Heavy template base code uses inheritance all the time. But here is no expectation of is-a relationship there, it is purely for code reuse. In template code, the is-a is better expressed as modelling a concept (which is done mostly via structural typing, not nominal typing).

It is unfortunate that there is no explicit way to distinguish OOP is-a from code reuse, other than the presence of virtual functions in base classes, but in my 15 years of professional use of c++ I can't remember it ever being an issue.