Description
(go 1.18 - details at the bottom)
affected/package: base
What did you do?
Attempt to make a generic map. A simple example is at https://go.dev/play/p/odFEAA0i5uZ?v=gotip
What did you expect to see?
It is possible to have a type parameter representing any type usable for the key of a map
What did you see instead?
It is not possible to have a type parameter representing any type usable for the key of a map. Specifically the key type any cannot be used in a type parameter, whereas it is possible to use directly as a key with map[].
For instance:
type MapBool[K any] struct {
m map[K]bool
}
Fails because K has a missing comparable constraint.
However, if we use
type MapBool[K comparable] struct {
m map[K]bool
}
then
type m3 = MapBool[any]
fails because any does not implement comparable
However
type m1 = map[any]bool
works just fine.
This makes it difficult (impossible?) to implement a generic with the same flexibility as map (FWIW I was trying to make a generic based sync.map). It seems that map requiring a comparable might be overconstrained as map[] appears to be able to successfully compare interface{} (i.e. any)
What version of Go are you using (go version)?
$ go version go1.18rc1
Does this issue reproduce with the latest release?
Yes
What operating system and processor architecture are you using (go env)?
go env Output
GO111MODULE="" GOARCH="amd64" GOBIN="" GOCACHE="/Users/amb/Library/Caches/go-build" GOENV="/Users/amb/Library/Application Support/go/env" GOEXE="" GOEXPERIMENT="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="darwin" GOINSECURE="" GOMODCACHE="/Users/amb/go/pkg/mod" GONOPROXY="" GONOSUMDB="" GOOS="darwin" GOPATH="/Users/amb/go" GOPRIVATE="" GOPROXY="https://proxy.golang.org,direct" GOROOT="/usr/local/go" GOSUMDB="sum.golang.org" GOTMPDIR="" GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64" GOVCS="" GOVERSION="go1.18rc1" GCCGO="gccgo" GOAMD64="v1" AR="ar" CC="clang" CXX="clang++" CGO_ENABLED="1" GOMOD="/Users/amb/go/src/github.com/abligh/gsyncmap/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 -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/_r/zj8cp1n90vjgfjgm2clvqvjh0000gn/T/go-build3224811868=/tmp/go-build -gno-record-gcc-switches -fno-common"