Is it possible to override a "services" function from module code?

If the app developer loses control of which class to use within a service because it can be overridden by any third-party modules that might be included via Composer, then it opens the application up to harmful attacks.

However, if the app-level Services config file takes precedence, then the app developer can always be sure they're specifying the correct class.

Extending a core class not a problem at all. The app developer can then choose to use that class by extending the App-level Services config. Your question was if the services config file methods could be overridden, if I understood correctly. And that, to me, is dangerous.

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