From: "mame (Yusuke Endoh)" <noreply@...>
Date: 2022-08-18T09:42:34+00:00
Subject: [ruby-core:109544] [Ruby master Bug#18955] Kernel#sprintf - %c ignores a non-ASCII character's encoding

Issue #18955 has been updated by mame (Yusuke Endoh).


At the dev-meeting, @akr proposed that the format `%c` behaves like `%s` (with the one-codepoint restriction) and @matz agreed with it.

----------------------------------------
Bug #18955: Kernel#sprintf - %c ignores a non-ASCII character's encoding
https://bugs.ruby-lang.org/issues/18955#change-98715

* Author: andrykonchin (Andrew Konchin)
* Status: Open
* Priority: Normal
* ruby -v: 3.0.3
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
I haven't found any similar existing issue so decided to create a new one.

I noticed that `sprintf("%c", string)` doesn't handle (in an expected way) a case when encodings of format sequence and string argument aren't the same and the string argument contains non-ASCII character.

In this case it seems to me that `sprintf` just uses binary representation of a character and assigns (or interprets with) encoding of the format sequence string.

I would expect that `sprintf` negotiates encoding and converts everything (the character and the format string) to the chosen one. And raises error when negotiation fails.

Examples to illustrate this behavior:

```ruby
format = "%c".encode("Windows-1251")
string = "��".encode(Encoding::KOI8_U)
r = sprintf(format, string)
r.encoding
# => #<Encoding:Windows-1251>

r == "��".encode("Windows-1251")
# => false

r.codepoints
# => [234]
string.codepoints
# => [234]
```

In this example the result's encoding is a format's encoding. But codepoint isn't changed and equals a codepoint of the character in the original string's encoding. But it should be different:

```ruby
"��".encode("Windows-1251").codepoints
# => [201]
```

Another example:

```ruby
string = "��".encode(Encoding::CP1252)
sprintf("%c", string)
# => in `sprintf': invalid byte sequence in UTF-8 (ArgumentError)
```

In this example the error means that `sprintf` doesn't encode properly a codepoint (of string's encoding) in UTF-8. It uses just raw bytes.



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