I can't count the number of C++ codebases I've worked on where either myself or some other engineer implemented an ad-hoc, incomplete "sum type-like" enumeration system, e.g. in linting, pre-compilers, etc. In my case, I've done it without even knowing what these were--my background is physics, not computer science. The concept isn't terribly difficult to understand, even if the jargon name for it isn't widely known, or is associated with more type-theory oriented languages.