What is solicitor-client privilege and what is the current state of the law on the issue?

In Descoteaux et al v. Mierzwinski and Attorney General of Quebec et al (1982), 1982 CanLII 22 (SCC), 141 D.L.R. (3d) 590, the court discussed the nature of solicitor-client privilege in the following language at p. 601: There is no denying that a person has a right to communicate with a legal adviser in all confidence, a right that it "founded upon the unique relationship of solicitor and client" (Solosky, ((1979), 1979 CanLII 9 (SCC), 50 C.C.C. (2d) 495; 105 D.L.R. (3d) 745; [1980] 1 S.C.R. 821) ...). It is a personal and extra-patrimonial right which follows a citizen throughout his dealings with others. Like other personal, extra-patrimonial rights, it gives rise to preventive and curative remedies provided for by law, depending on the nature of the agression (sic) threatening it or of which it was the object. Thus a lawyer who communicates a confidential communication to others without his client's authorization could be sued by his client for damages; or a third party who had accidentally seen the contents of a lawyer's file could be prohibited by injunction from disclosing them. And further at p. 603: The substantive rule Although the right to confidentiality first took the form of a rule of evidence, it is now recognized as having a much broader scope, as can be seen from the manner in which this court dealt with the issues raised in Solosky, ... . Further at pp. 604 and 605: It would, I think be useful for us to format the substantive rule, as the judges formerly did with the rule of evidence; it could, in my view, be stated as follows: 1. The confidentiality of communications between solicitor and client may be raised in any circumstances where such communications are likely to be disclosed without the client's consent. 2. Unless the law provides otherwise, when and to the extent that the legitimate exercise of a right would interfere with another person's right to have his communications with his lawyer kept confidential, the resulting conflict should be resolved in favour of protecting the confidentiality. 3. When the law gives someone the authority to do something which, in the circumstances of the case, might interfere with that confidentiality, the decision to do so and the choice of means of exercising that authority should be determined with a view to not interfering with it except to the extent absolutely necessary in order to achieve the ends sought by the enabling legislation.