Define a base class for all AST nodes. (#947)

Also implement code-generation to manage the resulting boilerplate.

executable_semantics/ast/static_scope.h

@@ -10,50 +10,33 @@
 #include <variant>
 #include <vector>
 
+#include "executable_semantics/ast/ast_node.h"
 #include "executable_semantics/ast/source_location.h"
 #include "executable_semantics/common/nonnull.h"
 
 namespace Carbon {
 
-class NamedEntityInterface {
+class NamedEntity : public virtual AstNode {
  public:
-  enum class NamedEntityKind {
-    // Includes variable definitions and matching contexts.
-    BindingPattern,
-    // Used by entries in choices.
-    ChoiceDeclarationAlternative,
-    // Used by continuations.
-    Continuation,
-    // Includes choices, classes, and functions. Variables are handled through
-    // BindingPattern.
-    Declaration,
-    // Used by functions.
-    GenericBinding,
-    // Used by entries in classes.
-    Member,
-  };
+  virtual ~NamedEntity() = 0;
 
-  NamedEntityInterface() = default;
-  virtual ~NamedEntityInterface() = default;
-
-  NamedEntityInterface(NamedEntityInterface&&) = delete;
-  auto operator=(NamedEntityInterface&&) -> NamedEntityInterface& = delete;
+  NamedEntity() = default;
 
   // TODO: This is unused, but is intended for casts after lookup.
-  virtual auto named_entity_kind() const -> NamedEntityKind = 0;
-  virtual auto source_loc() const -> SourceLocation = 0;
+  auto kind() const -> NamedEntityKind {
+    return static_cast<NamedEntityKind>(root_kind());
+  }
 };
 
 // The set of declared names in a scope. This is not aware of child scopes, but
 // does include directions to parent or related scopes for lookup purposes.
 class StaticScope {
  public:
-  void Add(std::string name, Nonnull<const NamedEntityInterface*> entity);
+  void Add(std::string name, Nonnull<const NamedEntity*> entity);
 
  private:
   // Maps locally declared names to their entities.
-  std::unordered_map<std::string, Nonnull<const NamedEntityInterface*>>
-      declared_names_;
+  std::unordered_map<std::string, Nonnull<const NamedEntity*>> declared_names_;
 
   // A list of scopes used for name lookup within this scope.
   // TODO: This is unused, but is intended for name lookup cross-scope.