From b3c5d94bd1e51b7eb7647735ee3e9cba4e16d5e1 Mon Sep 17 00:00:00 2001
From: "Yukihiro \"Matz\" Matsumoto" <matz@ruby.or.jp>
Date: Tue, 28 Apr 2020 12:03:32 +0900
Subject: Update `IO#ungetc` to keep `@buf` string; ref #4982

---
 mrbgems/mruby-io/mrblib/io.rb | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mrbgems/mruby-io/mrblib/io.rb b/mrbgems/mruby-io/mrblib/io.rb
index 5df1932df..c0cfdc403 100644
--- a/mrbgems/mruby-io/mrblib/io.rb
+++ b/mrbgems/mruby-io/mrblib/io.rb
@@ -182,9 +182,9 @@ class IO
   def ungetc(substr)
     raise TypeError.new "expect String, got #{substr.class}" unless substr.is_a?(String)
     if @buf.empty?
-      @buf = substr.dup
+      @buf.replace(substr)
     else
-      @buf = substr + @buf
+      @buf[0,0] = substr
     end
     nil
   end
-- 
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