Mac & Rails でreadlineなエラーが起きた時に対応したこと

ある日のこと、rspecを走らせたらエラーが発生

siukaido@siukaidos-MBA:~/sample$ bundle exec rspec
    Sorry, you can't use byebug without Readline. To solve this, you need to
    rebuild Ruby with Readline support. If using Ubuntu, try `sudo apt-get
    install libreadline-dev` and then reinstall your Ruby.

readline のバージョンが古いのかなぁと思い、brew upgrade をためしてみる

siukaido@siukaidos-MBA:~$ brew upgrade readline
Error: readline 7.0.3_1 already installed

・・・が、最新バージョンだった。
んー、どういうことだろうと思って info で見てみる。

siukaido@siukaidos-MBA:~$ brew info readline
readline: stable 7.0.3 (bottled) [keg-only]
Library for command-line editing
https://tiswww.case.edu/php/chet/readline/rltop.html
/usr/local/Cellar/readline/6.3.8 (46 files, 2.0MB)
  Poured from bottle on 2015-09-11 at 23:22:41
/usr/local/Cellar/readline/7.0.1 (46 files, 2MB)
  Poured from bottle on 2017-01-20 at 15:05:12
/usr/local/Cellar/readline/7.0.3_1 (46 files, 1.5MB) *
  Poured from bottle on 2017-03-22 at 11:26:03
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/readline.rb
==> Caveats
This formula is keg-only, which means it was not symlinked into /usr/local.

macOS provides the BSD libedit library, which shadows libreadline.
In order to prevent conflicts when programs look for libreadline we are
defaulting this GNU Readline installation to keg-only.


For compilers to find this software you may need to set:
    LDFLAGS:  -L/usr/local/opt/readline/lib
    CPPFLAGS: -I/usr/local/opt/readline/include

いくつかバージョンがあったので、とりあえず 6.3.8 にダウングレード。

siukaido@siukaidos-MBA:~$ brew switch readline 6.3.8
Cleaning /usr/local/Cellar/readline/6.3.8
Cleaning /usr/local/Cellar/readline/7.0.1
Cleaning /usr/local/Cellar/readline/7.0.3_1
Opt link created for /usr/local/Cellar/readline/6.3.8

したっけ、うまく動きましたとさ。
（ちなみに 7.0.1 だとダメでした）