In T 844/18 one of several applicants for certain US provisional applications from which priority was claimed was missing from the application leading to the patent-in-suit. No question of succession in title arose. Applying the long-established "all applicants" (or "same applicant") approach in line with the boards' case law, the opposition division held that priority had not been validly claimed and revoked the patent for lack of novelty. The board formulated the core issue thus: "A and B are applicants for the priority application. A alone is the applicant for the subsequent application. Is a priority claim valid even without any assignment of priority right from B to A?" The appellants' (patent proprietors') three lines of attack on the EPO's "all applicants" approach failed to convince the board, and the appeal was dismissed. The first question raised was whether entitlement to priority should be assessed by the EPO. The board concluded that the instances of the EPO are empowered and obliged to assess the validity of a priority right claim as required by Art. 87(1) EPC, and that this includes examining the issue of "who" concerning priority entitlement. Art. 87(1) EPC does not require that the "any person" who has filed the patent application is legally entitled to do so, merely that they have done so. The board also rejected arguments based on Art. 60(3) EPC by analogy. The EPO only carries out a formal assessment of the person who filed the application. This is, moreover, not inconsistent with its approach to succession in title, which the appellants claimed. The EPO only assesses that a successor in title is the successor in title of the original applicant, which indeed involves a substantial legal assessment but is not an assessment of legal entitlement to a priority right. 

Secondly, concerning the interpretation of "any person" in Art. 87(1) EPC, the board found the ordinary meaning of the term in this context to be ambiguous in all language versions (cf. Art. 4A(1) Paris Convention, Art. 31(1) Vienna Convention). It agreed with T 15/01 that the object and purpose of the Paris Convention (cf. Art. 31(1), 33(4) Vienna Convention) is to "safeguard, for a limited period, the interests of a patent applicant in his endeavour to obtain international protection for his invention", and that "the international priority provisions contained in the Paris Convention ... assist the applicant in obtaining international protection for his invention". The appellants relied on those statements in T 15/01 to argue that the Paris Convention's object and purpose favoured their (non-restrictive) interpretation of "any person". This would allow A alone to file another application in another country claiming the same invention without involving B, whereas the "all applicants" approach could lead to the loss of the priority right, and hence possibly of the patent application, where one of the applicants refused to join in as an applicant for the subsequent application. The board reasoned otherwise: the object and purpose of the Paris Convention cannot form the basis for favouring one or some persons to the detriment of all other persons who originally formed part of the group filing a patent application. Furthermore, an applicant can progress an application before the EPO without the active participation of the other applicants named in it. The current practice protects applicants from being "left out" and being forced to resort to international litigation to protect their rights. The board further noted that the priority provisions of the Paris Convention have remained essentially unchanged since 1883 and that there has been no EPO or national case law clearly adopting the appellants' interpretation. The bar for overturning long-established case law and practice should be very high because of the disruptive effects a change may have. The continuation of such long-standing and rationally based practices can be considered an aspect of legal certainty. 

Thirdly, with regard to the applicable law, the board held that it is not, as argued by the appellants, the national law of the place of filing of the priority application (here US law) that determines who qualifies as “any person”, but the Paris Convention, to which the US is a party and which is thus part of the "supreme Law of the Land" (Art. VI, clause 2, US Constitution). The board also drew on the travaux préparatoires of the Paris Convention from 1880 to conclude that this treaty determines who "any person" is, and that this determination is a purely formal one. The Paris Convention and the EPC provide self-contained definitions of the person who claims priority. This person is defined by the action that they have performed, i.e. filing a first application. Whether they are the inventors or are actually entitled to be the applicants for this patent are not issues requiring investigation under the Paris Convention. As to the use of US provisional applications to claim priority for a European patent application, the board noted that applicants wishing to do so should be aware of the difficulties that they might face. This was a consequence of the US's adhesion to the Paris Convention.