protocol Thing {
  associatedtype Foo
  associatedtype Bar
}

struct AnyThing<Foo, Bar>: Thing {}

enum Either<A, B> {
  case first(A)
  case second(B)
}

@_functionBuilder
struct ThingBuilder {
  public static func buildEither<A, B>(first value: A) -> Either<A, B>
  where A: Thing, B: Thing {
    .first(value)
  }

  public static func buildEither<A, B>(second value: B) -> Either<A, B>
  where A: Thing, B: Thing {
    .second(value)
  }
}

func build<A>(@ThingBuilder block: () -> A) -> A where A: Thing {
  block()
}

let x = build {
  if Bool.random() {
    AnyThing<Int, Int>()
  } else {
    AnyThing()
  }
}