Make a '::distributed' pseudo element the first-ever client who can accept a relative selector.

Source/core/css/CSSSelector.h

 /*
  * Copyright (C) 1999-2003 Lars Knoll (knoll@kde.org)
  *               1999 Waldo Bastian (bastian@kde.org)
  * Copyright (C) 2004, 2006, 2007, 2008, 2009, 2010, 2013 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 62 matching lines @@
             PagePseudoClass
         };
 
         enum Relation {
             Descendant = 0,
             Child,
             DirectAdjacent,
             IndirectAdjacent,
             SubSelector,
             ShadowPseudo,
-            ShadowDistributed
+            ShadowVirtualDescendant,
+            ShadowVirtualChild

[dglazkov, 2013/05/22 17:43:43]

Introducing "virtual" here is bugging me. What does the word mean in this
context? I understand that you're aiming for a "virtual scoping root", but worry
that it isn't clear.

Another thought: isn't adding the "virtual" bit into Relation sort of
overloading it?

         };
 
         enum PseudoType {
             PseudoNotParsed = 0,
             PseudoUnknown,
             PseudoEmpty,
             PseudoFirstChild,
             PseudoFirstOfType,
             PseudoLastChild,
             PseudoLastOfType,
@@ skipping 239 matching lines @@
         || m_match == CSSSelector::End;
 }
 
 inline bool CSSSelector::isDistributedPseudoElement() const
 {
     return m_match == PseudoElement && pseudoType() == PseudoDistributed;
 }
 
 inline bool CSSSelector::isShadowDistributed() const
 {
-    return m_relation == CSSSelector::ShadowDistributed;
+    return m_relation == CSSSelector::ShadowVirtualDescendant || m_relation == CSSSelector::ShadowVirtualChild;
 }
 
 inline void CSSSelector::setValue(const AtomicString& value)
 {
     ASSERT(m_match != Tag);
     ASSERT(m_pseudoType == PseudoNotParsed);
     // Need to do ref counting manually for the union.
     if (m_hasRareData) {
         if (m_data.m_rareData->m_value)
             m_data.m_rareData->m_value->deref();
@@ skipping 79 matching lines @@
     ASSERT(m_match != Tag);
     // AtomicString is really just an AtomicStringImpl* so the cast below is safe.
     // FIXME: Perhaps call sites could be changed to accept AtomicStringImpl?
     return *reinterpret_cast<const AtomicString*>(m_hasRareData ? &m_data.m_rareData->m_value : &m_data.m_value);
 }
 
 
 } // namespace WebCore
 
 #endif // CSSSelector_h