Issue #10584 has been reported by Geoff Nixon.

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Bug #10584: String.valid_encoding?, String.ascii_only? fails to account for BOM.
https://bugs.ruby-lang.org/issues/10584

* Author: Geoff Nixon
* Status: Open
* Priority: Normal
* Assignee: 
* Category: core
* Target version: current: 2.2.0
* ruby -v: ruby 2.2.0preview2 (2014-11-28 trunk 48628) [x86_64-darwin14]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN
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IMO:

- A Unicode (UTF-16, UTF-32) string with a valid BOM should not be considered a valid encoding if endianness is changed.

- A UTF-8 string with BOM should not consider the BOM as a codepoint.

~~~sh
> file utf-16be-file
utf-16be-file: POSIX shell script, Big-endian UTF-16 Unicode text executable

> file utf-16le-file
utf-16le-file: POSIX shell script, Little-endian UTF-16 Unicode text executable

> file utf-8-with-bom-file
utf-8-with-bom-file: POSIX shell script, UTF-8 Unicode (with BOM) text executable
~~~

~~~sh
> ruby -e "p File.binread('utf-16le-file').force_encoding('UTF-16BE').valid_encoding?"
true # false

> ruby -e "p File.binread('utf-16be-file').force_encoding('UTF-16LE').valid_encoding?"
true # false

> ruby -e "p File.read('utf-8-with-bom-file').ascii_only?"
false # true

> ruby -e "p File.read('utf-8-with-bom-file')[0]"
"" # '#'
~~~

No?

---Files--------------------------------
utf-8-with-bom-file (14 Bytes)
utf-16be-file (2.45 KB)
utf-16le-file (2.46 KB)


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