For product types, nominal versus structural typing is a design decision with a straightforward interpretation in each case; you can define two identical record types with the same fields in the same order, but different names; either they are or are not compatible; it's easy to see how each possibility leads to a coherent type system.

It's not clear to me that the same applies to sum types; the whole point of those is that you keep the tag names, so that you can create values and later distinguish them. But I have not been able to find any mention of this problem, in the discussion of nominal versus structural typing.

Is it the case that:

1. Of course nominal versus structural typing only applies to product types, sum types have to be nominal, this is obvious enough to go without saying, so no one bothers mentioning it.
2. Actually, sum types can be structural, in the following way that I had not thought of...
3. Something else?