Fix false negatives for Lint/UnreachableCode

This PR fixes false negatives for `Lint/UnreachableCode`
when using pattern matching.

changelog/fix_an_incorrect_autocorrect_for_lint_unreachable_code.md

@@ -0,0 +1 @@
+* [#12857](https://github.com/rubocop/rubocop/pull/12857): Fix false negatives for `Lint/UnreachableCode` when using pattern matching. ([@koic][])

lib/rubocop/cop/lint/unreachable_code.rb

@@ -71,7 +71,7 @@ def flow_expression?(node)
             expressions.any? { |expr| flow_expression?(expr) }
           when :if
             check_if(node)
-          when :case
+          when :case, :case_match
             check_case(node)
           else
             false
@@ -89,7 +89,9 @@ def check_case(node)
           return false unless else_branch
           return false unless flow_expression?(else_branch)
 
-          node.when_branches.all? { |branch| branch.body && flow_expression?(branch.body) }
+          branches = node.case_type? ? node.when_branches : node.in_pattern_branches
+
+          branches.all? { |branch| branch.body && flow_expression?(branch.body) }
         end
       end
     end

spec/rubocop/cop/lint/unreachable_code_spec.rb

@@ -97,6 +97,24 @@ def something
       RUBY
     end
 
+    it "registers an offense for `#{t}` in all `case` pattern branches" do
+      expect_offense(wrap(<<~RUBY))
+        case cond
+        in 1
+          something
+          #{t}
+        in 2
+          something2
+          #{t}
+        else
+          something3
+          #{t}
+        end
+        bar
+        ^^^ Unreachable code detected.
+      RUBY
+    end
+
     it "accepts code with conditional `#{t}`" do
       expect_no_offenses(wrap(<<~RUBY))
         #{t} if cond
@@ -182,5 +200,19 @@ def something
         bar
       RUBY
     end
+
+    it "accepts `#{t}` is in `case` pattern branch without else" do
+      expect_no_offenses(wrap(<<~RUBY))
+        case cond
+        in 1
+          something
+          #{t}
+        in 2
+          something2
+          #{t}
+        end
+        bar
+      RUBY
+    end
   end
 end