It's questionable I'll grant you. You're saying that because you can see the implementation of filter and see that it does nothing special then it isn't worth a test. I'm saying that if I couldn't see the implementation of filter and didn't know how it worked then I'd want a test for it. I'd much rather err on the side of a more comprehensive test suite for new APIs that we add that verifies all the guarantees we claim about them since it protects us from breaking add-ons in the future should we change the implementation of the API later.