title: What does "existential type" mean in Swift?
body: <p>I am reading <a href="https://github.com/apple/swift-evolution/blob/master/proposals/0244-opaque-result-types.md#introduction" rel="noreferrer">Swift Evolution proposal 244 (Opaque Result Types)</a> and don't understand what the following means:</p>
  <blockquote>
  <p>... existential type ...</p>
  <p>One could compose these transformations by
  using the <em><strong>existential type</strong></em> Shape instead of generic arguments, but
  doing so would imply more dynamism and runtime overhead than may be
  desired.</p>
  </blockquote>