With the recent addition of #pragma dynamic_branch it is now possible to use keywords in runtime branching style code in addition to the preprocessor code selection.
If a shader has multiple subshaders/passes with shared code, it is easy to forget to declare all the keywords that are required for the shared code. The current errors given in this situation do not really help the user. All that we tell is "undeclared identifier 'FOOBAR'". It would be much better if we could tell the user right away that they're missing the keyword declaration in this particular pass.