[swift-evolution] [Manifesto] Completing Generics

[plx]

# Missing Idea: "Type Predicates”

This is another way of attacking the “where clauses get too long” problem; it may already be implicitly part of “generic typealiases”.

Consider being able to write a declaration like this:

protocol SpecializedContainer {

  associatedtype Element: Comparable
  
  // potential declaration:
  typepredicate Compatible(S:SequenceType) = S.Generator.Element == Element

  // potential use syntaxes (tbd):
  func insertContentsOf<S:SequenceType where Compatible(S)>(elements: S)
  func insertContentsOf<Compatible(S)>(elements: S) // <- potential shorthand
  
}

…which isn’t the strongest motivating example on its own, but if you start to consider use cases where we will wind up having longer “where” clauses (and I think we will, over time) it may start paying its own way.

This probably falls under “minor feature” (and if added I’d hope the implementation picks a simple semantics for it), but something along these lines may do a decent job of addressing some of the same issues that motivate request to be able to write `Sequence<T>`.