non-const reference for function output parameter has been recommended by Google C++ style.

[MoFHeka]

According to https://git.code.oa.com/standards/cpp/issues/71
Warning 'Is this a non-const reference? If so, make const or use a pointer' may be changed.

[matyalatte]

Thanks for the proposal. Yeah, it would be better to fix.
I suppressed that kind of warnings to see other warnings but I forgot to re-enable it.
https://github.com/matyalatte/cpplint-cpp/blob/aacdd98a76c4e917d39cb31ae1e18d0b822003af/CPPLINT.cfg#L17C1-L17C27