builtin/error: late assignment of custom error type cause nil checking of the error variable not working

[bsthun]

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

$ go version
go version go1.18.3 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/bsthun/Library/Caches/go-build"
GOENV="/Users/bsthun/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Volumes/Data/Environments/gopath/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Volumes/Data/Environments/gopath"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/opt/go/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/opt/go/libexec/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.18.3"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/zc/z09bkbz52kjd29vyb4jjz7rw0000gn/T/go-build613377742=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

After declare an error variable with type of error, and assign its value with a function that returned custom error (a struct with an interface of error). It will have issue when checking nil (err != nil) as the statement will always returnd true even the real value of the variable is nil. You can reproduce the bug using playground: https://go.dev/play/p/otd6w4e7nK1.

What did you expect to see?

The condition statement should return proper value when doing nil check. (if the value of variable err is nil, then err != nil should return false)

What did you see instead?

Even err (which declare as a type error, then later assign to a custom error) has value of nil but the condition err != nil is always returned true.

[ZekeLu]

Under the covers, interfaces are implemented as two elements, a type T and a value V.
...
An interface value is nil only if the V and T are both unset, (T=nil, V is not set), In particular, a nil interface will always hold a nil type. If we store a nil pointer of type *int inside an interface value, the inner type will be *int regardless of the value of the pointer: (T=*int, V=nil). Such an interface value will therefore be non-nil even when the pointer value V inside is nil.

See https://go.dev/doc/faq#nil_error.

[seankhliao]

Unlike many projects, the Go project does not use GitHub Issues for general discussion or asking questions. GitHub Issues are used for tracking bugs and proposals only.

For questions please refer to https://github.com/golang/go/wiki/Questions