What is the test for severable conditions in a contract?

A full discussion of the law in respect to conditions contained in contracts, whether they are conditions precedent, or inserted for the benefit of one party's benefit alone, or whether the condition may be severable from the remainder of the contract, is to be found in Barnett v. Harrison (1976) 1975 CanLII 33 (SCC), 57 D.L.R. (3d) 225.

I can find nothing in the above decision or those other cases discussed in it that would appear to alter the principle laid down in such cases as Allsopp v. Orchard (1923) 1 Ch. 323, and Burgess v. Cox (1951) 1 Ch. 383 to the effect that before a waiver or a relinquishment of a benefit may be permitted so as to allow specific performance, there must first be established that there is in existence a contract which may be sued on.