If an application is made in one of member states of Paris convention, then it is entitled to 12-month priority in member states. But what if applicant didn't pay the filing fee to the receiving office (e.g. in this particular cas it's EPO) on time and application was deemed withdrawn (however, it was technically made)? Or if it was refused for some other reason?
Article 4A of the Paris Convention provides (in relevant part; quoting from the official English translation):
(1) Any person who has duly filed an application for a patent ... in one of the countries of the Union ... shall enjoy, for the purpose of filing in the other countries, a right of priority during the periods hereinafter fixed.
(3) By a regular national filing is meant any filing that is adequate to establish the date on which the application was filed in the country concerned, whatever may be the subsequent fate of the application.
So, all that is required is that the prior application got a filing date. Bodenhausen's Guide further clarifies, at page 40:
In many countries, the question of "what is sufficient to establish the date on which the application was filed" would now be governed by Article 5 of the Patent Law Treaty (PLT).
It is worth adding that a copy of the prior application would be included in the file wrapper of any application claiming priority from the prior application (and the applicant needs to provide the copy for that purpose).