From: "toy (Ivan Kuchin) via ruby-core" <ruby-core@...>
Date: 2025-01-31T17:02:20+00:00
Subject: [ruby-core:120850] [Ruby master Bug#21102] Unexpected encoding when concatenating ASCII string with ASCII compatible string with non ASCII encoding

Issue #21102 has been reported by toy (Ivan Kuchin).

----------------------------------------
Bug #21102: Unexpected encoding when concatenating ASCII string with ASCII compatible string with non ASCII encoding
https://bugs.ruby-lang.org/issues/21102

* Author: toy (Ivan Kuchin)
* Status: Open
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
The problem was noticed in code that is boiled down to:

```ruby
# encoding: UTF-8

str = "something"
p str.encoding # => #<Encoding:UTF-8>
p [nil, str].join.encoding # => #<Encoding:US-ASCII>
```

As `nil.to_s` is an empty string with encoding `ASCII` and `"something"` is `ASCII` compatible string, the result is a string with `ASCII` encoding.

Event simpler example is `p (nil.to_s + "something").encoding`. Confusing is that resulting encoding depends on order and on compatibility of string encodings:

```ruby
# encoding: UTF-8

str1 = "something" # ASCII compatible
str2 = "s�m�th�ng" # not ASCII compatible
p (nil.to_s + str1).encoding # => #<Encoding:US-ASCII>
p (nil.to_s + str2).encoding # => #<Encoding:UTF-8>
p (str1 + nil.to_s).encoding # => #<Encoding:UTF-8>
p (str2 + nil.to_s).encoding # => #<Encoding:UTF-8>
```

I would expect it to behave akin to summing integers and floats or rationals:
```ruby
p 1 + 1.0 # => 2.0
p 1.0 + 1 # => 2.0
p 1 + 1r # => (2/1)
p 1r + 1 # => (2/1)
```

So it is at least surprising to me.

#18579 is probably the most related, but also #14975 and #20594



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