Nil check for custom errors in some situations are incorrect

[vishen]

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

$ go version
go version go1.12.1 linux/amd64

Does this issue reproduce with the latest release?

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/pentecostjonathan/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/pentecostjonathan/go"
GOPROXY=""
GORACE=""
GOROOT="/home/pentecostjonathan/src/go"
GOTMPDIR=""
GOTOOLDIR="/home/pentecostjonathan/src/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/pentecostjonathan/src/coral-health/coralcloud/go.mod"
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-build298044920=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I have a custom error (implementing the Error interface). I have a function that calls two functions; the first function returns a nil error and and the second returns nil for my custom error. If I call the first function and assign it to err and then I call the second and accidentally assign it to the same err variable; a nil check always seems to return true.

type S struct{}

func (s *S) Error() string {
        return "error for S"
}

func structNil() *S {
        return nil
}

func errorNil() error {
        return nil
}

func main() {
        err := errorNil()
        fmt.Println(err != nil)
        err = structNil()
        fmt.Println(err != nil)
	err = errorNil()
	fmt.Println(err != nil)
}

false
true
false

https://play.golang.org/p/4edPo-lMy6G

I am unsure if this is expected or not, but it took me quite a while to track down exactly why my code was behaving weirdly. I realise it was a mistake on my behalf for assigning my custom error to an error type variable, but since the compiler seemed happy I didn't realise I was doing anything wrong.

What did you expect to see?

That all nil checks return the correct result.

What did you see instead?

A nil check on a custom error type always returns true.

[Bo0mer]

This is working as expected. See https://golang.org/doc/faq#nil_error.

[vishen]

Ah completely missed that, thanks!