This question is an open theological question; it is not a matter of doctrine. The Magisterium has not decided the question as to whether or not canonizations are infallible. It is the opinion of the majority of theologians that canonizations done by the Holy Father enter within the limits of his infallible teaching authority. In other words, the answer to this question is a matter of theological opinion, not a doctrine or dogma. There are no magisterial documents teaching that canonizations are infallible. The ordinary non-infallible Magisterium has no such teaching. The Popes and Councils of the Church have no such teaching. Regardless of how many or which theologians opined one way or the other on this topic, it is an open question.
See my article: Papal Infallibility and the Canonization of Saints
All of the teachings of the Magisterium are truths found explicitly or implicitly in Sacred Tradition or Sacred Scripture. The Magisterium can teach from natural law, but all of the truths of natural law are also found, at least implicitly, in Tradition and Scripture. So all that the Magisterium teaches must be found in Tradition or Scripture. Now according to the Second Vatican Council, Papal Infallibility “extends as far as the deposit of Revelation extends” (Lumen Gentium, n. 25). So unless a Saint is mentioned in Divine Revelation, his or her canonization cannot be an infallible teaching under Papal Infallibility.
My theological opinion is that canonizations (other than the Saints in Sacred Scripture) cannot fall under Papal Infallibility because the assertion that a particular person is a Saint is a judgment of the prudential order, not a teaching from Divine Revelation. The process for canonization does not examine Tradition and Scripture, to see if the individual is a Saint. Rather, the process judges temporal facts and personal testimony in order to reach a conclusion about the holiness of an individual. Such a decision is inherently prudential in nature, and therefore not a dogma or doctrine of the Church.
by
Ronald L. Conte Jr.
Roman Catholic theologian and
translator of the Catholic Public Domain Version of the Bible.
the reproach of Christ 
Your comments are not clear to me. Are you saying that the Church can err, to a limited extent, about judging someone’s virtues and that, therefore, a Saint is not really a Saint ?
The Church has two types of authority:
1. Magisterium – the teaching authority, concerns doctrine
2. temporal authority – concerns discipline and judgments of the prudential order
The teachings of the Magisterium are either infallible (no possibility of error) or non-infallible (limited possibility of error). The judgments of the temporal authority are generally fallible.
Yes, the Pope can err, to a limited extent, in his non-infallible teachings, and he can err, to a greater extent, in his judgments of the prudential order.
A Saint is a particularly holy person, with virtue to a high degree. The Church can err in judging the degree of holiness, because the Church is judging based on temporal evidence and the testimony of persons. I don’t know of any particular error concerning a canonization. But the canonized Saints vary in their degree of holiness. We cannot say that it is infallibly true that all are of sufficiently high degree of holiness to be considered truly Saints.
Ok, its a fair argument. But what about the fact that in the Papal proclamation, it is said a person is a Saint is also in Heaven ? According to your argument, there is a limited possibity of error that, in fact, this person is : a) not a Saint, b) not in Heaven, or C) is a Saint, but did not live the virtues heroically, but is still in Heaven.
When the Church says that a person is a Saint, the meaning is not merely that he or she is in Heaven. So it is absurd to say both that canonizations are infallible, but all it means is the person is in Heaven. The meaning of canonization is that the person had a saintly life. If that assertion about a person is true, then they must have lived a holy life. Even a great sinner can possibly repent and get into Heaven.