cmd/go: Go modules initialised by go1.21.1 cannot be built using go1.20.8

[soh-ew]

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

$ go version 1.21.1

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='arm64'
GOBIN=''
GOCACHE='/Users/enwei.soh/Library/Caches/go-build'
GOENV='/Users/enwei.soh/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/enwei.soh/gopath/pkg/mod'
GONOPROXY='none'
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/enwei.soh/gopath'
GOPRIVATE=''
GOPROXY='proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.21.1/libexec' 
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.21.1/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.21.1'
GCCGO='gccgo'
AR='ar'
CC='cc'
CXX='c++'
CGO_ENABLED='1'
GOMOD='/dev/null'
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 arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/40/90j35z011n14czs8pbg736rr0000gn/T/go-build3498449936=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

We are attempting to make builds for both current and previous Go language versions (1.21 and 1.20). However, this does not work for Go modules initialised by go1.21.1, or any go directive set to 1.21.1 in the go.mod file, as go1.20.8 does not recognise the go directive with a X.Y.Z format.

1. Run go mod init in an empty folder
2. add a dummy main file (e.g. https://go.dev/play/p/UPxh4-pRY1V)
3. Run GOTOOLCHAIN=go1.20.8 go build main.go

What did you expect to see?

main binary created.

What did you see instead?

go: errors parsing go.mod:
/Users/enwei.soh/gopath/src/testgo21/go.mod:3: invalid go version '1.21.1': must match format 1.23

[bcmills]

This is working as designed in #57001.

To select the minimum go version using Go 1.21 or higher, you can use go get go@1.20 or similar.

With older Go versions (back to Go 1.17), you can use go mod tidy -go=1.20.

[soh-ew]

Thanks for the response!

Just to clarify, does that mean that moving forward:

1. If I want to build for the latest and previous go versions (e.g. go1.22 and go1.21), my Go modules will need to enforce a maximum go directive of the previous go version go1.21
2. Extending from the above point, my go1.21 Go modules cannot use other modules as dependencies if these other modules declare the go directive to be higher than go1.21 (as per discussion in cmd/go: go get -u && go mod tidy behaves differently starting with Go 1.21.0, leading to unknown directive toolchain errors #62409 )

Is this understanding correct?

[seankhliao]

yes, besides the fact that the new versions should include patch: go1.21.0 and go1.22.0