There is a fundamental contradiction embedded in Canadian criminal law. Even before the Charter, a confession was supposed to be excluded from evidence unless the Crown could prove it was voluntary. And, at least in theory, a statement was only voluntary if it was made without any threat or inducement.
However, the dirty truth is that the whole criminal justice system is premised on getting admissions of guilt in return for stuff. The vast majority of charges never go to trial, and the only reason is because a guilty verdict after trial is worse news than a plea. If it weren't for this theoretically illegitimate expected risk/reward structure, the system would collapse.
So criminal law doctrine prohibits precisely what the crime control system is structured around.
In R. v. Spencer, the Supreme Court had to deal with this contradiction in the context of a confession provided by the accused with the hope of getting someone else out of trouble. Spencer and (perhaps) his lady friend were allegedly involved in a number of robberies on the West Coast. Spencer said he would confess if his inamorata was spared. The investigating officer promised nothing. Spencer confessed anyway. The trial judge let the confession in, resulting in a conviction. This was overturned on intermediate appeal.
The majority comes to the sensible conclusion that the confession should be allowed. I can't say they improve the doctrinal muddle, though.
Case Comment for R. v. Spencer, 2006 SCC 11
Photo credit Phillip Landreville, Supreme Court of Canada collection