iter: range over seq (variable of type iter.Seq[T]) must have one iteration variable

[j178]

Go version

go version devel go1.22-46ea4ab Sat Dec 9 21:48:06 2023 +0000 darwin/amd64

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

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/Users/jo/Library/Caches/go-build'
GOENV='/Users/jo/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT='rangefunc'
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/jo/go/pkg/mod'
GONOPROXY=''
GOOS='darwin'
GOPATH='/Users/jo/go'
GOPRIVATE=''
GOROOT='/Users/jo/sdk/gotip'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/jo/sdk/gotip/pkg/tool/darwin_amd64'
GOVCS=''
GOVERSION='devel go1.22-46ea4ab Sat Dec 9 21:48:06 2023 +0000'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='0'
GOMOD='/Users/jo/code/it/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch x86_64 -m64 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/sw/0cn9x8611_xdn6c20pxd96m40000ks/T/go-build14739506649094477967=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

package it

import "iter"

func Count[T any](seq iter.Seq[T]) (cnt int) {
	for range seq {  // does not compile
		cnt++
	}
	return
}

func Count2[T any](seq iter.Seq[T]) (cnt int) {
	for _ = range seq { // compiles ok
		cnt++
	}
	return
}

What did you expect to see?

This program compiles ok.

What did you see instead?

# command-line-arguments
./f.go:6:12: range over seq (variable of type iter.Seq[T]) must have one iteration variable

Since Count does care about the value, from previous experience of for-range, we tend to omit the blank identifier, but that is a compile error.

[callthingsoff]

I see a TODO here:
https://github.com/golang/go/blob/46ea4ab5cb87e9e5d443029f5f1a4bba012804d3/src/internal/types/testdata/spec/range.go#L29C10-L29C10

	// TODO: Would be nice to 'for range T.M' and 'for range (*T).PM' directly,
	// but there is no gofmt-friendly way to write the error pattern in the right place.

[prattmic]

cc @rsc

[rsc]

Duplicate of #65236.