Issue #7964 has been reported by brixen (Brian Ford).

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Bug #7964: Writing an ASCII-8BIT String to a StringIO created from a UTF-8 String
https://bugs.ruby-lang.org/issues/7964

Author: brixen (Brian Ford)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 
ruby -v: ruby 2.0.0p0 (2013-02-24 revision 39474) [x86_64-darwin10.8.0]


In the following script, an ASCII-8BIT String is written to a StringIO created with a UTF-8 String without error. However, a << b or a + b will raise an exception, as will writing an ASCII-8BIT String to a File with UTF-8 external encoding.

$ cat file_enc.rb 
# encoding: utf-8
require 'stringio'

a = "On a very cold morning, it was -8°F."
b = a.dup.force_encoding "ascii-8bit"

io = StringIO.new a
io.write(b)
p io.string.encoding

File.open "data.txt", "w:utf-8" do |f|
  f.write a
  f.write b
end

$ ruby2.0 -v file_enc.rb 
ruby 2.0.0p0 (2013-02-24 revision 39474) [x86_64-darwin10.8.0]
#<Encoding:UTF-8>
file_enc.rb:13:in `write': "\xC2" from ASCII-8BIT to UTF-8 (Encoding::UndefinedConversionError)
	from file_enc.rb:13:in `block in <main>'
	from file_enc.rb:11:in `open'
	from file_enc.rb:11:in `<main>'

$ ruby1.9.3 -v file_enc.rb 
ruby 1.9.3p327 (2012-11-10 revision 37606) [x86_64-darwin10.8.0]
#<Encoding:UTF-8>
file_enc.rb:13:in `write': "\xC2" from ASCII-8BIT to UTF-8 (Encoding::UndefinedConversionError)
	from file_enc.rb:13:in `block in <main>'
	from file_enc.rb:11:in `open'
	from file_enc.rb:11:in `<main>'


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