This may be a bit too pedantic, but I consider interface {} to be a way to do polymorphism via type classes. Interface defines an open class of types which implement some interface.

Sum types are a type definition defining something as A or B. Not “anything that quacks like a duck”. But concretely “one of this or one of that”. This enables different syntax, like the match expression to be used, in which you exhaustively list all the variants. The compiler doesn’t need to heap allocate enums because it knows the maximum size of a single value. The compiler and programmer can take advantage of the knowledge that there’s a closed set of values the type can hold. It’s not an open type class.

Result and Option are quite beautiful as sum types. But they’re horrible as type classes. Try implementing them using interface{} in Go. It’s not the same.

> Interface defines an open class of types

But can also define a closed set of types, perfectly satisfying "sum types".

> This enables different syntax, like the match expression to be used, in which you exhaustively list all the variants.

Go does not provide this out of the box, but that is not prerequisite for sum types. The mathematical theory says nothing about "the compiler must produce an error if the user doesn't match call cases". There is sufficient information provided by the sum types if you wish to add this to your build chain yourself, of course.