[ruby-core:123188] [Ruby Bug#21561] Wrong encoding for File.dirname result on Windows

8 Sep 2025

      Issue #21561 has been updated by byroot (Jean Boussier).

Backport changed from 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN to 3.2: WONTFIX, 3.3: REQUIRED, 3.4: REQUIRED
...
The source problem is the encoding US-ASCII for "."
Yes, but `File.expand_path` behavior is also suspicious.

Either way, I think returning a different encoding in a special case isn't a good idea. So I'm in favor of always using the same logic for the encoding selection: https://github.com/ruby/ruby/pull/14472

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Bug #21561: Wrong encoding for File.dirname result on Windows
https://bugs.ruby-lang.org/issues/21561#change-114519

* Author: thyresias (Thierry Lambert)
* Status: Open
* ruby -v: ruby 3.4.5 (2025-07-16 revision 20cda200d3) +PRISM [x64-mingw-ucrt]
* Backport: 3.2: WONTFIX, 3.3: REQUIRED, 3.4: REQUIRED
----------------------------------------
I placed the file containing the code below in a directory named "à_signaler" to show the effect on non-ascii characters:
```ruby
def dirname_encoding(file)
  h = {
    "file" => file,
    "File.dirname(file)" => File.dirname(file),
    "File.expand_path(file)" => File.expand_path(file),
    "File.expand_path(File.dirname(file))" => File.expand_path(File.dirname(file)),
    "File.dirname(File.expand_path(file))" => File.dirname(File.expand_path(file)),
  }
  puts
  h.each do |name, value|
    puts "#{name} = #{value.inspect} (#{value.encoding})"
  end
end

dirname_encoding "foo.bar"
dirname_encoding "foo/bar"
```
The result:
```
file = "foo.bar" (UTF-8)
File.dirname(file) = "." (US-ASCII)
File.expand_path(file) = "D:/à_signaler/foo.bar" (UTF-8)
File.expand_path(File.dirname(file)) = "D:/\xE0_signaler" (US-ASCII)
File.dirname(File.expand_path(file)) = "D:/à_signaler" (UTF-8)

file = "foo/bar" (UTF-8)
File.dirname(file) = "foo" (UTF-8)
File.expand_path(file) = "D:/à_signaler/foo/bar" (UTF-8)
File.expand_path(File.dirname(file)) = "D:/à_signaler/foo" (UTF-8)
File.dirname(File.expand_path(file)) = "D:/à_signaler/foo" (UTF-8)
```
The source problem is the encoding US-ASCII for "."
This may be linked to Bug #20699.

-- 
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