A 24-year-old patient consulted a dentist about a cavity in an upper jaw tooth, which appeared three months ago. The tooth had not ached before. Objectively: on the occlusal surface of the 15 tooth there is a small carious cavity within the mantle dentin filled with leftover food. Probing revealed that the cavity walls and floor were dense. The response to thermal stimuli is absent. What is the most likely diagnosis?