Can an in-custody witness be considered a witness in a criminal case?

Section 1127a, on which CALJIC No. 3.20 is patterned, defines an in-custody informant as "a person, other than a codefendant, percipient witness, accomplice, or coconspirator whose testimony is based upon statements made by the defendant while both the defendant and the informant are held within a correctional institution." (Italics added.) "In-custody informant witnesses have no personal knowledge of the crime, but testify that a defendant made an inculpatory statement to them while in proximity in a county jail or state prison, often in exchange for favorable treatment by law enforcement. In-custody percipient witnesses, by contrast, like other percipient witnesses, codefendants, accomplices, and coconspirators, testify on the basis of personal knowledge of the crime." (People v. Bivert (2011) 52 Cal.4th 96, 121.)