Fix debug platform inclusion for Windows

lib/ruby_lsp/setup_bundler.rb

@@ -152,7 +152,13 @@ def write_custom_gemfile
       end
 
       unless @dependencies["debug"]
-        parts << 'gem "debug", require: false, group: :development, platforms: :mri'
+        # The `mri` platform excludes Windows. We want to install the debug gem only on MRI for any operating system,
+        # but that constraint doesn't yet exist in Bundler. On Windows, we are manually checking if the engine is MRI
+        parts << if Gem.win_platform?
+          'gem "debug", require: false, group: :development, install_if: -> { RUBY_ENGINE == "ruby" }'
+        else
+          'gem "debug", require: false, group: :development, platforms: :mri'
+        end
       end
 
       if @rails_app && !@dependencies["ruby-lsp-rails"]

test/setup_bundler_test.rb

@@ -823,6 +823,36 @@ def test_is_resilient_to_gemfile_changes_in_the_middle_of_setup
     end
   end
 
+  def test_composed_bundle_includes_debug
+    Dir.mktmpdir do |dir|
+      Dir.chdir(dir) do
+        File.write(File.join(dir, "Gemfile"), <<~GEMFILE)
+          source "https://rubygems.org"
+        GEMFILE
+
+        env = {}
+
+        if Bundler.settings[:path]
+          env["BUNDLE_PATH"] = File.expand_path(Bundler.settings[:path])
+        end
+
+        Bundler.with_unbundled_env do
+          capture_subprocess_io do
+            system(env, "bundle install")
+            assert_path_exists("Gemfile.lock")
+
+            env.merge!(RubyLsp::SetupBundler.new(dir, launcher: true).setup!)
+          end
+
+          _stdout, stderr = capture_subprocess_io do
+            system(env, "bundle exec ruby -e 'require \"debug\"'")
+          end
+          assert_empty(stderr)
+        end
+      end
+    end
+  end
+
   private
 
   def with_default_external_encoding(encoding, &block)