errors: errors.Join does not skip nil error interface implementations

[VolkerLieber]

Go version

go1.23.0

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/root/.cache/go-build'
GOENV='/root/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/root/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/root/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/snap/go/10743'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/snap/go/10743/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.23.3'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/root/.config/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/root/errors-test/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1931872224=/tmp/go-build -gno-record-gcc-switches'

What did you do?

package main

import (
	"errors"
	"reflect"
	"testing"
)

// Error interface implementation
type TestError struct {
}

func (te TestError) Error() string {
	return "test error"
}

func TestJoin(t *testing.T) {
	err1 := errors.New("err1")
	err2 := errors.New("err2")

	var nilerror *TestError

	for _, test := range []struct {
		errs []error
		want []error
	}{{
		errs: []error{err1, nilerror, err2},
		want: []error{err1, err2},
	}} {
		got := errors.Join(test.errs...).(interface{ Unwrap() []error }).Unwrap()
		if !reflect.DeepEqual(got, test.want) {
			t.Errorf("Join(%v) = %v; want %v", test.errs, got, test.want)
		}
		if len(got) != cap(got) {
			t.Errorf("Join(%v) returns errors with len=%v, cap=%v; want len==cap", test.errs, len(got), cap(got))
		}
	}
}

What did you see happen?

Join([err1 err2]) = [err1 err2]; want [err1 err2]

What did you expect to see?

ok

The err!=nil check returns false for all passed nil pointers to an error interface implementation, resulting in the nil error being included in the constructed error.

[gabyhelp]

Related Issues

• errors: add example around unwrapping errors.Join #69586 (closed)
• errors: Using `errors.Join` on a `joinError` should expand the existing joined error #60209 (closed)
• errors: Is(nil) behaves unexpectedly #40442 (closed)
• Nil check for custom errors in some situations are incorrect #31873 (closed)
• Nil-ness of a pointer is lost when converted to an interface #42306 (closed)
• error interface, is nil or not? #33994 (closed)
• Nil pointer to struct implementing error interface is nil, but error(x) == nil returns false #24947 (closed)
• Nil is not nill in some cases #70364 (closed)
• `err != nil` when `err == nil` with custom `error` type #35292 (closed)
• builtin/error: late assignment of custom error type cause nil checking of the error variable not working #53768 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[VolkerLieber]

Ok, thanks to the linked tickets, it seems like this is "intended": https://go.dev/doc/faq#nil_error
This still seems to me like a bug rather than a feature.
What do you think?

[seankhliao]

It's consistent with how other error checks work