Since the Pope gave his own answer to this question on Sunday, there has been a lot of discussion about the exact situations under which it is possible for protestant Christians to request communion from a Catholic minister. I thought I would bring together in one place the major magisterial sources in answer to this question.
First, Pope Saint John Paul II in Eucharistia in Ecclesia 45:
“45. While it is never legitimate to concelebrate in the absence of full communion, the same is not true with respect to the administration of the Eucharist under special circumstances, to individual persons belonging to Churches or Ecclesial Communities not in full communion with the Catholic Church. In this case, in fact, the intention is to meet a grave spiritual need for the eternal salvation of an individual believer, not to bring about an intercommunion which remains impossible until the visible bonds of ecclesial communion are fully re-established.”
Second, John Paul II again from Ut Unum Sint 45-46:
“45… These are signs of convergence which regard various aspects of the sacramental life. Certainly, due to disagreements in matters of faith, it is not yet possible to celebrate together the same Eucharistic Liturgy. And yet we do have a burning desire to join in celebrating the one Eucharist of the Lord, and this desire itself is already a common prayer of praise, a single supplication. Together we speak to the Father and increasingly we do so “with one heart”. At times it seems that we are closer to being able finally to seal this “real although not yet full” communion. A century ago who could even have imagined such a thing?
“46. In this context, it is a source of joy to note that Catholic ministers are able, in certain particular cases, to administer the Sacraments of the Eucharist, Penance and Anointing of the Sick to Christians who are not in full communion with the Catholic Church but who greatly desire to receive these sacraments, freely request them and manifest the faith which the Catholic Church professes with regard to these sacraments. Conversely, in specific cases and in particular circumstances, Catholics too can request these same sacraments from ministers of Churches in which these sacraments are valid. The conditions for such reciprocal reception have been laid down in specific norms; for the sake of furthering ecumenism these norms must be respected.”
That final paragraph is footnoted to, among other sources, Canon Law 844.4. which says:
“If the danger of death is present or if, in the judgement of the diocesan bishop or conference of bishops, some other grave necessity urges it, Catholic ministers administer these same sacraments licitly also to other Christians not having full communion with the Catholic Church, who cannot approach a minister of their own community and who seek such on their own accord, provided that they manifest Catholic faith in respect to these sacraments and are properly disposed.”
The danger of death is a condition quite specifically mentioned here, but John Paul II does not explicitly mention it, and neither does Pope Benedict in Sacaramentum Caritatis 56:
“56. The subject of participation in the Eucharist inevitably raises the question of Christians belonging to Churches or Ecclesial Communities not in full communion with the Catholic Church. In this regard, it must be said that the intrinsic link between the Eucharist and the Church’s unity inspires us to long for the day when we will be able to celebrate the Holy Eucharist together with all believers in Christ, and in this way to express visibly the fullness of unity that Christ willed for his disciples (cf. Jn 17:21). On the other hand, the respect we owe to the sacrament of Christ’s Body and Blood prevents us from making it a mere “means” to be used indiscriminately in order to attain that unity. (172) The Eucharist in fact not only manifests our personal communion with Jesus Christ, but also implies full communio with the Church. This is the reason why, sadly albeit not without hope, we ask Christians who are not Catholic to understand and respect our conviction, which is grounded in the Bible and Tradition. We hold that eucharistic communion and ecclesial communion are so linked as to make it generally (my emphasis) impossible for non-Catholic Christians to receive the former without enjoying the latter. There would be even less sense in actually concelebrating with ministers of Churches or ecclesial communities not in full communion with the Catholic Church. Yet it remains true that, for the sake of their eternal salvation, individual non-Catholic Christians can be admitted to the Eucharist, the sacrament of Reconciliation and the Anointing of the Sick. But this is possible only in specific, exceptional situations and requires that certain precisely defined conditions be met (173). These are clearly indicated in the Catechism of the Catholic Church (174) and in its Compendium (175). Everyone is obliged to observe these norms faithfully.”
Footnote 173 is to Canon Law 844.3-4. The Catechism reference is to 1398 – 1401.
“1401. When, in the ordinary’s judgement, a grave necessity arises, Catholic ministers may give the sacraments of Eucharist, Penance, and Anointing of the Sick to other Christians not in full communion with the Catholic Church, who ask for them of their own will, provided they give evidence of holding the Catholic faith regarding the sacraments and possess the required dispositions.”
Again the reference is to Canon Law 844.4. The Compendium reference is to 293:
“293. Catholic ministers make give holy communion licitly to members of the Oriental churches which are not in full communion with the Catholic Church whenever they ask for it of their own will and possess the required dispositions. Catholic ministers made licitly give holy communion to members of other ecclesial communities only if, in grave necessity, they ask for it of their own will, possess the required dispositions, and give evidence of holding the Catholic faith regarding the sacrament.”
No footnote is given (that’s not the Compendium’s style), but you notice that “grave necessity” is not defined.
So, there you have it. As far as I know, those are the major places in the Catholic Magisterium where the topic of communion for Christians not in communion with the Holy See are addressed.