encoding/gob: Encoder.Encode fails to encode maps with keys of type url.URL

[ghost]

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

$ go version
go version go1.12.5 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
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/xxx/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/xxx/gocode"
GOPROXY=""
GORACE=""
GOROOT="/home/xxx/go"
GOTMPDIR=""
GOTOOLDIR="/home/xxx/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
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-build478246212=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/8-8Qws7ogyH

What did you expect to see?

Gob encoder successfully encodes a value of type map[url.URL]something.

What did you see instead?

There's an error: gob: unaddressable value of type *url.URL, despite the fact that the map keys are of type url.URL, not *url.URL.

[mvdan]

Perhaps this is just the error being confusing. One cannot take the address of map keys or values directly, so that's probably the error that the encoder is hitting. Perhaps it could copy the key value to then take the copy's address, but I wonder if that could be unexpected behavior. For example, what if the key type is large and copying is a non-trivial amount of work?

Funnily enough, the spec makes no mention of map key/values not being addressable. It seems to be a limitation of the current Go implementation.

[mvdan]

To clarify my previous comment - the String method uses a pointer receiver, so it can only be called on a non-pointer value if it's addressable. See https://golang.org/pkg/net/url/#URL.String

[ghost]

With a custom type: https://play.golang.org/p/jZJr0eYPal5

The problem seems to be the pointer receiver in MarshalBinary. With a non-pointer receiver encoding/gob works fine, even if I add a String method with a pointer receiver: https://play.golang.org/p/dK7SsqahtAd