Outside of Facebook, by default:

• the typechecker is automatically ran on page requests
• a typechecker error is fatal

This means that this code can not be ran if there are any references to undefined classes in the tree, including classes that are generated by codegen, so, it seems that:

• codegen files should be committed to the repo
• codegen files need to be created before any references to them are added

By comparison, https://github.com/hhvm/user-documentation approach (which has a much less clean implementation than this) is:

• manually written .hhi files are committed to the repo, and do not use hack-specific features
• PHP code is generated as a build step