cmd/compile: comparable does not match each comparable type

[blaubaer]

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

$ go version
go version devel go1.18-a064a4f29a Sat Feb 26 01:16:03 2022 +0000 windows/amd64

Does this issue reproduce with the latest release?

With the latest revision on master (a064a4f29a97a4fc7398d1ac9d7c53c5ba0bc646).

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

go env Output

$ go env
set GO111MODULE=on
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\protected\AppData\Local\go-build
set GOENV=C:\Users\protected\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\protected\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\protected\go
set GOPRIVATE=
set GOWORK=
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -fmessage-length=0 -fdebug-prefix-map=C:\Users\protected\AppData\Local\Temp\go-build4145725796=/tmp/go-build -gno-record-gcc-switches

What did you do?

language.Tag as Map key

package main

import (
	"fmt"

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

func main() {
	aMap := map[language.Tag]string{
		language.German: "foo",
	}
	fmt.Println(aMap)
}

language.Tag as comparable generic parameter

package main

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

func doSomething[K comparable](with map[K]string) {
	fmt.Println("comparable", with)
}

func main() {
	aMap := map[language.Tag]string{
		language.German: "foo",
	}
	doSomething(aMap)
}

The following code

What did you expect to see?

1. language.Tag as Map key: Is accepted by the compiler.
2. language.Tag as comparable generic parameter: Is accepted by the compiler.

What did you see instead?

1. language.Tag as Map key: Is accepted by the compiler.
2. language.Tag as comparable generic parameter: Compiler refuses compiling with

   foo_broken.go:16:13: "golang.org/x/text/language".Tag does not implement comparable
   

ℹ️ I'm pretty sure that this was working at least ~1 week ago with the master. With revision a289e9ce7514a34cd930469322395bf0e89b59ea) it was working.

[thanm]

Biesction points to 163da6feb525a98dab5c1f01d81b2c705ead51ea as the bad commit, introduced in CL 387055.

@griesemer @findleyr

[findleyr]

The reason for this is that Tag contains fullTag, which is an interface. This is discussed in more detail in #51257 (comment)

This is a confusing UX, particularly when the interface is buried deep inside the type. Here the interface is only one level deep, but one can imagine how confusing it could be if the interface were inside several layers of nested structs.

IMO this warrants reconsideration.

[blaubaer]

Please correct me if I'm wrong. The specs defines the same rules for map keys as for stuff that implements comparable, or? If so either the map implementation in Go is broken in general or how 163da6feb525a98dab5c1f01d81b2c705ead51ea change it was too restrictive. I would highly recommend to align both behaviors.

[findleyr]

Please correct me if I'm wrong. The specs defines the same rules for map keys as for stuff that implements comparable

To clarify: no, map keys treat all interfaces as comparable, whereas the comparable builtin treats only interfaces that are statically known to be comparable as comparable.

[ianlancetaylor]

Closing as dup of #50646 and #51257 and #51338.

[changkun]

P.S. I guess Ian was meant to say #51338 other than #51138.

[ianlancetaylor]

Thanks, I edited my note.