[ruby-core:111026] [Ruby master Bug#18899] Inconsistent argument handling in IO#set_encoding

Issue #18899 has been updated by javanthropus (Jeremy Bopp).


Please also see #18995 for another example of the intricate implementation behaving unexpectedly.  During my own investigation, I discovered that using `"-"` for the internal encoding name is silently ignored.  According to the comments in the code, `"-"` is used to indicate no conversion, but it's completely undocumented for the method.  If you use `"-"` for the external encoding name, you get similarly divergent behavior as reported for this issue if you pass `"-:utf-8"` vs. `"-"`, `"utf-8"`.

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Bug #18899: Inconsistent argument handling in IO#set_encoding
https://bugs.ruby-lang.org/issues/18899#change-100280

* Author: javanthropus (Jeremy Bopp)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-linux]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
`IO#set_encoding` behaves differently when processing a single String argument than it does when processing 2 arguments (whether Strings or Encodings) in the case where the external encoding is being set to binary and the internal encoding is being set to any other encoding.

This script demonstrates the resulting values of the external and internal encodings for an IO instance given different ways to equivalently call `#set_encoding`:

```ruby
#!/usr/bin/env ruby


def show(io, args)
  printf(
    "args: %-50s  external encoding: %-25s  internal encoding: %-25s\n",
    args.inspect,
    io.external_encoding.inspect,
    io.internal_encoding.inspect
  )
end

File.open('/dev/null') do |f|
  args = ['binary:utf-8']
  f.set_encoding(*args)
  show(f, args)

  args = ['binary', 'utf-8']
  f.set_encoding(*args)
  show(f, args)

  args = [Encoding.find('binary'), Encoding.find('utf-8')]
  f.set_encoding(*args)
  show(f, args)
end
```

This behavior is the same from Ruby 2.7.0 to 3.1.2.



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