This really doesn't seem like a reinventing to me (from either the Rust or C++ side). "Reinventing" implies ignoring history in the way you mention. Tagged uni

This really doesn't seem like a reinventing to me (from either the Rust or C++ side). "Reinventing" implies ignoring history in the way you mention. Tagged unions are prevalent in so many languages that the designers of both C++17 and Rust are bound to know about them. This is "borrowing" (or "stealing" :p). The original Rust was ML-like and had an OCaml compiler, so Rust's enums definitely descended from those. I can't talk for C++ for sure, but like I said it's common in so many languages that they're bound to have derived inspiration from them.