dev-ruby/bundlerのemergeに失敗する。

rubyのアップデート中、dev-ruby/bundlerのアップデートに失敗しました。

/etc/portage # emerge -u dev-ruby/bundler

Calculating dependencies... done!

>>> Verifying ebuild manifests

>>> Emerging (1 of 1) dev-ruby/bundler-2.1.4::gentoo
 * bundler-2.1.4.gem BLAKE2B SHA512 size ;-) ...                                                              [ ok ]
>>> Unpacking source...
 * Running unpack phase for all ...
 * Unpacking .gem file... ...                                                                                 [ ok ]
 * Uncompressing metadata ...                                                                                 [ ok ]
 * Unpacking data.tar.gz ...                                                                                  [ ok ]
>>> Source unpacked in /var/tmp/portage/dev-ruby/bundler-2.1.4/work
>>> Preparing source in /var/tmp/portage/dev-ruby/bundler-2.1.4/work ...
 * Running prepare phase for all ...
 * Running prepare phase for all ...
 * Running source copy phase for ruby25 ...
 * Running source copy phase for ruby26 ...
>>> Source prepared.
>>> Configuring source in /var/tmp/portage/dev-ruby/bundler-2.1.4/work ...
>>> Source configured.
>>> Compiling source in /var/tmp/portage/dev-ruby/bundler-2.1.4/work ...
 * Running compile phase for all ...
Traceback (most recent call last):
        2: from /usr/local/bin/rdoc:23:in `<main>'
        1: from /usr/lib64/ruby/site_ruby/2.5.0/rubygems.rb:302:in `activate_bin_path'
/usr/lib64/ruby/site_ruby/2.5.0/rubygems.rb:283:in `find_spec_for_exe': can't find gem rdoc (>= 0.a) with executable rdoc (Gem::GemNotFoundException)
 * ERROR: dev-ruby/bundler-2.1.4::gentoo failed (compile phase):
 *   failed to (re)build documentation
 *
 * Call stack:
 *     ebuild.sh, line  125:  Called src_compile
 *   environment, line 2041:  Called ruby-ng_src_compile
 *   environment, line 1681:  Called _ruby_invoke_environment 'all' 'all_ruby_compile'
 *   environment, line  615:  Called all_ruby_compile
 *   environment, line  699:  Called all_fakegem_compile
 *   environment, line  662:  Called die
 * The specific snippet of code:
 *                   rdoc ${RUBY_FAKEGEM_DOC_SOURCES} || die "failed to (re)build documentation";
 *
 * If you need support, post the output of `emerge --info '=dev-ruby/bundler-2.1.4::gentoo'`,
 * the complete build log and the output of `emerge -pqv '=dev-ruby/bundler-2.1.4::gentoo'`.
 * The complete build log is located at '/var/tmp/portage/dev-ruby/bundler-2.1.4/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/dev-ruby/bundler-2.1.4/temp/environment'.
 * Working directory: '/var/tmp/portage/dev-ruby/bundler-2.1.4/work/all/bundler-2.1.4'
 * S: '/var/tmp/portage/dev-ruby/bundler-2.1.4/work/all/bundler-2.1.4'

>>> Failed to emerge dev-ruby/bundler-2.1.4, Log file:

>>>  '/var/tmp/portage/dev-ruby/bundler-2.1.4/temp/build.log'

rdocが失敗しているようです。
で、「 `find_spec_for_exe': can't find gem rdoc (>= 0.a) with executable rdoc (Gem::GemNotFoundException)」は設定されているバージョンと実際の実行ファイルのバージョン違いで発生するそうです。
ebuildでそんなことあるかいな? と、思って、rdocの位置を疑いました。

# whereis rdoc
rdoc: /usr/bin/rdoc /usr/local/bin/rdoc
 # which rdoc
/usr/local/bin/rdoc

あらあら、/usr/local/binに入っているrdocを使おうとしていますわ。
よっこしてみましょう。

 # mv /usr/local/bin/rdoc /usr/local/bin/rdoc.bak

これでアップデートできました。
この/usr/local/bin/rdocをリネームすることでどのような影響がでるかは未知数です。