Description

Right now in Spring unit tests fuzzer when initially creating mocks doesn't mock any methods. Only when it gets feedback from the concrete execution about invoked methods it does the actual mocking of these method. Although, even after that it may still choose not to mock them (e.g. if they are too deep, by default fuzzer recursion depth limit is set to 4).

To complete concrete execution when invoked method of the mock is not mocked, we create answers at runtime. For most types we use mocks as default answers, to achieve maximum coverage and make discovery of methods to mock faster.

However, that sometimes causes us to mock types that should not be normally mocked (e.g. List and Optional).

That is bad because mocks of such types can have a nonsensical behavior (e.g. we can have List that returns false when isEmpty() is called on it, while returning 0 when size() is called on it), also, sometimes mocks are unable to cover certain instructions, for example, if we mock Optional we won't be able to cover lambda that is passed to orElseGet method).