From c0390a00b5a7614528b8766c2ed1bae0868c8d0a Mon Sep 17 00:00:00 2001
From: Koichi ITO <koic.ito@gmail.com>
Date: Thu, 5 Aug 2021 12:34:18 +0900
Subject: [PATCH] [Fix #9980] Fix a false positive for
 `Style/IdenticalConditionalBranches`

Fixes #9980.

This PR fixes a false positive for `Style/IdenticalConditionalBranches`
when assigning to a variable used in a condition.
---
 ...or_style_identical_conditional_branches.md |  1 +
 .../style/identical_conditional_branches.rb   | 15 ++++--
 .../identical_conditional_branches_spec.rb    | 47 +++++++++++++++++++
 3 files changed, 59 insertions(+), 4 deletions(-)
 create mode 100644 changelog/fix_false_positive_for_style_identical_conditional_branches.md

diff --git a/changelog/fix_false_positive_for_style_identical_conditional_branches.md b/changelog/fix_false_positive_for_style_identical_conditional_branches.md
new file mode 100644
index 00000000000..9d8a4da0465
--- /dev/null
+++ b/changelog/fix_false_positive_for_style_identical_conditional_branches.md
@@ -0,0 +1 @@
+* [#9980](https://github.com/rubocop/rubocop/issues/9980): Fix a false positive for `Style/IdenticalConditionalBranches` when assigning to a variable used in a condition. ([@koic][])
diff --git a/lib/rubocop/cop/style/identical_conditional_branches.rb b/lib/rubocop/cop/style/identical_conditional_branches.rb
index 0eaf35d3367..60185e7891d 100644
--- a/lib/rubocop/cop/style/identical_conditional_branches.rb
+++ b/lib/rubocop/cop/style/identical_conditional_branches.rb
@@ -124,14 +124,21 @@ def check_branches(node, branches)
           return if branches.any?(&:nil?)
 
           tails = branches.map { |branch| tail(branch) }
-          check_expressions(node, tails, :after_condition) if duplicated_expressions?(tails)
+          check_expressions(node, tails, :after_condition) if duplicated_expressions?(node, tails)
 
           heads = branches.map { |branch| head(branch) }
-          check_expressions(node, heads, :before_condition) if duplicated_expressions?(heads)
+          check_expressions(node, heads, :before_condition) if duplicated_expressions?(node, heads)
         end
 
-        def duplicated_expressions?(expressions)
-          expressions.size > 1 && expressions.uniq.one?
+        def duplicated_expressions?(node, expressions)
+          unique_expressions = expressions.uniq
+          return false unless expressions.size >= 1 && unique_expressions.one?
+
+          unique_expression = unique_expressions.first
+          return true unless unique_expression.assignment?
+
+          lhs = unique_expression.child_nodes.first
+          node.condition.child_nodes.none? { |n| n.source == lhs.source if n.variable? }
         end
 
         def check_expressions(node, expressions, insert_position) # rubocop:disable Metrics/MethodLength
diff --git a/spec/rubocop/cop/style/identical_conditional_branches_spec.rb b/spec/rubocop/cop/style/identical_conditional_branches_spec.rb
index 2a9d993727b..434c796f107 100644
--- a/spec/rubocop/cop/style/identical_conditional_branches_spec.rb
+++ b/spec/rubocop/cop/style/identical_conditional_branches_spec.rb
@@ -96,6 +96,21 @@
     end
   end
 
+  context 'on if..else with identical bodies and assigning to a variable used in `if` condition' do
+    it "doesn't register an offense" do
+      expect_no_offenses(<<~RUBY)
+        x = 0
+        if x == 0
+          x += 1
+          foo
+        else
+          x += 1
+          bar
+        end
+      RUBY
+    end
+  end
+
   context 'on case with identical bodies' do
     it 'registers an offense and corrects' do
       expect_offense(<<~RUBY)
@@ -233,6 +248,22 @@
     end
   end
 
+  context 'on case..when with identical bodies and assigning to a variable used in `case` condition' do
+    it "doesn't register an offense" do
+      expect_no_offenses(<<~RUBY)
+        x = 0
+        case x
+        when 1
+          x += 1
+          foo
+        when 42
+          x += 1
+          bar
+        end
+      RUBY
+    end
+  end
+
   context 'when using pattern matching', :ruby27 do
     context 'on case-match with identical bodies' do
       it 'registers an offense and corrects' do
@@ -259,6 +290,22 @@
           do_x
         RUBY
       end
+
+      context 'on case..in with identical bodies and assigning to a variable used in `case` condition' do
+        it "doesn't register an offense" do
+          expect_no_offenses(<<~RUBY)
+            x = 0
+            case x
+            in 1
+              x += 1
+              foo
+            in 42
+              x += 1
+              bar
+            end
+          RUBY
+        end
+      end
     end
 
     context 'when one of the case-match branches is empty' do