[ruby-core:109688] [Ruby master Bug#18973] Kernel#sprintf: %c allows codepoints above 127 for 7-bits ASCII encoding

Issue #18973 has been updated by mame (Yusuke Endoh).


We brielfly discussed this issue at the dev meeting. @naruse said it should behave like `String#<<` as follows. @nobu said he would try to implement this.

```ruby
s = "".force_encoding("US-ASCII")
s << 128
p s          #=> "\x80"
p s.encoding #=> #<Encoding:ASCII-8BIT>

s = "".force_encoding("US-ASCII")
s << 256 #=> 256 out of char range (RangeError)
```

----------------------------------------
Bug #18973: Kernel#sprintf: %c allows codepoints above 127 for 7-bits ASCII encoding
https://bugs.ruby-lang.org/issues/18973#change-98912

* Author: andrykonchin (Andrew Konchin)
* Status: Open
* Priority: Normal
* ruby -v: 3.0.3
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
I've noticed the following behavior:

```ruby
sprintf("%c".encode("US-ASCII"), 128)
=> "\x80"

sprintf("%c".encode("US-ASCII"), 128).valid_encoding?
=> false
```

Specifying codepoints 128-255 for ASCII encoded formatting sequence leads to a broken string.

```ruby
sprintf("%c".encode("US-ASCII"), 255)
=> "\xFF"
sprintf("%c".encode("US-ASCII"), 256)
(irb):17:in `sprintf': 256 out of char range (RangeError)
```

Specifying codepoint greater that 255 causes the expected exception `out of char range`.

I suppose this exception should be raised for codepoints 128-255 as well (for ASCII encoding).



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