swift234

Implement support for nested generics in the AST and type
checker. There are a few related sets of changes here:

  - Generic parameter lists have a link  to their "outer" generic
  parameter lists, so its easy to establish the full generic context
  of an entity.
  - Bound and unbound generic types now carry a parent type, so that
  we distinguish between, e.g., X<Int>.Inner<Int> and
  X<Double>.Inner<Int>. Deduction, substitution, canonicalization,
  etc. cope with the parent type.
  - Opening of polymorphic types now handles multiple levels of
  generic parameters when needed (e.g., when we're substituting into
  the base).

Note that the generics module implied by this representation restricts
what one can do with requirements clauses in nested generics. For
example, one cannot add requirements to outer generic parameters or
their associated types, e.g., this is ill-formed:

  struct X<T : Range> {
    func f<U requires T.Element : Range>() {}
  }

The restriction has some precedent (e.g., in C#), but could be
loosened by rearchitecting how we handle archetypes in nested
generics. The current approach is more straightforward.


Swift SVN r2568