Support Ruby 2.7's pattern matching for Lint/DuplicateBranch

changelog/new_add_pattern_matching_doc_and_test_for_lint_duplicate_branch.md

@@ -0,0 +1 @@
+* [#9930](https://github.com/rubocop/rubocop/pull/9930): Support Ruby 2.7's pattern matching for `Lint/DuplicateBranch` cop. ([@koic][])

lib/rubocop/cop/lint/duplicate_branch.rb

@@ -4,7 +4,7 @@ module RuboCop
   module Cop
     module Lint
       # This cop checks that there are no repeated bodies
-      # within `if/unless`, `case-when` and `rescue` constructs.
+      # within `if/unless`, `case-when`, `case-in` and `rescue` constructs.
       #
       # With `IgnoreLiteralBranches: true`, branches are not registered
       # as offenses if they return a basic literal value (string, symbol,
@@ -97,6 +97,7 @@ def on_branching_statement(node)
         end
         alias on_if on_branching_statement
         alias on_case on_branching_statement
+        alias on_case_match on_branching_statement
         alias on_rescue on_branching_statement
 
         private

rubocop.gemspec

@@ -35,7 +35,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency('rainbow', '>= 2.2.2', '< 4.0')
   s.add_runtime_dependency('regexp_parser', '>= 1.8', '< 3.0')
   s.add_runtime_dependency('rexml')
-  s.add_runtime_dependency('rubocop-ast', '>= 1.7.0', '< 2.0')
+  s.add_runtime_dependency('rubocop-ast', '>= 1.8.0', '< 2.0')
   s.add_runtime_dependency('ruby-progressbar', '~> 1.7')
   s.add_runtime_dependency('unicode-display_width', '>= 1.4.0', '< 3.0')
 

spec/rubocop/cop/lint/duplicate_branch_spec.rb

@@ -65,6 +65,36 @@
     end
   end
 
+  shared_examples_for 'literal case-match allowed' do |type, value|
+    context "when returning a #{type} in multiple branches", :ruby27 do
+      it 'allows branches to be duplicated' do
+        expect_no_offenses(<<~RUBY)
+          case foo
+          in x then #{value}
+          in y then #{value}
+          else #{value}
+          end
+        RUBY
+      end
+    end
+  end
+
+  shared_examples_for 'literal case-match disallowed' do |type, value|
+    context "when returning a #{type} in multiple branches", :ruby27 do
+      it 'registers an offense' do
+        expect_offense(<<~RUBY, value: value)
+          case foo
+          in x then #{value}
+          in y then #{value}
+          ^^^^^^^^^^^{value} Duplicate branch body detected.
+          else #{value}
+          ^^^^ Duplicate branch body detected.
+          end
+        RUBY
+      end
+    end
+  end
+
   shared_examples_for 'literal rescue allowed' do |type, value|
     context "when returning a #{type} in multiple branches" do
       it 'allows branches to be duplicated' do
@@ -400,7 +430,7 @@
   context 'with IgnoreConstantBranches: true' do
     let(:cop_config) { { 'IgnoreConstantBranches' => true } }
 
-    %w[if case rescue].each do |keyword|
+    %w[if case case-match rescue].each do |keyword|
       context "with `#{keyword}`" do
         it_behaves_like "literal #{keyword} allowed", 'constant', 'MY_CONST'