The function is MustParse which panics intentionally:

MustParse is like Parse, but panics if the given BCP 47 tag cannot be parsed. It simplifies safe initialization of Tag values.

closing as working as intended, please comment/reopen if you disagree

@seankhliao the function being called is Matcher.Match (interface). That function makes no mention of MustParse, and note that ParseAcceptLanguage successfully parsed the language, but calling matcher.Match causes the panic.

Also note that the panic is not "failed to parse", it's an index out of bounds exception...

@seankhliao also note there's no direct calls to MustParse or a panic anywhere in those functions. Furthermore, calling MustParse with the language "us-zz" does not panic: https://play.golang.org/p/A3M_rOYffT-

This is a bug, please re-open.

Reopening for evaluation.

I see, but is there a way to trigger it from the exposed api?

Yes, that's what led me to file a bug. It's currently impacting a production service: https://github.com/google/exposure-notifications-verification-server/blob/79e397e0f917c78f1d3fe388beeb25e14928095f/internal/i18n/i18n.go#L93

right, reproducer using public api:

package main

import (
	"fmt"

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

func main() {
	desired, _, err := language.ParseAcceptLanguage("en-ZZ")
	fmt.Println(desired, err)

	matcher := language.NewMatcher([]language.Tag{language.English})
	t, i, c := matcher.Match(desired...)
	fmt.Println(t, i, c)
}

CC: @mpvl

This was handed off by the COVID project. Any chance this can get a bump in priority for a fix? They have a workaround using recover(), but my understanding is that it's kinda kludgy.

Change https://golang.org/cl/305469 mentions this issue: language: fix off-by-one error