text/template: slice function does not work when value is wrapped in interface

[icza]

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

$ go version
go version go1.13.3 linux/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="/home/icza/.cache/go-build"
GOENV="/home/icza/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY="xxx"
GONOSUMDB="xxx"
GOOS="linux"
GOPATH="/home/icza/gows"
GOPRIVATE="xxx"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/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-build339874310=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Tried to use the slice template function introduced in Go 1.13.

Playground:

t := template.Must(template.New("").Parse(`{{slice .x 0 2}}`))

m1 := map[string][]int{
	"x": []int{1, 2, 3, 4},
}
fmt.Println(t.Execute(os.Stdout, m1))

m2 := map[string]interface{}{
	"x": []int{1, 2, 3, 4},
}
fmt.Println(t.Execute(os.Stdout, m2))

It only works if value is not wrapped in an interface value (e.g. interface{}). In my opinion, a concrete slice value stored in an interface should be "extracted" and used.

What did you expect to see?

[1 2]<nil>
[1 2]<nil>

What did you see instead?

[1 2]<nil>
template: :1:2: executing "" at <slice .x 0 2>: error calling slice: reflect: call of reflect.Value.Slice on interface Value

[mvdan]

Thanks for reporting this; I think I've spotted the bug.

[gopherbot]

Change https://golang.org/cl/211877 mentions this issue: text/template: indirect interfaces before slicing

[gopherbot]

Change https://golang.org/cl/212117 mentions this issue: text/template: make reflect.Value indirections more robust