title: Understanding captured conversion
body: <p>I am trying to undertand how captured conversion works for wildcard types. There is a section in <a href="http://docs.oracle.com/javase/specs/jls/se8/html/jls-5.html#jls-5.1.10">JLS</a> explaining that:</p>
  
  <blockquote>
    <p>Let <code>G</code> name a generic type declaration (§8.1.2, §9.1.2) with <code>n</code> type parameters <code>A1,...,An</code> with corresponding bounds <code>U1,...,Un</code>.</p>
    
    <p>There exists a capture conversion from a parameterized type
    <code>G&lt;T1,...,Tn&gt;</code> (§4.5) to a parameterized type <code>G&lt;S1,...,Sn&gt;</code>, where, for <code>1 ≤ i ≤ n</code> :</p>
    
    <ul>
    <li><p>If <code>Ti</code> is a wildcard type argument (§4.5.1) of the form <code>?</code>, then <code>Si</code> is a
    fresh type variable whose upper bound is <strong><code>Ui[A1:=S1,...,An:=Sn]</code></strong> and
    whose lower bound is the <code>null</code> type (§4.1).</p></li>
    <li><p>If <code>Ti</code> is a wildcard type argument of the form <code>? extends Bi</code>, then <code>Si</code> is
    a fresh type variable whose upper bound is <code>glb(Bi,
    Ui[A1:=S1,...,An:=Sn])</code> and whose lower bound is the <code>null</code> type.</p>
    
    <p><code>glb(V1,...,Vm)</code> is defined as <code>V1 &amp; ... &amp; Vm</code>.</p>
    
    <p>It is a compile-time error if, for any two classes (not interfaces) <code>Vi</code>
    and <code>Vj</code>, <code>Vi</code> is not a subclass of <code>Vj</code> or vice versa.</p></li>
    <li><p>If <code>Ti</code> is a wildcard type argument of the form <code>? super Bi</code>, then <code>Si</code> is a
    fresh type variable whose upper bound is <strong><code>Ui[A1:=S1,...,An:=Sn]</code></strong> and
    whose lower bound is <code>Bi</code>.</p></li>
    <li><p>Otherwise, <code>Si = Ti</code>.</p></li>
    </ul>
  </blockquote>
  
  <p>The thing that is not clear to me is <code>Ui[A1:=S1,...,An:=Sn]</code>. What does it mean? I could not find a definition for that searching through the JLS.</p>