cmd/vet: declared but not used

[sabey]

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

go version go1.10 linux/amd64

Does this issue reproduce with the latest release?

yes

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

amd64

What did you do?

https://play.golang.org/p/w8ySRRDrihm

package main

type Server struct {
	Counter func()
}

func main() {
	counter := 0
	server := &Server{
		Counter: func() {
			counter = 1
		},
	}
	for i := 0; i < 100; i++ {
		server.Counter()
	}
}

go vet // fails, didn't fail pre go1.10
go run main.go // no problem

if you change counter = 1 to counter++ or counter = counter + 1 there is no error

What did you expect to see?

no error

What did you see instead?

./main.go:8:2: counter declared but not used
vet: typecheck failures

[dgryski]

Assignment-only expressions don't count as "use" -- you need a read. Both ++ and counter + 1 are reads.

[ALTree]

The fact that gc should issue an error (and it doesn't) is #3059.

Also this is not vet reporting an issue, but go/types correctly raising an error during typechecking. I guess vet was changed during the 1.10 cycle to print typechecking errors instead of ignoring them.

[ALTree]

Found the discussion, is at #21287 (cmd/vet: decide how to handle type checking failure). Final decision was:

So the decision here is "go vet" will be fixed so that it can always typecheck, and then failure to typecheck will be a fatal error.

Explicitly mentioning this change in the 1.10 release notes would have been nice, but I don't see anything in https://golang.org/doc/go1.10#vet

Anyway, vet side, this is working as expected. We still have the gc issue I linked above.

[ianlancetaylor]

I don't think there is anything to do here. The vet error is correct, even if the compiler proper doesn't catch it. Closing.