Can statements made by a defense alibi witness be discovered by the prosecution?

In People v. Thornton, supra, 88 Cal.App.3d 795, 152 Cal.Rptr. 77, the issue was what statements, if any, made to defense investigators by defense alibi witnesses can be discovered by the prosecution. Division Three of this court concluded that such discovery constitutes an unconstitutional violation of a defendant's privilege against self-incrimination even if the statements only impeach the testimony of the alibi witnesses and do not inculpate the defendant. The court further held, however, that the error was not reversible. One justice concurred that the judgment should be affirmed, but disagreed with the majority holding that the discovery permitted by the trial court violated the defendant's privilege against self-incrimination.