err != nil when err == nil with custom error type

[jamoozy]

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

$ go version
go version go1.13.3 linux/amd64

Though I think this works on all recent versions ... ?

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/jamoozy/.cache/go-build"
GOENV="/home/jamoozy/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/jamoozy/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/jamoozy/lib/go1.13.3"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/jamoozy/lib/go1.13.3/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/jamoozy/silo/go/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-build549669925=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/wYWX4cDk3rc

What did you expect to see?

I expected err == nil to return true.

What did you see instead?

I saw err == nil: false in the output. I suspect this has to do with what is mentioned in the laws of reflection: that instead of err having a pure nil value, it's pointing to the (value, type) tuple (nil, *Err). However, I don't understand why this would be desirable.

Thank you for your time!

[gertcuykens]

Correct because, interface has type and value
https://golang.org/doc/faq#nil_error

If you look at it from a interface perspective it just follows the rules as all other interfaces

[jamoozy]

Many thanks for the response! I'm still curious why this is the desired behavior, though. What kind of code relies on nil comparisons acting this way?

[ianlancetaylor]

You may want to look at #22729.

Basically, nil is overloaded. It is the zero value of both pointer types and interface types (and also map, slice, function, and channel types). It is entirely reasonable to store a nil pointer into a variable of interface type. Such a value is no longer the zero value of its type, so comparing it with nil (this time the nil interface value, not the nil pointer value) must be false.