The problem is we need the distinction between "author of the code" unknown ( in which case whatever symbol such as T is enough), and "user of the code" unknown

The problem is we need the distinction between "author of the code" unknown ( in which case whatever symbol such as T is enough), and "user of the code" unknown ( in which case relying on dynamic casting or interfaces is fine). What go doesn't provide is 1/ unknown but 2/ not. Which happens when you're creating a lib to be used later.