From 983674667a21ee2e4e6a43282507858634dce907 Mon Sep 17 00:00:00 2001
From: Vipul A M <vipulnsward@gmail.com>
Date: Mon, 29 Dec 2014 18:31:34 +0530
Subject: [PATCH] When trying to access a character on a string buffer object
 via `:[]`, if the object being accessed currently returns `html_safe?` as
 true, we used to set  `@html_safe` variable as true on new object created.
 When doing something like

x = 'Hello'.html_safe
x[/a/, 1]

would throw an error on ruby 2.2, since when nothign gets matched nil is returned by the code and it tries to set   `@html_safe` value to true,
which would error since starting 2.2 nil is frozen.

This change adds a safety net to avoid setting `@html_safe = true` on frozen objects.

Fixes #18235
---
 .../lib/active_support/core_ext/string/output_safety.rb  | 4 +++-
 activesupport/test/safe_buffer_test.rb                   | 9 +++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/activesupport/lib/active_support/core_ext/string/output_safety.rb b/activesupport/lib/active_support/core_ext/string/output_safety.rb
index ba92afd5f4..24d0f9319f 100644
--- a/activesupport/lib/active_support/core_ext/string/output_safety.rb
+++ b/activesupport/lib/active_support/core_ext/string/output_safety.rb
@@ -150,7 +150,9 @@ module ActiveSupport #:nodoc:
       else
         if html_safe?
           new_safe_buffer = super
-          new_safe_buffer.instance_variable_set :@html_safe, true
+          unless new_safe_buffer.respond_to?(:frozen?) && new_safe_buffer.frozen?
+            new_safe_buffer.instance_variable_set :@html_safe, true
+          end
           new_safe_buffer
         else
           to_str[*args]
diff --git a/activesupport/test/safe_buffer_test.rb b/activesupport/test/safe_buffer_test.rb
index efa9d5e61f..fca4b45276 100644
--- a/activesupport/test/safe_buffer_test.rb
+++ b/activesupport/test/safe_buffer_test.rb
@@ -165,4 +165,13 @@ class SafeBufferTest < ActiveSupport::TestCase
     x = 'foo %{x} bar'.html_safe % { x: 'qux' }
     assert x.html_safe?, 'should be safe'
   end
+
+  test 'Should not affect frozen objects when accessing characters' do
+    x = 'Hello'.html_safe
+    assert_nothing_raised do
+      x[/a/, 1]
+    end
+  end
+
+
 end