[Fix #10421] Make Style/DefWithParentheses aware of endless method …

…definition

Fixes #10421.

This PR makes `Style/DefWithParentheses` aware of endless method definition.

`def foo() do_something end` will continue to allow because `def foo do_something end`
is a syntax error.
On the other hand, be aware that the endless method definition's parentheses can be omitted.

changelog/fix_make_style_def_with_parentheses_aware_of_endless_method_definition.md

@@ -0,0 +1 @@
+* [#10421](https://github.com/rubocop/rubocop/issues/10421): Make `Style/DefWithParentheses` aware of endless method definition. ([@koic][])

lib/rubocop/cop/style/def_with_parentheses.rb

@@ -11,35 +11,41 @@ module Style
       #
       #   # bad
       #   def foo()
-      #     # does a thing
+      #     do_something
       #   end
       #
       #   # good
       #   def foo
-      #     # does a thing
+      #     do_something
       #   end
       #
-      #   # also good
-      #   def foo() does_a_thing end
+      #   # bad
+      #   def foo() = do_something
+      #
+      #   # good
+      #   def foo = do_something
+      #
+      #   # good (without parentheses it's a syntax error)
+      #   def foo() do_something end
       #
       # @example
       #
       #   # bad
       #   def Baz.foo()
-      #     # does a thing
+      #     do_something
       #   end
       #
       #   # good
       #   def Baz.foo
-      #     # does a thing
+      #     do_something
       #   end
       class DefWithParentheses < Base
         extend AutoCorrector
 
         MSG = "Omit the parentheses in defs when the method doesn't accept any arguments."
 
         def on_def(node)
-          return if node.single_line?
+          return if node.single_line? && !node.endless?
           return unless !node.arguments? && (node_arguments = node.arguments.source_range)
 
           add_offense(node_arguments) do |corrector|

spec/rubocop/cop/style/def_with_parentheses_spec.rb

@@ -29,6 +29,19 @@ def Test.func
     RUBY
   end
 
+  context 'Ruby >= 3.0', :ruby30 do
+    it 'reports an offense for endless method definition with empty parens' do
+      expect_offense(<<~RUBY)
+        def foo() = do_something
+               ^^ Omit the parentheses in defs when the method doesn't accept any arguments.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        def foo = do_something
+      RUBY
+    end
+  end
+
   it 'accepts def with arg and parens' do
     expect_no_offenses(<<~RUBY)
       def func(a)