spec: document/explain which interfaces implement comparable #50646
Description
What version of Go are you using (go version)?
$ go version go version go1.18beta1 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="on" GOARCH="amd64" GOBIN="" GOCACHE="/home/bobg/.cache/go-build" GOENV="/home/bobg/.config/go/env" GOEXE="" GOEXPERIMENT="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="linux" GOINSECURE="" GOMODCACHE="/home/bobg/go/pkg/mod" GONOPROXY="" GONOSUMDB="" GOOS="linux" GOPATH="/home/bobg/go" GOPRIVATE="" GOPROXY="https://proxy.golang.org,direct" GOROOT="/home/bobg/sdk/go1.18beta1" GOSUMDB="sum.golang.org" GOTMPDIR="" GOTOOLDIR="/home/bobg/sdk/go1.18beta1/pkg/tool/linux_amd64" GOVCS="" GOVERSION="go1.18beta1" GCCGO="gccgo" GOAMD64="v1" AR="ar" CC="gcc" CXX="g++" CGO_ENABLED="1" GOMOD="/home/bobg/go/src/github.com/bobg/modver/go.mod" GOWORK="" 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-build3923542930=/tmp/go-build -gno-record-gcc-switches"
What did you do?
https://go.dev/play/p/S-ijx-XOlu8?v=gotip
var (
anyType = types.Universe.Lookup("any").Type()
comparableType = types.Universe.Lookup("comparable").Type()
)
fmt.Println(types.AssignableTo(comparableType, anyType))
fmt.Println(types.AssignableTo(anyType, comparableType))What did you expect to see?
true
false
That is, I (naively?) expected the same check that prevents me using non-comparable types as map keys to tell me that I cannot assign a value of an unconstrained type to a variable of a comparable type.
What did you see instead?
true
true