[AST] Introduce ArgumentList

include/swift/AST/ASTWalker.h

@@ -18,6 +18,7 @@
 
 namespace swift {
 
+class ArgumentList;
 class Decl;
 class Expr;
 class ClosureExpr;
@@ -244,6 +245,30 @@ class ASTWalker {
   /// traversal is terminated and returns failure.
   virtual bool walkToParameterListPost(ParameterList *PL) { return true; }
 
+  /// This method is called when first visiting an argument list before walking
+  /// into its arguments.
+  ///
+  /// \param ArgList The argument list to walk.
+  ///
+  /// \returns a pair indicating whether to visit the arguments, along with
+  /// the argument list that should replace this argument list in the tree. If
+  /// the latter is null, the traversal will be terminated.
+  ///
+  /// The default implementation returns \c {true, ArgList}.
+  virtual std::pair<bool, ArgumentList *>
+  walkToArgumentListPre(ArgumentList *ArgList) {
+    return {true, ArgList};
+  }
+
+  /// This method is called after visiting the arguments in an argument list.
+  /// If it returns null, the walk is terminated; otherwise, the
+  /// returned argument list is spliced in where the old argument list
+  /// previously appeared.
+  ///
+  /// The default implementation always returns the argument list.
+  virtual ArgumentList *walkToArgumentListPost(ArgumentList *ArgList) {
+    return ArgList;
+  }
 
 protected:
   ASTWalker() = default;