x/text/language: Cannot parse ISO/IEC 15897 (C, POSIX, etc.) as language tags

[wking]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

~/.local/lib/go/src/golang.org/x/text $ go version
go version go1.10.2 linux/arm64
~/.local/lib/go/src/golang.org/x/text $ git describe
v0.3.0-48-g67e48ad

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

$ go env GOARCH
arm64

What did you do?

$ cat main.go 
package main

import (
        "fmt"

        "golang.org/x/text/language"
)

func main() {
        tag, err := language.Parse("POSIX")
        fmt.Printf("%v\n%v\n", tag, err)
}
$ go run main.go 
und
language: tag is not well-formed

What did you expect to see?

A POSIX language tag.

What did you see instead?

A "tag is not well-formed" error. Passing C instead of POSIX produces the same error.

Neither of these subtags are in the IANA registry, so the current errors are strictly compliant with the current BCP 47 claim. But POSIX defines a collation order which the current behavior does not provide access to. As far as I can tell, there's currently no way to create a Collator that will sort using the POSIX rules. Do folks who need to support them need to roll their own sorter, or should x/text/language be extended to support locales from the ISO/IEC 15897 registry? Is this related to the posix variant discussed here, here, and here (but not registered or grandfathered?)? Maybe I should be translating LC_COLLATE=POSIX and LC_COLLATE=C to a und-u-va-posix language tag for sorting? See also the distinction between languages and locales in RFC 2277. This may be related to this TODO?

[mpvl]

Go's BCP 47 tags support passing POSIX using "en-US-u-va-posix".
Passing such a tag to collator should select posix sorting. Does that work for you?