Bug 1617084 - Return nullptr_t instead of a full iterator from end(). r=bzbarsky

This is legal with C++17. It's not too important for the ancestor iterators
because they're just a pointer anyway, but it's nice for
ShadowIncludingTreeIterator, which has an AutoTArray and what not.

Depends on D63594

Differential Revision: https://phabricator.services.mozilla.com/D64355

--HG--
extra : moz-landing-system : lando

dom/base/AncestorIterator.h

@@ -38,17 +38,14 @@ namespace dom {
     explicit Inclusive##name_(const nsINode& aNode)                      \
         : mCurrent(const_cast<nsINode*>(&aNode)) {}                      \
     Self& begin() { return *this; }                                      \
-    Self end() { return {}; }                                            \
-    bool operator!=(const Self& aOther) const {                          \
-      return mCurrent != aOther.mCurrent;                                \
-    }                                                                    \
+    std::nullptr_t end() const { return nullptr; }                       \
+    bool operator!=(std::nullptr_t) const { return !!mCurrent; }         \
     void operator++() { mCurrent = mCurrent->method_(); }                \
     nsINode* operator*() { return mCurrent; }                            \
                                                                          \
     MUTATION_GUARD(Inclusive##name_)                                     \
                                                                          \
    protected:                                                            \
-    Inclusive##name_() : mCurrent(nullptr) {}                            \
     explicit Inclusive##name_(nsINode* aCurrent) : mCurrent(aCurrent) {} \
     nsINode* mCurrent;                                                   \
   };                                                                     \

dom/base/FilteredNodeIterator.h

@@ -24,6 +24,7 @@ class FilteredNodeIterator : public Iter {
   }
 
   FilteredNodeIterator& begin() { return *this; }
+  using Iter::end;
 
   void operator++() {
     Iter::operator++();
@@ -31,14 +32,13 @@ class FilteredNodeIterator : public Iter {
   }
 
   using Iter::operator!=;
+
   T* operator*() {
     nsINode* node = Iter::operator*();
     MOZ_ASSERT(!node || T::FromNode(node));
     return static_cast<T*>(node);
   }
 
-  FilteredNodeIterator end() { return FilteredNodeIterator(); }
-
  private:
   void EnsureValid() {
     while (true) {

dom/base/ShadowIncludingTreeIterator.h

@@ -45,20 +45,15 @@ class ShadowIncludingTreeIterator {
   // it goes.
   ShadowIncludingTreeIterator& begin() { return *this; }
 
-  ShadowIncludingTreeIterator end() { return ShadowIncludingTreeIterator(); }
+  std::nullptr_t end() const { return nullptr; }
 
-  bool operator!=(const ShadowIncludingTreeIterator& aOther) {
-    return mCurrent != aOther.mCurrent;
-  }
+  bool operator!=(std::nullptr_t) const { return !!mCurrent; }
 
   void operator++() { Next(); }
 
   nsINode* operator*() { return mCurrent; }
 
  private:
-  // Constructor used only for end() to represent a drained iterator.
-  ShadowIncludingTreeIterator() : mCurrent(nullptr) {}
-
   void Next() {
     MOZ_ASSERT(mCurrent, "Don't call Next() after we have no current node");