Learning something related to the company project is not a side project but it directs to be a main part for the project. It is the key role for the project and it may or may not be allowed depending on company type, size and its policy.
Developer working on their own project for some company hours is strictly disallowed by any company.
Working on something cool or developers own projects are so called side projects and it must be accomplished at company off hours (in home). It is encouraged for any developers.
It would only be good idea if and only if company provides the environment for learning something new for the developers. It would benefit both parties, employers and employees.