appending two slices with default ends modifies base slice

[xeviknal]

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

$ go version
1.13.7

Does this issue reproduce with the latest release?

Yes, go1.14

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/xcanalma/.cache/go-build"
GOENV="/home/xcanalma/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/xcanalma/.gvm/pkgsets/go1.14/global"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/xcanalma/.gvm/gos/go1.14"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/xcanalma/.gvm/gos/go1.14/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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-build278629003=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/ys0shvSiHkW

What did you expect to see?

I expect that both append and slice[] operator don't modify the slice variable.

[2 3 5 7 11 13]
[2 3 11 13]
[2 3 5 7 11 13]

What did you see instead?

[2 3 5 7 11 13]
[2 3 11 13]
[2 3 11 13 11 13]

[renthraysk]

If don't want to modify the backing memory of a slice with append, limit the capacity, which will force append to allocate a new slice plus memory.

s2 := append(s[0:2:2], s[4:]...)

[dmitshur]

I believe the behavior you're seeing matches what is written in the Go specification. Do you see an inconsistency?

Append operates on a slice, and a slice has an underlying array. If two slices share common parts of an underlying array, then modifying one slice can cause the other slice to have different data. To avoid that, the two slices should not share the same underlying array.

Also see https://golang.org/wiki/SliceTricks.

[xeviknal]

Thanks for the explanation. Once the behind the scenes of a slice is clear, the append methods makes more sense. However, it looks like the passed array is read-only and a new array is returned.

Didn't know the 3-index operand. Interesting one. Thanks and closing the issue.