In the case of a saint who lived after the close of revelation, there is no question of the person's sanctity being a part of the deposit of faith, so the canonization cannot be the kind of act of the infallible magisterium spoken of by Vatican I.
"The secondary object of infallibility is truths of the Christian teaching on faith and morals, which are not formally revealed, but which are closely connected with the teaching of revelation. (Sent. certa.)
"This doctrine if a necessary consequence of the doctrine of infallibility which has the purpose of preserving and of truly interpreting the deposit of holy faith. The Church could not achieve this purpose if she could not infallibly decide regarding doctrines and acts which are intimately linked with revelation. She may exercise her power in these matters either positively or negatively by the rejection of error opposed to the truth.
"To the secondary object of infallibility belong: α) Theological conclusions derived from a formally revealed truth by aid of natural truth of reason. β) Historical facts on the determination of which the certainty of a truth of revelation depends (facta dogmatica). γ) Natural truths of reason which are intimately connected with revelation. δ) The canonization of saints, that is, the final judgment that a member of the Church has been assumed into eternal bliss and may be the object of general veneration. The veneration shown to the saints is, as St. Thomas teachers, "to a certain extent a confession of the faith, in which we believe in the glory of the saints" (Quodl. 9, 16). If the Church could err in her opinion, consequences would arise which would be incompatible with the sanctity of the Church." (Ott, Fundamentals of Catholic Dogma
, p. 299)
What Ott states was confirmed by the Holy Roman and Universal Inquisition:
"The second proposition of the Professio fidei
states: 'I also firmly accept and hold each and everything definitively proposed by the Church regarding teaching on faith and morals.' The object taught by this formula includes all those teachings belonging to the dogmatic or moral area, which are necessary for faithfully keeping and expounding the deposit of faith, even if they have not been proposed by the Magisterium of the Church as formally revealed.
"Such doctrines can be defined solemnly by the Roman Pontiff when he speaks 'ex cathedra' or by the College of Bishops gathered in council, or they can be taught infallibly by the ordinary and universal Magisterium of the Church as a 'sententia definitive tenenda. Every believer, therefore, is required to give firm and definitive assent to these truths, based on faith in the Holy Spirit's assistance to the Church's Magisterium, and on the Catholic doctrine of the infallibility of the Magisterium in these matters. Whoever denies these truths would be in a position of rejecting a truth of Catholic doctrine and would therefore no longer be in full communion with the Catholic Church.
"The truths belonging to this second paragraph can be of various natures, thus giving different qualities to their relationship with revelation. There are truths which are necessarily connected with revelation by virtue of an historical relationship; while other truths evince a logical connection that expresses a stage in the maturation of understanding of revelation which the Church is called to undertake. The fact that these doctrines may not be proposed as formally revealed, insofar as they add to the data of faith elements that are not revealed or which are not yet expressly recognized as such, in no way diminishes their definitive character, which is required at least by their intrinsic connection with revealed truth. Moreover, it cannot be excluded that at a certain point in dogmatic development, the understanding of the realities and the words of the deposit of faith can progress in the life of the Church, and the Magisterium may proclaim some of these doctrines as also dogmas of divine and catholic faith.
[. . .]
"With regard to those truths connected to revelation by historical necessity and which are to be held definitively, but are not able to be declared as divinely revealed, the following examples can be given: the legitimacy of the election of the Supreme Pontiff or of the celebration of an ecumenical council, the canonizations of saints (dogmatic facts), the declaration of Pope Leo XIII in the Apostolic Letter Apostolicae Curae
on the invalidity of Anglican ordinations." (Congregation for the Doctrine of the Faith, Doctrinal Commentary on the Concluding Formula of the Professio Fidei
, 6-7, 11)