Merge pull request #9976 from dvandersluis/issue/9975

[Fix #9975] Parentheses are always required for `Style/MethodDefParentheses` when a forwarding argument (`...`) is used

changelog/fix_parentheses_are_always_required_when_a.md

@@ -0,0 +1 @@
+* [#9975](https://github.com/rubocop/rubocop/issues/9975): Parentheses are always required for `Style/MethodDefParentheses` when a forwarding argument (`...`) is used. ([@dvandersluis][])

lib/rubocop/cop/style/method_def_parentheses.rb

@@ -96,7 +96,7 @@ class MethodDefParentheses < Base
         MSG_MISSING = 'Use def with parentheses when there are parameters.'
 
         def on_def(node)
-          return if node.endless?
+          return if forced_parentheses?(node)
 
           args = node.arguments
 
@@ -129,6 +129,15 @@ def correct_definition(def_node, corrector)
           corrector.insert_after(arguments_range, ')')
         end
 
+        def forced_parentheses?(node)
+          # Regardless of style, parentheses are necessary for:
+          # 1. Endless methods
+          # 2. Argument lists containing a `forward-arg` (`...`)
+          # Removing the parens would be a syntax error here.
+
+          node.endless? || node.arguments.any?(&:forward_arg_type?)
+        end
+
         def require_parentheses?(args)
           style == :require_parentheses ||
             (style == :require_no_parentheses_except_multiline && args.multiline?)

spec/rubocop/cop/style/method_def_parentheses_spec.rb

@@ -91,6 +91,14 @@ def self.test(param); end
         def self.test param; end
       RUBY
     end
+
+    it 'requires parens for forwarding', :ruby27 do
+      expect_no_offenses(<<~RUBY)
+        def foo(...)
+          bar(...)
+        end
+      RUBY
+    end
   end
 
   shared_examples 'endless methods' do
@@ -112,6 +120,12 @@ def foo(x) = x
           def foo(x) = bar(x)
         RUBY
       end
+
+      it 'accepts parens with `forward-arg`' do
+        expect_no_offenses(<<~RUBY)
+          def foo(...)= bar(...)
+        RUBY
+      end
     end
   end