async_co’:
test.vala:12:69: error: ‘_tmp2_’ undeclared (first use in this function)
     xyzzy.notify["b"].disconnect(on_b);

Looking at the generated C code:

_data_->_tmp2_ = 0U;
g_signal_parse_name ("notify::b", G_TYPE_OBJECT, &_data_->_tmp1_, &_tmp2_, TRUE);

The address is being taken of _tmp2_ when it should be _data_->_tmp2_.

I'll attach the test case as a separate file.