cmd/vet: unable to detect unreachable code

[ghost]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   go version go1.7rc1 windows/amd64
2. What operating system and processor architecture are you using (go env)?
   set GOARCH=amd64
   set GOBIN=
   set GOEXE=.exe
   set GOHOSTARCH=amd64
   set GOHOSTOS=windows
   set GOOS=windows
   set GOPATH=C:/work/go/
   set GORACE=
   set GOROOT=c:\go
   set GOTOOLDIR=c:\go\pkg\tool\windows_amd64
   set CC=gcc
   set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=c:\temp\go-build108228690=/tmp/go-build -gno-record-gcc-switches
   set CXX=g++
   set CGO_ENABLED=1
3. What did you do?
   go tool vet -unreachable main.go

// main.go
package main
const flag = false
func main() {
    if flag {
        println("<<unreachable code>>")
    }
}

1. What did you expect to see?
   a warning from vet
2. What did you see instead?
   no warnings

[josharian]

I don't think we'd want vet to complain about such code. That's a fairly typical way to add debugging code, particularly in hot functions where you can't accept any runtime overhead when not debugging.

[ghost]

It turns out that it is impossible to determine what code was eliminated?

package main
const erraneous = (1 << 0) - 1
func main() {
    if erraneous == 1 {
            println("dead code")
    }
}

Code is silently eliminated by the compiler. And no warnings from vet.
Maybe it makes sense to add -report-eliminated-code flag to the compiler or vet? Just an idea.

[josharian]

We try fairly strenuously to avoid flags.

[robpike]

Working as intended, as @josharian explained.