Type Pack Unification

[ghost]

I'm wondering why variadic type packs cannot be unified with generic type packs, like in the following example:

local function variadic_identity(...: any): ...any
   return ...
end

local function generic_call<T...>(...: T...): T...
   return variadic_identity(...)
end

where variadic_identity(...) has the type error "Failed to unify variadic packs".

Why does this happen? Is this a subtyping related limitation of the unification process? Can this could be worked on without introducing backwards incompatibilities?

Forgive me if this has been asked or answered before; from my searches I haven't seen much related to the topic.