[Fix #11040] Fix a false positive for Style/IfUnlessModifier

Fixes #11040.

This PR fixes a false positive for `Style/IfUnlessModifier`
when `defined?`'s argument value is undefined.

changelog/fix_a_false_positive_for_style_if_unless_modifier.md

@@ -0,0 +1 @@
+* [#11040](https://github.com/rubocop/rubocop/issues/11040): Fix a false positive for `Style/IfUnlessModifier` when `defined?`'s argument value is undefined. ([@koic][])

lib/rubocop/cop/style/if_unless_modifier.rb

@@ -11,6 +11,20 @@ module Style
       # cop. The tab size is configured in the `IndentationWidth` of the
       # `Layout/IndentationStyle` cop.
       #
+      # NOTE: It is allowed when `defined?` argument has an undefined value,
+      # because using the modifier form causes the following incompatibility:
+      #
+      # [source,ruby]
+      # ----
+      # unless defined?(undefined_foo)
+      #   undefined_foo = 'default_value'
+      # end
+      # undefined_foo # => 'default_value'
+      #
+      # undefined_bar = 'default_value' unless defined?(undefined_bar)
+      # undefined_bar # => nil
+      # ----
+      #
       # @example
       #   # bad
       #   if condition
@@ -51,6 +65,8 @@ def self.autocorrect_incompatible_with
         end
 
         def on_if(node)
+          return if defined_nodes(node).any? { |n| defined_argument_is_undefined?(node, n) }
+
           msg = if single_line_as_modifier?(node) && !named_capture_in_condition?(node)
                   MSG_USE_MODIFIER
                 elsif too_long_due_to_modifier?(node)
@@ -65,6 +81,24 @@ def on_if(node)
 
         private
 
+        def defined_nodes(node)
+          if node.condition.defined_type?
+            [node.condition]
+          else
+            node.condition.each_descendant.select(&:defined_type?)
+          end
+        end
+
+        def defined_argument_is_undefined?(if_node, defined_node)
+          defined_argument = defined_node.first_argument
+          return false unless defined_argument.lvar_type? || defined_argument.send_type?
+
+          if_node.left_siblings.none? do |sibling|
+            sibling.respond_to?(:lvasgn_type?) && sibling.lvasgn_type? &&
+              sibling.name == defined_argument.node_parts[0]
+          end
+        end
+
         def autocorrect(corrector, node)
           replacement = if node.modifier_form?
                           last_argument = node.if_branch.last_argument if node.if_branch.send_type?

spec/rubocop/cop/style/if_unless_modifier_spec.rb

@@ -371,6 +371,81 @@ def f
     end
   end
 
+  context 'using `defined?` in the condtion' do
+    it 'registers for argument value is defined' do
+      expect_offense(<<~RUBY)
+        value = :custom
+
+        unless defined?(value)
+        ^^^^^^ Favor modifier `unless` usage when having a single-line body. Another good alternative is the usage of control flow `&&`/`||`.
+          value = :default
+        end
+      RUBY
+
+      expect_correction(<<~RUBY)
+        value = :custom
+
+        value = :default unless defined?(value)
+      RUBY
+    end
+
+    it 'registers for argument value is `yield`' do
+      expect_offense(<<~RUBY)
+        unless defined?(yield)
+        ^^^^^^ Favor modifier `unless` usage when having a single-line body. Another good alternative is the usage of control flow `&&`/`||`.
+          value = :default
+        end
+      RUBY
+
+      expect_correction(<<~RUBY)
+        value = :default unless defined?(yield)
+      RUBY
+    end
+
+    it 'registers for argument value is `super`' do
+      expect_offense(<<~RUBY)
+        unless defined?(super)
+        ^^^^^^ Favor modifier `unless` usage when having a single-line body. Another good alternative is the usage of control flow `&&`/`||`.
+          value = :default
+        end
+      RUBY
+
+      expect_correction(<<~RUBY)
+        value = :default unless defined?(super)
+      RUBY
+    end
+
+    it 'accepts `defined?` when argument value is undefined' do
+      expect_no_offenses(<<~RUBY)
+        other_value = do_something
+
+        unless defined?(value)
+          value = :default
+        end
+      RUBY
+    end
+
+    it 'accepts `defined?` when argument value is undefined and the first condition' do
+      expect_no_offenses(<<~RUBY)
+        other_value = do_something
+
+        unless defined?(value) && condition
+          value = :default
+        end
+      RUBY
+    end
+
+    it 'accepts `defined?` when argument value is undefined and the last condition' do
+      expect_no_offenses(<<~RUBY)
+        other_value = do_something
+
+        unless condition && defined?(value)
+          value = :default
+        end
+      RUBY
+    end
+  end
+
   context 'with implicit match conditional' do
     let(:body) { 'b' * 36 }