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.