The bindings are marked to return a IOError. The original function returns just an entry from the GIOError enum, so Vala generates code that tries to assign an enum to a GError which of course fails:

int main() {
    var f = IOError.from_errno (-12);
    return 0;
}

gint _vala_main (void) {
        gint result = 0;
        GError* _tmp0_ = NULL;
        GError* _tmp1_;
        GError* f;
        _tmp0_ = g_io_error_from_errno (-12);
        _tmp1_ = _g_error_copy0 (_tmp0_);
        f = _tmp1_;
        result = 0;
        _g_error_free0 (f);
        return result;
}