Just some quick impressions and questions
It wasn't clear to me at first, but am I understanding the proposal right that this is essentially a new take on the "structs as entity variants" approach (see, e.g., this old thread)?
Esp. for cases with duplicated types this proposal seems safer over anonymous enums: I.e., in a enum Foo(Thing, Thing) I know the intended order is Foo-like, whereas one source of an anonymous enum(Thing, Thing) may or may not produce the same index order as another source.
Can index-based TupleEnum::0 be used as a constructor function like with regular enum tags?
Can you use explicit discriminant values with this proposal? If yes, there might be confusion between the positional index and the actual discriminant value.
Looking at the linked syn code makes me want to have ECS (entitiy component system) support in the language... 