[1718627440]

The list of UB categories and rules is not infinite. The list of UB programs is, as is the list of all non UB programs.

[gpderetta]

It is not obvious to me that the list of categories is not infinite (unless the final category is "everything else" of course)

[1718627440]

To be undefined behaviour, it must at least be valid syntax. The syntax is described in a finite document. Also it only gets executed by a finite machine, that has a finite number of finite descriptive documents.