What is the test for disclosing the work product of counsel to a witness?

McEachern C.J. cautioned in Delgamuukw v. B.C., supra at p. 78 that: Generally speaking, I accept that documents and communications which may relate to the substance of the evidence or to the credibility of the witness must be disclosed when he enters the witness box. In this connection credibility must be given a limited or narrow construction because almost anything might relate to credibility and this aspect of the matter must not be an open door to free‑roaming cross‑examination. What the American cases call the "work product of counsel" should be protected, even if it has been conveyed to the witness, unless it appears that it is likely to affect the evidence or the credibility of the witness.