cmd/compile: confusing errors when forgetting an import for an anonymous field

[mvdan]

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

go version devel +a0e7f12771 Sat Sep 29 10:07:58 2018 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes - checked 1.11.

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/mvdan/go/cache"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/mvdan/go/land:/home/mvdan/go"
GOPROXY=""
GORACE=""
GOROOT="/home/mvdan/tip"
GOTMPDIR=""
GOTOOLDIR="/home/mvdan/tip/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-build472648887=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Forget to add an import for an anonymous struct field.

What did you expect to see?

What I see when the field isn't anonymous:

$ cat f.go
package p

type t struct {
        F sync.Mutex
}
$ go build f.go
# command-line-arguments
./f.go:4:4: undefined: sync

What did you see instead?

A confusing error about sync, and a redundant error about Mutex:

$ cat f.go
package p

type t struct {
        sync.Mutex
}
$ go build f.go
# command-line-arguments
./f.go:4:2: sync is not a package
./f.go:4:2: undefined: Mutex

/cc @griesemer

[griesemer]

Indeed. Could use a better error message. Not urgent (ok to postpone if we don't get to it for 1.12).

[hellozee]

I would like to give it a try, if anyone could give me some pointers?

[griesemer]

FWIW, go/types reports: "undeclared name: sync" which seems clearer.

@hellozee Not sure if this is a great "starter issue" in the compiler as it seems pretty central to how name lookup works - but I'm just guessing; haven't looked into this yet. But basically 1) you'd have to find which part of the code reports "sync is not a package" and why it's not complaining about sync not being defined instead. And 2) mark the respective node with Broke or SetDiag so that we don't get a 2nd error.

[hellozee]

@griesemer sure

[hellozee]

@griesemer the files in question would be src/cmd/compile/internal/gc/noder.go , packname method of the noder structure. And src/cmd/compile/internal/gc/typecheck.go, typecheck1 function.
Am I going in the right direction?

[griesemer]

@hellozee Yes, but it seems like this issue requires a bit more thought since the problem appears during "noding". It may be possible to avoid this with some restructuring, but that wouldn't be a "starter issue".

[hellozee]

Ohh better to leave it to experts then, thanks for heads up though @griesemer

[gopherbot]

Change https://golang.org/cl/144259 mentions this issue: cmd/compile: better errors for structs with conflicting fields and methods

Edit: referred to wrong issue in CL by mistake.

[gopherbot]

Change https://golang.org/cl/144261 mentions this issue: cmd/compile: better error for embedded field referring to missing import