Syntax issue with Go compiler when using two-value assignment from map to struct member

[peteArnt]

Please answer these questions before submitting your issue. Thanks!

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

go version devel +73497c7 Thu Nov 10 01:34:03 2016 +0000 linux/amd64

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

GOARCH="amd64"
GOBIN="/home/peter/mygo/bin"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/peter/mygo"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="/usr/bin/gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build003323332=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

https://play.golang.org/p/dIcyfHcos7

What did you expect to see?

A successful compilation

What did you see instead?

tmp/sandbox205861443/main.go:18: non-name x.S on left side of :=

It appears that using the idiomatic map lookup pattern, there is a problem assigning the retrieved map value to a structure member (as opposed to a simple local variable). This occurs when a status variable (i.e. "ok") is used with the ":=" operator.
x.S, ok := m[2]
where: x is a local structure with member S, and ok is created within the statement due to the :=
See the playground link above.

[bradfitz]

That's how the language is defined. This isn't an implementation bug.

[peteArnt]

Brad,
Thanks for your timely response; however I am still confused about this.
This seems inconsistent. In one case I am assigning into a declared struct (x); in the other I am assigning to a declared scalar (s). Used in the context of v, ok := m[n] where v is either x.S or s, it looks like you're telling me I can only use a scalar variable for v? If this is truly how it is defined, obviously I'll abide by that. I just do not see why there's a distinction.
After looking at the language spec for a while, it still wasn't obvious to me that it was defined like this.

If there is a better forum to discuss this topic, I'd be glad to migrate. Let me know.
Thanks,
-Pete

[zigo101]

@peteArnt
this problem is not related to map. It is a famous known design behaviour/issue/flaw (or whatever).
But now the implementation is really consistent with Go spec: the right of := must be pure identifiers, can't be object fields, array/slice/map elements or pointer dereferences.

https://golang.org/ref/spec#Short_variable_declarations

#6842

[bradfitz]

If there is a better forum to discuss this topic, I'd be glad to migrate. Let me know.

See https://golang.org/wiki/Questions