Yes, you read about std::variant on a blog and think that it is a sum type. Then you try it out and realize that it's a thin (type-safe) wrapper over tagged unions that is at least three times slower and has about 5 unreadable alternatives that replace simple switch statements.
Then you find out that members of a "variant" are not really variant members but just the individual types that can be assigned to a union. For example, assigning to a non-const reference does not work (and obviously cannot work once you realize that std::variant is just syntax sugar over a tagged union).
Most of these new additions since C++11 are just leaky abstractions and wrappers.