Fallback to dynamically defining node type predicates

While we have tests which enforce that we're calling
`def_node_type_predicate` for all known `Parser::Meta::NODE_TYPES`, it
is possible for a host application to use a newer version of `parser`,
which might support additional nodes, and for the application to attempt
to access those nodes in custom cops.

To preserve the previous forward compatibility, we fallback to
generating any missing methods. They won't be documented, but at least
they'll work.

The tests will enforce that if rubocop-ast bumps its Parser version, all
node type predicates are generated via `dev_node_type_predicate`.

lib/rubocop/ast/node.rb

@@ -139,12 +139,15 @@ def #{recursive_kind}                                     # def recursive_litera
 
       # Define a +_type?+ predicate method for the given node type.
       private_class_method def self.def_node_type_predicate(name, type = name)
+        @node_types_with_documented_predicate_method << type
+
         class_eval <<~RUBY, __FILE__, __LINE__ + 1
           def #{name}_type?      # def block_type?
             @type == :#{type}    #   @type == :block
           end                    # end
         RUBY
       end
+      @node_types_with_documented_predicate_method = []
 
       # @see https://www.rubydoc.info/gems/ast/AST/Node:initialize
       def initialize(type, children = EMPTY_CHILDREN, properties = EMPTY_PROPERTIES)
@@ -317,6 +320,14 @@ def send_type?
         # separately to make this check as fast as possible.
         false
       end
+      @node_types_with_documented_predicate_method << :send
+
+      # Ensure forward compatibility with new node types, by defining methods for unknown node types too.
+      # Note these won't get auto-generated documentation, which is why we prefer defining them above.
+      (Parser::Meta::NODE_TYPES - @node_types_with_documented_predicate_method).each do |node_type|
+        method_name = :"#{node_type.to_s.gsub(/\W/, '')}_type?"
+        define_method(method_name) { false }
+      end
 
       # @!endgroup
 

spec/rubocop/ast/node_spec.rb

@@ -1110,13 +1110,34 @@ class << expr
     end
   end
 
-  describe '*_type? methods on Node' do
-    Parser::Meta::NODE_TYPES.each do |node_type|
-      method_name = "#{node_type.to_s.gsub(/\W/, '')}_type?"
+  Parser::Meta::NODE_TYPES.each do |node_type|
+    node_name = node_type.to_s.gsub(/\W/, '')
+    method_name = :"#{node_name}_type?"
 
-      it "is not of #{method_name}" do
+    describe "##{method_name}" do
+      it 'is false' do
         expect(described_class.allocate.public_send(method_name)).to be(false)
       end
+
+      it 'is documented' do
+        is_documented = described_class
+                        .instance_variable_get(:@node_types_with_documented_predicate_method)
+                        .include?(node_type)
+
+        failure_message = <<~MSG
+          #{described_class.name}##{method_name} is not documented as it was generated automatically as a fallback.
+
+          To fix this, define it using the following macro instead:
+
+              class #{described_class.name} < #{described_class.superclass.name}
+                # ...
+                def_node_type_predicate :#{node_name}#{unless node_type.to_s == node_name
+                                                         ", :#{node_type}"
+                                                       end}
+        MSG
+
+        expect(is_documented).to be(true), failure_message
+      end
     end
   end
 end