Here is what past popes have said on the subject:
Pope Pius IX, Letter to Archbishop Scherr of Munich in 1863:
"We desire to reassure ourselves that they did not mean to limit the obligation, which strictly binds Catholic teachers and writers, to those things only which are proposed by the infallible judgment of the Church as dogmas of faith to be believed by everybody. In a like manner, We are convinced that it was not their intention to state that the perfect adherence to revealed truths (which they regard as absolutely necessary for true progress in science and for refuting errors) can be maintained, if the submission of faith is given only to those dogmas expressly defined by the Church. The reason for this is the following: even supposing that we are treating of that subjection which is to be made by an explicit act of divine faith, this must not be limited to those things which have been defined in the express decrees of the ecumenical councils or of the Roman Pontiffs of this See; but it must also be extended to those things which, through the ordinary teaching of the whole Church throughout the world, are proposed as divinely revealed and, as a result, by universal and constant consent of Catholic theologians are held to be matters of faith. "
Pope Pius XII, Humani Generis, 1950 (Denz. 2313):
“It is not to be thought that what is set down in Encyclical Letters does not demand assent in itself, because in this the popes do not exercise the supreme power of their magisterium. For these matters are taught by the ordinary magisterium, regarding which the following is pertinent: “He who heareth you, heareth me.” [Luke 10:16]; and usually what is set forth and inculcated in the Encyclical Letters, already pertains to Catholic doctrine. But if the Supreme Pontiffs in their acts, after due consideration, express an opinion on a hitherto controversial matter, it is clear to all that this matter, according to the mind and will of the same Pontiffs, cannot any longer be considered a question of free discussion among the theologians.”