[ruby-core:120972] [Ruby master Bug#21102] Unexpected encoding when concatenating ASCII string with ASCII compatible string with non ASCII encoding

Issue #21102 has been updated by naruse (Yui NARUSE).

Status changed from Open to Rejected

This behavior is for the case when a string is used as as buffer.
In that case the first string is the buffer, and following strings are inpu=
ts. Therefore the encoding of the buffer should be respected as far as it c=
an.

If you have a real problem which prevents development, please feedback agai=
n.

----------------------------------------
Bug #21102: Unexpected encoding when concatenating ASCII string with ASCII =
compatible string with non ASCII encoding
https://bugs.ruby-lang.org/issues/21102#change-111873

* Author: toy (Ivan Kuchin)
* Status: Rejected
* 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 =3D "something"
p str.encoding # =3D> #<Encoding:UTF-8>
p [nil, str].join.encoding # =3D> #<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 i=
s that resulting encoding depends on order and on compatibility of string e=
ncodings:

```ruby
# encoding: UTF-8

str1 =3D "something" # ASCII compatible
str2 =3D "s=F6m=E9th=EDng" # not ASCII compatible
p (nil.to_s + str1).encoding # =3D> #<Encoding:US-ASCII>
p (nil.to_s + str2).encoding # =3D> #<Encoding:UTF-8>
p (str1 + nil.to_s).encoding # =3D> #<Encoding:UTF-8>
p (str2 + nil.to_s).encoding # =3D> #<Encoding:UTF-8>
```

I would expect it to behave akin to summing integers and floats or rational=
s:
```ruby
p 1 + 1.0 # =3D> 2.0
p 1.0 + 1 # =3D> 2.0
p 1 + 1r # =3D> (2/1)
p 1r + 1 # =3D> (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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