It is the Pope's approval of the Universal Church's veneration of a saint which is protected from error by the Holy Ghost, regardless of the formal or informal processes used.

Ladislaus, are you going to respond to my question of the different levels of infallibility or infallible authority or infallible assent (however one wants to phrase it)?

I already anticipated and addressed the issue of canonizations which could be considered dogmatic facts (eg., biblical saints, or saints such as St Jerome):

Rare though such saints be, their canonizations are infallible not because the pope canonized them, but because of the infallibility of scripture and tradition.

Conversely, the great majority of saints (and all recently canonized) have absolutely no basis in scripture or tradition (ie., the faith) and consequently, there could never be an obligation to assent to their sanctity (even though I believe the traditional saints are in fact saints), because they, not being part of the deposit of faith, could by definition never be de fide.

Fides ecclesiastica is a classification of those Roman Catholic dogmas which are Church teachings, definitively decided on by the Magisterium, but not as being Divine revelations properly speaking.  They are considered infallible and irrevocable because, although they are not "truths of faith" (De Fide), they are nevertheless "closely related to them".

If infallible declarations must pertain to faith (ie., scripture and tradition) and morals, by definition, we are speaking of revelation!

The idea that a Catholic could be bound to beliefs having no basis in scripture or tradition is so revolutionary and bizarre that I do not believe I have ever heard even the most extreme sedevacantist advance the idea.

If infallible declarations must pertain to faith (ie., scripture and tradition) and morals, by definition, we are speaking of revelation!

The initial point that needs to be made in this discussion is that the infallibility of canonisations is not taught by the magisterium of the Church. Belief in their infallibility is not therefore required of Catholics. This point is agreed on by theologians, as can be illustrated by the teaching of a standard manual of theology; van Noort, Castelot and Murphy's Dogmatic Theology vol. II: Christ's Church (Cork: Mercier Press, 1958). These authors follow the traditional and very important practice of attaching a theological note to every thesis that they advance. These notes specify the degree of authority possessed by each thesis, and the corresponding obligation to believe that is laid upon Catholics. The highest note is 'de fide': it belongs to propositions that must be believed with the assent of theological faith, and that cannot be knowingly and pertinaciously rejected without committing the sin of heresy. The lowest note is 'sententia communis', which, as Ludwig Ott states, means 'doctrine which in itself belongs to the field of free opinions, but which is accepted by theologians generally' (Ludwig Ott, Fundamentals of Catholic Dogma, 6th ed. (St. Louis, Mo.: Herder, 1964), p. 10).

Van Noort, Castelot, and Murphy specify that the canonisations in question are the final and definitive decrees by which the supreme pontiff declares that someone has been admitted to heaven and is to be venerated by everyone. The decree of authority that they attribute to the claim that such canonisations are infallible is 'sententia communis', the common opinion of theologians (van Noort, Castelot and Murphy, p. 117). Their evaluation of the authority of this claim is the more significant because they themselves agree with the assertion that such canonisations are infallible. There can thus be no intention on their part of minimising the authority of a claim with which they disagree. The assertion that canonisations are infallible thus belongs to the field of free opinions. It is not one that Catholics have an obligation to accept.