reflect: reflect.Value.IsZero results false positive

[changkun]

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

$ go version
go version go1.13 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=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/changkun/Library/Caches/go-build"
GOENV="/Users/changkun/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/changkun/dev/golang"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.13/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.13/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/changkun/dev/sched/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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/fr/hk6cn7m95bn7n0p054d35fc80000gp/T/go-build674759185=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

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

What did you expect to see?

The expected output is true because reflect.New creates a zero value

What did you see instead?

Actual output is false.

[mvdan]

reflect.New returns a pointer to a new value, so it's by definition non-nil, and non-zero.

[changkun]

@mvdan This is quite non-intuitive.
What is the decisive reason for reflect.New that creates a pointer to the value?
What is the correct way to check the newly created (actual) value by reflect.New is a zero value?

[mvdan]

See the docs at https://golang.org/pkg/reflect/#New. It works exactly like new, and has been like that for years, so we can't change it even if we wanted to.

If you want to check if the data behind the pointer is zero, do ptr.Elem().IsZero(). You should check the documentation to see why that works.

In the future, please try asking questions about reflect in the forums or Slack before filing bugs. The package takes a while to master, so most problems tend to not be bugs in Go itself.