[Fix #9603] Fix a false positive for Style/SoleNestedConditional

Fixes #9603.

This PR fixes a false positive for `Style/SoleNestedConditional`
when using nested modifier on value assigned in condition.

changelog/fix_false_positive_for_style_sole_nested_conditional.md

@@ -0,0 +1 @@
+* [#9603](https://github.com/rubocop/rubocop/issues/9603): Fix a false positive for `Style/SoleNestedConditional` when using nested modifier on value assigned in condition. ([@koic][])

lib/rubocop/cop/style/sole_nested_conditional.rb

@@ -43,6 +43,7 @@ def on_if(node)
           return if node.ternary? || node.else? || node.elsif?
 
           if_branch = node.if_branch
+          return if use_variable_assignment_in_condition?(node.condition, if_branch)
           return unless offending_branch?(if_branch)
 
           message = format(MSG, conditional_type: node.keyword)
@@ -53,6 +54,21 @@ def on_if(node)
 
         private
 
+        def use_variable_assignment_in_condition?(condition, if_branch)
+          assigned_variables = assigned_variables(condition)
+
+          assigned_variables && if_branch&.if_type? &&
+            assigned_variables.include?(if_branch.condition.source)
+        end
+
+        def assigned_variables(condition)
+          assigned_variables = condition.assignment? ? [condition.children.first.to_s] : []
+
+          assigned_variables + condition.descendants.select(&:assignment?).map do |node|
+            node.children.first.to_s
+          end
+        end
+
         def offending_branch?(branch)
           return false unless branch
 

spec/rubocop/cop/style/sole_nested_conditional_spec.rb

@@ -428,6 +428,22 @@ def foo
     RUBY
   end
 
+  it 'does not register an offense when using nested modifier on value assigned in single condition' do
+    expect_no_offenses(<<~RUBY)
+      if var = foo
+        do_something if var
+      end
+    RUBY
+  end
+
+  it 'does not register an offense when using nested modifier on value assigned in multiple conditions' do
+    expect_no_offenses(<<~RUBY)
+      if cond && var = foo
+        do_something if var
+      end
+    RUBY
+  end
+
   context 'when the inner condition has a send node without parens' do
     context 'in guard style' do
       it 'registers an offense and corrects' do