x/text/language: Match() choses the wrong Tag

[Flamefork]

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

$ go version
go version go1.14 linux/amd64

Does this issue reproduce with the latest release?

Yes.

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build210008815=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/_qlIzcAWD5F

package main

import (
	"fmt"
	"golang.org/x/text/language"
)

func main() {
	matcher := language.NewMatcher([]language.Tag{
		language.MustParse("en-GB"),
		language.MustParse("fr-FR"),
		language.MustParse("es-ES"),
	})
	tag, _, confidence := matcher.Match(
		language.MustParse("en"),
		language.MustParse("fr"),
		language.MustParse("en-US"),
	)
	fmt.Printf("best match: %s, confidence: %s\n", tag, confidence)
}

What did you expect to see?

best match: en-GB, confidence: High

What did you see instead?

best match: fr-FR, confidence: Exact

[ianlancetaylor]

CC @mpvl

[ameowlia]

👋 Hi @Flamefork,

I recently investigated a nearly identical bug here. Please go there to read lots of details.

My short summary is:

• This is working as expected (even if it is very surprising)
• If a person lists the languages+local they know and the languages are not continuous (like you have in the example with english then french then english), then it is assumed they speak all of those languages equally well and it will not give extra preference to the first exact language match it finds.
• "en" defaults to "en-US", which is not an exact match.
• "fr" defaults to "fr-FR", which is an exact match. This is the first exact match so this is the one chosen.