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Strongly discouraged.  If there is something specific you need, inject that.  ExecOperations, FileSystemOperations, ProjectLayout, etc.

tbh not even sure if that works - Gradle checks a bunch of stuff and prevents injection of invalid types.

I don't think it does. As he explicitly provides the instance to give to the constructor, any type should work.

But typically an extension should never need a project instance. So it depends on what you do with that project instance, but most probably it is bad practice, yes.