Fix an error for Style/MultilineMethodSignature

changelog/fix_error_for_style_multiline_method_signature.md

@@ -0,0 +1 @@
+* [#9644](https://github.com/rubocop/rubocop/pull/9644): Fix an error and an incorrect auto-correct for `Style/MultilineMethodSignature` when line break after opening parenthesis. ([@koic][])

lib/rubocop/cop/style/multiline_method_signature.rb

@@ -40,21 +40,28 @@ def on_def(node)
         def autocorrect(corrector, node)
           arguments = node.arguments
           joined_arguments = arguments.map(&:source).join(', ')
-          last_line_source_of_arguments = processed_source[arguments.last_line - 1].strip
+          last_line_source_of_arguments = last_line_source_of_arguments(arguments)
 
           if last_line_source_of_arguments.start_with?(')')
             joined_arguments = "#{joined_arguments}#{last_line_source_of_arguments}"
 
             corrector.remove(range_by_whole_lines(arguments.loc.end, include_final_newline: true))
           end
 
-          corrector.replace(arguments_range(node), joined_arguments)
+          corrector.remove(arguments_range(node))
+          corrector.insert_after(arguments.loc.begin, joined_arguments)
+        end
+
+        def last_line_source_of_arguments(arguments)
+          processed_source[arguments.last_line - 1].strip
         end
 
         def arguments_range(node)
-          range_between(
+          range = range_between(
             node.first_argument.source_range.begin_pos, node.last_argument.source_range.end_pos
           )
+
+          range_with_surrounding_space(range: range, side: :left)
         end
 
         def opening_line(node)

spec/rubocop/cop/style/multiline_method_signature_spec.rb

@@ -17,6 +17,25 @@ def foo(bar)
         RUBY
       end
 
+      it 'registers an offense and corrects when line break after opening parenthesis' do
+        expect_offense(<<~RUBY)
+          class Foo
+            def foo(
+            ^^^^^^^^ Avoid multi-line method signatures.
+              arg
+          )
+            end
+          end
+        RUBY
+
+        expect_correction(<<~RUBY)
+          class Foo
+            def foo(arg)
+            end
+          end
+        RUBY
+      end
+
       context 'when method signature is on a single line' do
         it 'does not register an offense for parameterized method' do
           expect_no_offenses(<<~RUBY)