Ruby

I was expecting the behavior in 2.5. That's what is correct, right?

I tested with matrix gem, and 2.4 behaves like 2.5, which is to say it loads the gem only if gem 'matrix' is called. Note that 2.4 didn't ship with default matrix gem (it was std lib), but it did ship with defaut bigdecimal gem.

I also realized that it's not possible to load the 0.1.0 matrix gem in 2.6.0 because it shipped with that version of the local gem. That in itself is a problem, as the public gem matrix v0.1.0 is actually older than the local gem that has shipped with Ruby 2.6.0. I will release v1.0.0 to avoid this issue, and I will have to be careful when mirroring the gem.

I think the behavior of 2.5 can be thought of a bug because the behavior for .so file isn't consistent with one for .rb file.
You can see the inconsistency in the results below.

$ ruby -rbigdecimal/util -ve 'p $LOADED_FEATURES.grep(/bigdecimal/)' ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-linux] ["/usr/local/lib/ruby/gems/2.5.0/gems/bigdecimal-1.4.2/lib/bigdecimal.so", "/usr/local/lib/ruby/gems/2.5.0/gems/bigdecimal-1.4.2/lib/bigdecimal.rb", "/usr/local/lib/ruby/gems/2.5.0/gems/bigdecimal-1.4.2/lib/bigdecimal/util.so", "/usr/local/lib/ruby/gems/2.5.0/gems/bigdecimal-1.4.2/lib/bigdecimal/util.rb"] 

In this case, bigdecimal/util is required, and the gem-installed version of bigdecimal/util.rb is loaded.
You can see the gem-installed version of bigdecimal.so is also loaded.

The behavior is changed when bigdecimal is required before bigdecimal/util.

$ ruby -rbigdecimal -rbigdecimal/util -ve 'p $LOADED_FEATURES.grep(/bigdecimal/)' ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-linux] /usr/local/lib/ruby/gems/2.5.0/gems/bigdecimal-1.4.2/lib/bigdecimal.rb:7: warning: method redefined; discarding old new Traceback (most recent call last):	5: from /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'	4: from /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'	3: from /usr/local/lib/ruby/gems/2.5.0/gems/bigdecimal-1.4.2/lib/bigdecimal/util.rb:9:in `<top (required)>'	2: from /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:34:in `require'	1: from /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:130:in `rescue in require' /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:130:in `require': /usr/local/lib/ruby/gems/2.5.0/gems/bigdecimal-1.4.2/lib/bigdecimal/util.so: undefined symbol: rmpd_util_str_to_d - /usr/local/lib/ruby/gems/2.5.0/gems/bigdecimal-1.4.2/lib/bigdecimal/util.so (LoadError) 

In this case, LoadError is occurred by the symbol resolution failure of bigdecimal/util.so. This file is newly introduced since bigdecimal-1.4.0.
Although bigdecimal.so in bigdecimal-1.4.2 has rmpd_util_str_to_d, it isn't loaded because the default-gem version of bigdecimal.so is loaded by -rbigdecimal option.

The difference between the default-gem and gem-installed versions is whether bigdecimal.rb exists. Although I've not investigated the implementation of Ruby 2.5's require` method yet, I consider that Ruby 2.5 won't search in the gem-installed version if there is .so file but there isn't .rb file.

I could reproduce them. But I also reproduce it with Ruby 2.5.3 with RubyGems 2.6.14.3 bundled with Ruby 2.4.

~ > ruby -v ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-darwin18] ~ > gem -v 2.6.14.3 ~ > gem list bigdecimal *** LOCAL GEMS *** bigdecimal (1.4.3, default: 1.3.4) ~ > ruby -e "require 'bigdecimal'; p \$LOADED_FEATURES.grep(/bigdecimal/).first" "/Users/hsbt/.rbenv/versions/2.5.3/lib/ruby/2.5.0/x86_64-darwin18/bigdecimal.bundle" 

It seems this issue produced by Ruby core without RubyGems maybe.

@mrkn (Kenta Murata)

Can you try with Ruby 2.5 applied r66867?

@hsbt (Hiroshi SHIBATA)
I've confirmed that Ruby 2.5 with r66867 loads bigdecimal installed by gem, correctly.