For example, you could declare a function as Foo(thing string | []int). In other to use "thing" you'd need to do a type assertion. That'd allow "generic" functions where the author could decide which types to support, without going all the way to generics, which tend to open a few cans of worms. Same thing for structs and such. Obviously this has an impact on internal layouts; union values would need an internal type tag similar to interfaces.

With this in mind, I would hope that Go could get better pattern matching with destructuring; switch statements currently only allow you to extract one value, and it's confusing that a single name is shared between all cases. But I'd love to be able to do something like:

    switch thing {
      case s := string:
        ...
      case [x, y] := []int if len(thing) == 2:
        ...
    }

If we could get proper Rust-style enums on top of that (with appropriate pattern matching) that'd also be neat.