Drop in react version not loaded from asset pipeline

[hphu3]

Steps to reproduce

1. Add a custom react.js file and/or JSXTransformer.js into your project's asset pipeline.
2. Observe the custom react version is not loaded.

Expected behavior

According to the docs the custom files should be loaded:
https://github.com/reactjs/react-rails/blob/master/VERSIONS.md#drop-in-version

Actual behavior

Files are not loaded.

System configuration

Sprockets or Webpacker version: Sprockets
React-Rails version: 2.5.0
Rect_UJS version: default
Rails version: 5.1
Ruby version: 2.3.3

---

Looking at Railtie, it seems like we read the paths from the AssetVariant class https://github.com/reactjs/react-rails/blob/master/lib/react/rails/railtie.rb#L92-L93 but the AssetVariant only looks at files in the gem root: https://github.com/reactjs/react-rails/blob/master/lib/react/rails/asset_variant.rb#L22-L23

It could also be I am misunderstanding the feature since it should work currently if I drop in custom files into the gem's directories.

[BookOfGreg]

Bug confirmed.

Reproduction starting from a basic sprockets setup:

ruby -v # ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-darwin18]
rails -v # Rails 5.2.2.1
rails new test-app
cd test-app
echo "gem 'react-rails'" >> Gemfile
bundle
rails g react:install
rails g react:component hello name:string
rails g controller hello index
echo "<%= react_component 'Hello', {name: 'you'}%>" >> app/views/hello/index.html.erb

rails s and visit http://localhost:3000/hello/index to verify.
Opening up dev console and running React.version shows 16.8.6

For me the easiest place to get other pre-built versiond of react happens to be this gem, so adding in your reproduction:

mkdir -p app/assets/vendor
curl -L https://github.com/reactjs/react-rails/raw/v2.4.7/lib/assets/react-source/production/react.js -o app/assets/vendor/react.js

2.4.7 should be 16.4.2 but...
Opening up dev console and running React.version still shows 16.8.6

[BookOfGreg]

Aha! However:

rm app/assets/javascripts/server_rendering.js

Opening up dev console and running React.version now shows 16.4.2

Maybe the server_rendering is being badly behaved. If you're not using SSR then Sprockets does let you swap out the react version at will. Still a bug but less severe.

[hphu3]

Going off of @BookOfGreg's findings, I was able to get it working by dropping in react-server.js alongside the react.js from this repo. Perhaps the documentation can be updated to reflect this.