[ruby-core:109646] [Ruby master Bug#18973] Kernel#sprintf: %c allows codepoints above 127 for 7-bits ASCII encoding

Issue #18973 has been updated by Eregon (Benoit Daloze).


I noticed https://github.com/ruby/ruby/blob/master/benchmark/app_aobench.rb seems to rely on this behavior.
But that is easily fixed by using `# coding: BINARY` instead of `# coding: US-ASCII`.

I think it would be good to fix this issue, so `sprintf("%c".encode("US-ASCII"), 128)` is `out of char range (RangeError)`, just like it is an exception for:
```ruby
> 128.chr(Encoding::US_ASCII)
(irb):2:in `chr': invalid codepoint 0x80 in US-ASCII (RangeError)
```

----------------------------------------
Bug #18973: Kernel#sprintf: %c allows codepoints above 127 for 7-bits ASCII encoding
https://bugs.ruby-lang.org/issues/18973#change-98869

* 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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