cmd/go2go: cannot create a map of which key is a type parameter of a generic type.

[YoshikiShibata]

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

$ go version
go version devel +85ed317eb2 Thu Jul 2 00:33:43 2020 +0000 darwin/amd64

Does this issue reproduce with the latest release?

N/A

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/yoshiki/Library/Caches/go-build"
GOENV="/Users/yoshiki/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/yoshiki/gocode/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/yoshiki/gocode:/Users/yoshiki/exercises/gpl:/Users/yoshiki/oak"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/yoshiki/tools/go2"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/yoshiki/tools/go2/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/r3/f30_9n8958j4ndrjyl1k7mp40000gn/T/go-build832033261=/tmp/go-build -gno-record-gcc-switches -fno-common"
GOROOT/bin/go version: go version devel +85ed317eb2 Thu Jul 2 00:33:43 2020 +0000 darwin/amd64
GOROOT/bin/go tool compile -V: compile version devel +85ed317eb2 Thu Jul 2 00:33:43 2020 +0000
uname -v: Darwin Kernel Version 19.5.0: Tue May 26 20:41:44 PDT 2020; root:xnu-6153.121.2~2/RELEASE_X86_64
ProductName:    Mac OS X
ProductVersion: 10.15.5
BuildVersion:   19F101
lldb --version: lldb-1103.0.22.10
Apple Swift version 5.2.4 (swiftlang-1103.0.32.9 clang-1103.0.32.53)

What did you do?

I tried to create a map of which key is a type parameter of a generic type.

package main

import "fmt"

type Foo(type T) struct {
}

func (f *Foo(T)) bar() {
    seen := make(map[T]bool)
    fmt.Printf("seen: %T\n", seen)
}

func main() {
    f := &Foo(int){}
    f.bar()
}

What did you expect to see?

Can compile

What did you see instead?

$ go tool go2go run map.go2
type checking failed for main
/var/folders/r3/f30_9n8958j4ndrjyl1k7mp40000gn/T/go2go-run727879803/map.go2:9:19: invalid map key type T

[tdakkota]

T should be comparable

package main

import "fmt"

type Foo(type T comparable) struct {
}

func (f *Foo(T)) bar() {
    seen := make(map[T]bool)
    fmt.Printf("seen: %T\n", seen)
}

func main() {
    f := &Foo(int){}
    f.bar()
}

https://go2goplay.golang.org/p/mswUb5qvKUX

[ianlancetaylor]

@tdakkota is right. I don't see a bug here.

[YoshikiShibata]

I see. Thanks!