From 0ea6529964fd3f139c67082a497ad1efd71a7684 Mon Sep 17 00:00:00 2001
From: Jean Boussier <jean.boussier@gmail.com>
Date: Tue, 19 May 2026 13:25:18 +0200
Subject: [PATCH] Fix compatibility with `--enable-frozen-string-literal`

This has been a supported option of Ruby since Ruby 2.3, and
even without it, mutating a string literal emits a warning.
---
 .github/workflows/ruby.yml       | 4 ++++
 lib/docx/containers/paragraph.rb | 2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/.github/workflows/ruby.yml b/.github/workflows/ruby.yml
index 056a003..6f9d6e7 100644
--- a/.github/workflows/ruby.yml
+++ b/.github/workflows/ruby.yml
@@ -22,6 +22,7 @@ jobs:
           - 3.2
           - 3.3
           - 3.4
+          - 4.0
           - head
 
     steps:
@@ -36,3 +37,6 @@ jobs:
       env:
         COVERALLS_REPO_TOKEN: ${{ secrets.COVERALLS_REPO_TOKEN }}
       run: bundle exec rake spec
+      env:
+        RUBYOPT: "--enable-frozen-string-literal --debug-frozen-string-literal"
+
diff --git a/lib/docx/containers/paragraph.rb b/lib/docx/containers/paragraph.rb
index f51496e..536e8bf 100755
--- a/lib/docx/containers/paragraph.rb
+++ b/lib/docx/containers/paragraph.rb
@@ -44,7 +44,7 @@ def to_s
 
         # Return paragraph as a <p></p> HTML fragment with formatting based on properties.
         def to_html
-          html = ''
+          html = +''
           text_runs.each do |text_run|
             html << text_run.to_html
           end