There are a few classes that it is generally a bad idea to mock: trying to mock them is usually a code smell indicating that a test is not testing the correct part of the application, and mocking them usually leads to problems.

Some of these are:

• System
• Collections (particularly HashMap)
• Data classes, and in general value objects
• Classes that deal with I/O, like File or Path (you should be wrapping them in a layer that only deals with I/O rather than mixing it with business logic)

Mockk should at least log a warning when trying to mock one of these classes, but ideally it should also have a configuration flag that disables mocking them altogether and has it throw an exception when a user tries to.

We could maybe even make the list of non-mockable classes configurable.