https://git.ruby-lang.org/ruby.git/commit/?id=ddb2acbba6

From ddb2acbba60302201f01175f019d8ecd99934add Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@r...>
Date: Sun, 19 Jul 2020 16:08:03 +0900
Subject: [ruby/stringio] No compatibility check in US-ASCII case

https://github.com/ruby/stringio/commit/59df1c8293

diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 9ab8537..d55af51 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -1429,14 +1429,15 @@ strio_write(VALUE self, VALUE str) https://github.com/ruby/ruby/blob/trunk/ext/stringio/stringio.c#L1429
     long len, olen;
     rb_encoding *enc, *enc2;
     rb_encoding *const ascii8bit = rb_ascii8bit_encoding();
+    rb_encoding *usascii = 0;
 
     if (!RB_TYPE_P(str, T_STRING))
 	str = rb_obj_as_string(str);
     enc = get_enc(ptr);
     enc2 = rb_enc_get(str);
-    if (enc != enc2 && enc != ascii8bit) {
+    if (enc != enc2 && enc != ascii8bit && enc != (usascii = rb_usascii_encoding())) {
 	VALUE converted = rb_str_conv_enc(str, enc2, enc);
-	if (converted == str && enc2 != ascii8bit) { /* conversion failed */
+	if (converted == str && enc2 != ascii8bit && enc2 != usascii) { /* conversion failed */
 	    rb_enc_check(rb_enc_from_encoding(enc), str);
 	}
 	str = converted;
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index ab448f0..54f5716 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -195,9 +195,15 @@ class TestStringIO < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/stringio/test_stringio.rb#L195
     s = StringIO.new.set_encoding(conversion_encoding)
     s.write(convertible)
     assert_equal(conversion_encoding, s.string.encoding)
+
     s = StringIO.new.set_encoding(Encoding::UTF_8)
     s.write("foo".force_encoding("ISO-8859-1"), convertible)
     assert_equal(Encoding::UTF_8, s.string.encoding)
+
+    s = StringIO.new.set_encoding(Encoding::US_ASCII)
+    s.write("foo".force_encoding("US-ASCII"), convertible)
+    assert_equal(Encoding::UTF_8, s.string.encoding)
+
     all_assertions do |a|
       [
         inconvertible,
-- 
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