Enable BlockNode#argument_list for numblocks.

changelog/change_enable_blocknodeargument_list_for.md

@@ -0,0 +1 @@
+* [#155](https://github.com/rubocop-hq/rubocop-ast/pull/155): Enable `BlockNode#argument_list` for `numblock`s. ([@dvandersluis][])

lib/rubocop/ast/node/block_node.rb

@@ -40,9 +40,11 @@ def arguments
       #
       # @return [Array<Node>]
       def argument_list
-        return [].freeze unless arguments?
-
-        arguments.argument_list
+        if numblock_type?
+          numbered_arguments
+        else
+          arguments.argument_list
+        end
       end
 
       # The body of this block.
@@ -130,6 +132,18 @@ def lambda?
       def void_context?
         VOID_CONTEXT_METHODS.include?(method_name)
       end
+
+      private
+
+      # Numbered arguments of this `numblock`.
+      def numbered_arguments
+        return [].freeze unless numblock_type?
+
+        max_param = children[1]
+        1.upto(max_param).map do |i|
+          ArgNode.new(:arg, [:"_#{i}"])
+        end.freeze
+      end
     end
   end
 end

spec/rubocop/ast/block_node_spec.rb

@@ -52,25 +52,37 @@
   describe '#argument_list' do
     subject(:argument_list) { block_node.argument_list }
 
+    let(:names) { argument_list.map(&:name) }
+
     context 'with no arguments' do
       let(:source) { 'foo { bar }' }
 
       it { is_expected.to be_empty }
     end
 
     context 'all argument types' do
-      let(:source) { 'foo { |a, b = 42, (c, *d), e:, f: 42, **g, &h| nil }' }
+      let(:source) { 'foo { |a, b = 42, (c, *d), e:, f: 42, **g, &h; i| nil }' }
 
-      it { expect(argument_list.size).to eq(8) }
+      it { expect(names).to eq(%i[a b c d e f g h i]) }
     end
 
+    # rubocop:disable Naming/VariableNumber
     context '>= Ruby 2.7', :ruby27 do
       context 'using numbered parameters' do
-        let(:source) { 'foo { _1 }' }
+        context 'with skipped params' do
+          let(:source) { 'foo { _7 }' }
+
+          it { expect(names).to eq(%i[_1 _2 _3 _4 _5 _6 _7]) }
+        end
+
+        context 'with sequential params' do
+          let(:source) { 'foo { _1 + _2 }' }
 
-        it { is_expected.to be_empty }
+          it { expect(names).to eq(%i[_1 _2]) }
+        end
       end
     end
+    # rubocop:enable Naming/VariableNumber
   end
 
   describe '#arguments?' do