Fix an error for Minitst/AssertOperator and Minitest/RefuteOperator

.github/workflows/test.yml

@@ -0,0 +1,32 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  oldest_supported_rubocop:
+    runs-on: ubuntu-latest
+    name: The oldest supported RuboCop version
+    steps:
+      - uses: actions/checkout@v4
+      - name: Use the oldest supported RuboCop
+        run: |
+          sed -e "/gem 'rubocop', github: 'rubocop\/rubocop'/d" -i Gemfile
+          cat << EOF > Gemfile.local
+            gem 'rubocop', '1.39.0' # Specify the oldest supported RuboCop version
+          EOF
+      - name: set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 2.7
+          bundler-cache: true
+      - name: test
+        run: bundle exec rake

changelog/fix_error_for_minitest_assert_operator_and_refute_operator.md

@@ -0,0 +1 @@
+* [#275](https://github.com/rubocop/rubocop-minitest/pull/275): Make `Minitest/AssertMatch` aware of `assert_operator` when running with Ruby 2.7. ([@koic][])

lib/rubocop/cop/minitest/assert_operator.rb

@@ -40,7 +40,7 @@ def on_send(node)
 
         def build_new_arguments(node)
           lhs, op, rhs = *node.first_argument
-          new_arguments = "#{lhs.source}, :#{op}, #{rhs.source}"
+          new_arguments = +"#{lhs.source}, :#{op}, #{rhs.source}"
 
           if node.arguments.count == 2
             new_arguments << ", #{node.last_argument.source}"

lib/rubocop/cop/minitest/refute_operator.rb

@@ -40,7 +40,7 @@ def on_send(node)
 
         def build_new_arguments(node)
           lhs, op, rhs = *node.first_argument
-          new_arguments = "#{lhs.source}, :#{op}, #{rhs.source}"
+          new_arguments = +"#{lhs.source}, :#{op}, #{rhs.source}"
 
           if node.arguments.count == 2
             new_arguments << ", #{node.last_argument.source}"