[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.

Author: koic

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 }