Trying to figure out a way say type are in class extends a generic type (IVisitor), _without_ filling in the R type parameter of visitor from the class type arg, but instead from a method named accept(). This would then enable use of visitor pattern without coupling accept() to what visit methods exist, and therefore what node types exist. This …

test.dart

abstract class IVisitor<R> {
  // visitX() methods not declared here, but
  // on implementing class, so we can add new node types,
  // without coupling the code calling accept() to that knowledge.
}

abstract class NodeVisitor<R> implements IVisitor<R> {
  R visitParen(Node token);
}

abstract class Node< T<R> extends IVisitor<R> > {
  R accept<R>(T<R> visitor);
}

abstract class Element implements Node<NodeVisitor> {
  @override
  R accept<R>(NodeVisitor<R> visitor)
  {

  }
}

void main()
{
  Node? nullableNode;
  Node node = (nullableNode)!;

  NodeVisitor<int>? nullableVisitor;
  NodeVisitor<int> visitor = (nullableVisitor)!;

  print( node.accept(visitor) + "testing that Dart knows statically that R is actually an int");
}