Starting to implement Variadic Generics

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@Douglas_Gregor @Chris_Lattner3
I was writing my document about Variadic Generics when I stumbled across Chris's post on the Introduce (static) callables thread. In the post, Chris mentions a long term goal of unifying nominal and structural types. If I'm correct, this desire is shared among multiple Swift users and other thread refers to this feature, too.

At the moment I'm thinking to represent Variadic Generics as tuples inside types and function bodies, much like variadic function arguments are passed as Arrays.
But if / when we have structural types for tuples and / or functions, because they will need Variadic Generics (I assume), they might look like this:

struct Tuple<T...> {
  // Tuple implementation
}

class Function<Args..., Result> {
  // Function implementation
}

But at this point it seems to me that we have a chicken-and-egg problem: Tuple definition requires Variadic Generics, but Variadic Generics are represented as tuples inside the type using them, so...

struct Tuple<T...> {
  // This is... a tuple?
  private var members: (T...)

  init(_ members: (T...)) {
    self.members = members
  }
}

So, looking at the (far) future, what can we do here? The only solution I can think of ATM is to actually not represent Variadic Generics as tuples, but as something else like a "parameter vector" or C++'s "parameter pack".

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