From 576bdec03f0d58847690a0607c788ada433ce60f Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@ruby-lang.org>
Date: Tue, 30 Aug 2022 18:12:08 +0900
Subject: [Bug #18973] Promote US-ASCII to ASCII-8BIT when adding 8-bit char

---
 string.c | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)

(limited to 'string.c')

diff --git a/string.c b/string.c
index 564812ae51..951aeca6dd 100644
--- a/string.c
+++ b/string.c
@@ -3481,17 +3481,13 @@ rb_str_concat(VALUE str1, VALUE str2)
         return rb_str_append(str1, str2);
     }
 
-    encidx = rb_enc_to_index(enc);
-    if (encidx == ENCINDEX_ASCII_8BIT || encidx == ENCINDEX_US_ASCII) {
-        /* US-ASCII automatically extended to ASCII-8BIT */
+    encidx = rb_ascii8bit_appendable_encoding_index(enc, code);
+    if (encidx >= 0) {
         char buf[1];
         buf[0] = (char)code;
-        if (code > 0xFF) {
-            rb_raise(rb_eRangeError, "%u out of char range", code);
-        }
         rb_str_cat(str1, buf, 1);
-        if (encidx == ENCINDEX_US_ASCII && code > 127) {
-            rb_enc_associate_index(str1, ENCINDEX_ASCII_8BIT);
+        if (encidx != rb_enc_to_index(enc)) {
+            rb_enc_associate_index(str1, encidx);
             ENC_CODERANGE_SET(str1, ENC_CODERANGE_VALID);
         }
     }
@@ -3524,6 +3520,26 @@ rb_str_concat(VALUE str1, VALUE str2)
     return str1;
 }
 
+int
+rb_ascii8bit_appendable_encoding_index(rb_encoding *enc, unsigned int code)
+{
+    int encidx = rb_enc_to_index(enc);
+
+    if (encidx == ENCINDEX_ASCII_8BIT || encidx == ENCINDEX_US_ASCII) {
+        /* US-ASCII automatically extended to ASCII-8BIT */
+        if (code > 0xFF) {
+            rb_raise(rb_eRangeError, "%u out of char range", code);
+        }
+        if (encidx == ENCINDEX_US_ASCII && code > 127) {
+            return ENCINDEX_ASCII_8BIT;
+        }
+        return encidx;
+    }
+    else {
+        return -1;
+    }
+}
+
 /*
  *  call-seq:
  *    prepend(*other_strings)  -> string
-- 
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