What is the legal test for “reading in”?

In Schachter v. Canada, 1992 CanLII 74 (SCC), [1992] 2 S.C.R. 679, Lamer C.J.O. described the limited circumstances in which it is appropriate to apply the constitutional remedy of “reading in”. He stated, at p.682: The purpose of reading in is to be as faithful as possible within the requirements of the Constitution to the scheme enacted by the legislature. In some cases, of course, it will not be a safe assumption that the legislature would have enacted the constitutionally permissible part of its enactment without the impermissible part. There reading in would not be appropriate. Just as reading in is sometimes required in order to respect the purposes of the legislature, it is also sometimes required in order to respect the purposes of the Charter. Reading in therefore is a legitimate remedy akin to severance and should be available under s. 52 in cases where it is an appropriate technique to fulfill the purposes of the Charter and at the same time minimize the interference of the court with the parts of legislation that do not themselves violate the Charter.