Expand use of IndexBase (#2436)

Initially I'd added this to lexer, this includes parser and semantics. Also adds ComparableIndexBase to unify a few common cases where <> comparisons are supported.

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/parser/parse_tree.h

@@ -42,7 +42,7 @@ namespace Carbon {
 // applied.
 class ParseTree {
  public:
-  class Node;
+  struct Node;
   class PostorderIterator;
   class SiblingIterator;
 
@@ -241,60 +241,8 @@ class ParseTree {
 //
 // That said, nodes can be compared and are part of a depth-first pre-order
 // sequence across all nodes in the parse tree.
-class ParseTree::Node {
- public:
-  // Node handles are default constructable, but such a node cannot be used
-  // for anything. It just allows it to be initialized later through
-  // assignment. Any other operation on a default constructed node is an
-  // error.
-  Node() = default;
-
-  friend auto operator==(Node lhs, Node rhs) -> bool {
-    return lhs.index_ == rhs.index_;
-  }
-  friend auto operator!=(Node lhs, Node rhs) -> bool {
-    return lhs.index_ != rhs.index_;
-  }
-  friend auto operator<(Node lhs, Node rhs) -> bool {
-    return lhs.index_ < rhs.index_;
-  }
-  friend auto operator<=(Node lhs, Node rhs) -> bool {
-    return lhs.index_ <= rhs.index_;
-  }
-  friend auto operator>(Node lhs, Node rhs) -> bool {
-    return lhs.index_ > rhs.index_;
-  }
-  friend auto operator>=(Node lhs, Node rhs) -> bool {
-    return lhs.index_ >= rhs.index_;
-  }
-
-  // Returns an opaque integer identifier of the node in the tree. Clients
-  // should not expect any particular semantics from this value.
-  //
-  // TODO: Maybe we can switch to stream operator overloads?
-  [[nodiscard]] auto index() const -> int { return index_; }
-
-  // Prints the node index.
-  auto Print(llvm::raw_ostream& output) const -> void;
-
-  // Returns true if the node is valid; in other words, it was not default
-  // initialized.
-  auto is_valid() -> bool { return index_ != InvalidValue; }
-
- private:
-  friend ParseTree;
-  friend PostorderIterator;
-  friend SiblingIterator;
-
-  // Value for uninitialized nodes.
-  static constexpr int InvalidValue = -1;
-
-  // Constructs a node with a specific index into the parse tree's postorder
-  // sequence of node implementations.
-  explicit Node(int index) : index_(index) {}
-
-  // The index of this node's implementation in the postorder sequence.
-  int32_t index_ = InvalidValue;
+struct ParseTree::Node : public ComparableIndexBase {
+  using ComparableIndexBase::ComparableIndexBase;
 };
 
 // A random-access iterator to the depth-first postorder sequence of parse nodes
@@ -320,15 +268,15 @@ class ParseTree::PostorderIterator
   auto operator*() const -> Node { return node_; }
 
   auto operator-(const PostorderIterator& rhs) const -> int {
-    return node_.index_ - rhs.node_.index_;
+    return node_.index - rhs.node_.index;
   }
 
   auto operator+=(int offset) -> PostorderIterator& {
-    node_.index_ += offset;
+    node_.index += offset;
     return *this;
   }
   auto operator-=(int offset) -> PostorderIterator& {
-    node_.index_ -= offset;
+    node_.index -= offset;
     return *this;
   }
 
@@ -373,7 +321,7 @@ class ParseTree::SiblingIterator
 
   using iterator_facade_base::operator++;
   auto operator++() -> SiblingIterator& {
-    node_.index_ -= std::abs(tree_->node_impls_[node_.index_].subtree_size);
+    node_.index -= std::abs(tree_->node_impls_[node_.index].subtree_size);
     return *this;
   }