[ruby-core:125802] [Ruby Bug#18995] IO#set_encoding sometimes set an IO's internal encoding to the default external encoding

22 Jun 2026

      Issue #18995 has been updated by javanthropus (Jeremy Bopp).

Contrast the reported behavior with an apparently equivalent call using a single, colon separated argument:
```ruby
def show(io)
  printf(
    "external encoding: %-25p  internal encoding: %-25p\n",
    io.external_encoding,
    io.internal_encoding
  )
end

Encoding.default_external = 'iso-8859-1'
Encoding.default_internal = 'iso-8859-2'

File.open('/dev/null') do |f|
  f.set_encoding('utf-8', nil)
  show(f)                             # f.internal_encoding is iso-8859-2, as expected

  f.set_encoding('utf-8:invalid')     # <-- The only difference is using a single string with colon separated encodings
  show(f)                             # f.internal_encoding is iso-8859-2, acceptable

  Encoding.default_external = 'iso-8859-3'
  Encoding.default_internal = 'iso-8859-4'
  show(f)                             # f.internal_encoding is iso-8859-2, expected given the above behavior
end
```

----------------------------------------
Bug #18995: IO#set_encoding sometimes set an IO's internal encoding to the default external encoding
https://bugs.ruby-lang.org/issues/18995#change-117719

* Author: javanthropus (Jeremy Bopp)
* Status: Open
* ruby -v: ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-linux]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
This script demonstrates the behavior:

```ruby
def show(io)
  printf(
    "external encoding: %-25p  internal encoding: %-25p\n",
    io.external_encoding,
    io.internal_encoding
  )
end

Encoding.default_external = 'iso-8859-1'
Encoding.default_internal = 'iso-8859-2'

File.open('/dev/null') do |f|
  f.set_encoding('utf-8', nil)
  show(f)                             # f.internal_encoding is iso-8859-2, as expected

  f.set_encoding('utf-8', 'invalid')
  show(f)                             # f.internal_encoding is now iso-8859-1!

  Encoding.default_external = 'iso-8859-3'
  Encoding.default_internal = 'iso-8859-4'
  show(f)                             # f.internal_encoding is now iso-8859-3!
end
```

In the 1st case, we see that the IO's internal encoding is set to the current setting of Encoding.default_internal. In the 2nd case, the IO's internal encoding is set to Encoding.default_external instead. The 3rd case is more interesting because it shows that the IO's internal encoding is actually following the current setting of Encoding.default_external. It didn't just copy it when #set_encoding was called. It changes whenever Encoding.default_external changes.

What should the correct behavior be?

-- 
https://bugs.ruby-lang.org/