How would you suggest tests around:

    void func() {
        printEvens(someCall().stream().filter(n -> n % 2 == 0).toList());
    }

    void printEvens(List<Integer> nums) {
        nums.stream().filter(n -> n % 2 == 0).forEach(n -> System.out.println(n));
    }

I am unaware of any linter or static analyzer that would flag this.

What's more, unit tests to test the code for printEvens (there exists one) pass because they're working properly... and the unit test that calls the calling function passes because it is working properly too.

Alternatively, write the failing test for this code.

A redundant filter() isn't observable (except in execution time).

You could pick it up if you were to explicitly track whether it's being called redundantly but it'd be very hard and by the time you'd thought of doing that you'd certainly have already manually checked the code for it.

what happened to not testing implementation details?