> Just call them "tagged union" or "variant" and suddenly "sum types" are not so rare anymore ;)

Nah, this is a false dichotomy. If a language doesn't have product types but instead resort to declaring variables separately, you don't call that "structs". Similarly, calling tagged unions a sum type is kind of misleading. They are both programming languages, I am sure I can do whatever I can do in A using B.

Why is a "struct" such a powerful concept? Because it is correct by construction. When I declare struct A, I have everything that A should contain. It is impossible to say, oops, I forgot about A::b. Similarly, a sum type as a concept is only useful when the abstraction is very, very solid.

Tagged unions or variants (in C and C++ respectively) is nothing like that. I have a variant A, I checked that that it is B, but oops, I casted it as C. Its your typical TOCTOU (or LOCLOU for line of check, line of use, I guess). std::optional is also like, same with pointers. Proper sum types, like Rust, it is literally impossible for me to get C by mistake.

Obviously, all of this minus some underlying implementation details, unsafe code yadda yadda.

Sorry if I sound aggressive, but I am just tired of "we have A at home, A at home (the most cursed shit)".

Does go have enums? Yea... just declare some global constants like so bro.

SomeEnum_A = 0

SomeEnum_B = 1

SomeEnum_C = 2

Yea..."enums"..

Edit: Oh how can I forget, we had this kind of issue just TODAY in production. No wonder I am so pissed off.