[Fix #10321] Make Style/MethodDefParentheses aware of anonymous blo…

…ck forwarding

Fixes #10321.

This PR fixes a false positive for `Style/MethodDefParentheses` when using
`EnforcedStyle: require_no_parentheses` and Ruby 3.1's anonymous block forwarding.

changelog/fix_false_positive_for_style_method_def_parentheses.md

@@ -0,0 +1 @@
+* [#10321](https://github.com/rubocop/rubocop/issues/10321): Make `Style/MethodDefParentheses` aware of Ruby 3.1's anonymous block forwarding. ([@koic][])

lib/rubocop/cop/style/method_def_parentheses.rb

@@ -6,8 +6,13 @@ module Style
       # This cop checks for parentheses around the arguments in method
       # definitions. Both instance and class/singleton methods are checked.
       #
-      # This cop does not consider endless methods, since parentheses are
-      # always required for them.
+      # Regardless of style, parentheses are necessary for:
+      #
+      # 1. Endless methods
+      # 2. Argument lists containing a `forward-arg` (`...`)
+      # 3. Argument lists containing an anonymous block forwarding (`&`)
+      #
+      # Removing the parens would be a syntax error here.
       #
       # @example EnforcedStyle: require_parentheses (default)
       #   # The `require_parentheses` style requires method definitions
@@ -133,9 +138,9 @@ def forced_parentheses?(node)
           # Regardless of style, parentheses are necessary for:
           # 1. Endless methods
           # 2. Argument lists containing a `forward-arg` (`...`)
+          # 3. Argument lists containing an anonymous block forwarding (`&`)
           # Removing the parens would be a syntax error here.
-
-          node.endless? || node.arguments.any?(&:forward_arg_type?)
+          node.endless? || node.arguments.any?(&:forward_arg_type?) || anonymous_block_arg?(node)
         end
 
         def require_parentheses?(args)
@@ -163,6 +168,12 @@ def unwanted_parentheses(args)
             unexpected_style_detected 'require_parentheses'
           end
         end
+
+        def anonymous_block_arg?(node)
+          return false unless (last_argument = node.arguments.last)
+
+          last_argument.blockarg_type? && last_argument.name.nil?
+        end
       end
     end
   end

spec/rubocop/cop/style/method_def_parentheses_spec.rb

@@ -99,6 +99,14 @@ def foo(...)
         end
       RUBY
     end
+
+    it 'requires parens for anonymous block forwarding', :ruby31 do
+      expect_no_offenses(<<~RUBY)
+        def foo(&)
+          bar(&)
+        end
+      RUBY
+    end
   end
 
   shared_examples 'endless methods' do