text/template: index/element switched for iter.Seq2 in range

[bep]

Go version

go1.24.2 darwin/arm64

Output of go env in your module/workspace:

AR='ar'
CC='clang'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='clang++'
GCCGO='gccgo'
GO111MODULE=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/bep/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/bep/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/_g/j3j21hts4fn7__h04w2x8gb40000gn/T/go-build279183488=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/bep/dev/go/bep/temp/go.mod'
GOMODCACHE='/Users/bep/dev/gomod_cache'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/bep/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='on'
GOTELEMETRYDIR='/Users/bep/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.24.2'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

package main

import (
	"bytes"
	"fmt"
	"iter"
	"log"
	"slices"
	"strings"
	"text/template"
)

type MyStrings []string

func (c MyStrings) Backward() iter.Seq2[int, string] {
	return slices.Backward(c)
}

func main() {
	data := map[string]interface{}{
		"abc": MyStrings{"a", "b", "c"},
	}
	tpl := `
Range: {{ range .abc }}{{ . }}{{ end }}
Range $i, $e: {{ range $i, $e := .abc }}{{ $e }}{{ end }}
Range $i, $e dot: {{ range $i, $e := .abc }}{{ . }}{{ end }}
Range $e dot: {{ range $e := .abc }}{{ . }}{{ end }}
Backward: {{ range .abc.Backward }}{{ . }}{{ end }}
Backward $i, $e: {{ range  $i, $e := .abc.Backward }}{{ $e }}{{ end }}
Backward $i, $e dot: {{ range  $i, $e := .abc.Backward }}{{ . }}{{ end }}
Backward $e dot: {{ range $e := .abc.Backward }}{{ . }}{{ end }}
`

	var buf bytes.Buffer
	tmpl, err := template.New("").Parse(tpl)
	if err != nil {
		log.Fatal(err)
	}

	if err := tmpl.Execute(&buf, data); err != nil {
		log.Fatal(err)
	}
	result := strings.TrimSpace(buf.String())
	fmt.Println(result)
}

What did you see happen?

Range: abc
Range $i, $e: abc
Range $i, $e dot: abc
Range $e dot: abc
Backward: 210
Backward $i, $e: cba
Backward $i, $e dot: cba
Backward $e dot: 210

What did you expect to see?

Range: abc
Range $i, $e: abc
Range $i, $e dot: abc
Range $e dot: abc
Backward: cba
Backward $i, $e: cba
Backward $i, $e dot: cba
Backward $e dot: cba

[gabyhelp]

Related Issues

• iter: `iter.Seq` returns incorrect values for subsequent next calls #64646 (closed)
• cmd/compile: memory corruption when setting outer values in functional iterators #70035 (closed)
• cmd/compile: ICE compiling rangefunc for < 1.23 in combination with Cgo #67497 (closed)
• affected/package: #57101 (closed)
• import/path: issue title #69851 (closed)
• text/template: key/value assignment is reversed within range loop #60801 (closed)
• cmd/vet: index of of range error running test with GOEXPERIMENT=aliastypeparams #71665 (closed)
• testing: iterators break t.Helper() #72794
• panic when range-func return nil #69042 (closed)
• error with range can't traverse "\xd5\xbc\xbd\xbe" #25306 (closed)

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

[qiulaidongfeng]

Existing implementation conforms to the specification
See https://go.dev/ref/spec#For_statements

Range expression                                       1st value                2nd value

function, 2 values  f  func(func(K, V) bool)           key      k  K            v          V

[bep]

@qiulaidongfeng this issue is about the range func in Go Templates. If you apply the Go Language spec to Go Templates, you will find many discrepancies.

Some additional arguments to my initial post:

• A range over a map/slice or a range over a basic iterator of that same map/slice should behave the same. This is how it works for regular Go, and anything else would be a big surprise.
• This difference in behaviour requires that the end user knows/cares about the difference between a slice/map and an iterator, which becomes especially problematic in a more dynamically typed scripting language.
• It also means that it's not possible to replace slices/maps with a struct without breaking the API, which is incredibly unfortunate.

[dmitshur]

CC @robpike, @mvdan.

[qiulaidongfeng]

See https://go.dev/doc/go1.24#minor_library_changes

Templates now support range-over-func and range-over-int.

not to support other new semantics.
I think the description in the go specification is the definition of range-over-func.

By the way, it has been released by go1.24, so, according to https://go.dev/doc/go1compat, for backward compatibility, we should not change.