@Override
        public Boolean unionShape(UnionShape shape) {
            for (MemberShape member : shape.members()) {
                if (memberShape(member)) {
                    return true;
                }
            }

            return false;
        }
Ensures that list, set, and map shapes are not directly recursive,
meaning that if they do have a recursive reference to themselves,
one or more references that form the recursive path travels through
a structure or union shape. And ensures that structure members are
not infinitely mutually recursive using the required trait.

<p>This check removes an entire class of problems from things like
code generators where a list of itself or a list of maps of itself
is impossible to define.
smithy-model/src/main/java/software/amazon/smithy/model/validation/validators/ShapeRecursionValidator.java