go: "go" directive in modules does not prevent using new stdlib code

[Groxx]

Go version

go version go1.24.2 darwin/arm64

Output of go env in your module/workspace:

❯ go env
AR='ar'
CC='cc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='c++'
GCCGO='gccgo'
GO111MODULE=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/stevenl/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/stevenl/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/zp/sqm92pj94sq7whfh1rl3yjd00000gn/T/go-build3995360884=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/stevenl/go/src/go.uber.org/cadence/go.mod'
GOMODCACHE='/Users/stevenl/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/stevenl/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.24.2/libexec'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/stevenl/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.24.2/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.24.2'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

go build ./... in a project using 1.24-specific stdlib packages/methods, but a 1.21-specific go.mod file.

What did you see happen?

Build (and run) succeeded.

Demo in the playground: https://go.dev/play/p/qqJqFz5G8et

What did you expect to see?

Given the go 1.21 version in go.mod, I expected the build to fail as if it was built using go 1.21, e.g. GOTOOLCHAIN=go1.21.1 go build ./... which produces errors like

prog.go:5:2: package weak is not in std (/usr/local/go-faketime/src/weak)

But this does not happen.

This too-old-version build failure is currently reproducible in the playground with the previous go version, e.g. https://go.dev/play/p/qqJqFz5G8et?v=goprev which uses go 1.23, but I'm not sure how to target specific versions to make a stable link.

The go module reference seems to imply the go version should protect against new Go things:

The go directive affects use of new language features:

• For packages within the module, the compiler rejects use of language features introduced after the version specified by the go directive. For example, if a module has the directive go 1.12, its packages may not use numeric literals like 1_000_000, which were introduced in Go 1.13.

But perhaps this is only for "language features" like generics, not "language features" like the standard library? TBH I'm not sure that distinction makes much sense, the two are inseparable for essentially all users.

go vet also does not complain about this, nor does go mod tidy try to upgrade the target version. Currently this seems to mean that we need to set up a manual test matrix to build with multiple Go versions, just to check something that can be statically determined.
This seems... subpar?

[seankhliao]

As the play snippet shows, it's a vet check, not a build error.
See previously #46136

[gabyhelp]

Related Issues

• Should the `go` directive in `go.mod` a semver version (including patch), or not? #68971 (closed)
• cmd/go: Go modules initialised by go1.21.1 cannot be built using go1.20.8 #63279 (closed)
• cmd/go: using newer standard library APIs with an incompatible go version in go.mod does not produce error #47802
• cmd/go: go get -u && go mod tidy behaves differently starting with Go 1.21.0, leading to unknown directive toolchain errors #62409 (closed)
• go/types: error "type instantiation requires go1.18 or later" is unclear when go version comes from go.mod #52880
• go build ignores GoVersion in go.mod file #58420 (closed)
• golang.org/x/*: go directive of 1.23.0 is forcing toolchain directives in anything not using exactly go1.23.0 #71864 (closed)
• go mod tidy fails in older version of Go if new stdlib package is imported #43400 (closed)
• modules: "updates to go.mod needed" when I test 1.20 code using 1.21+ #65751 (closed)

Related Documentation

• Go Modules Reference > go.mod files > go directive

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

[Groxx]

The snippet doesn't seem to show it being a vet check? It just outputs

demo of https://github.com/golang/go/issues/73603

Program exited.

Also running this locally and then go vet . does not produce any output at all:

❯ bat *
───────┬─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
       │ File: go.mod
───────┼─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
   1   │ module example.com
   2   │
   3   │ go 1.21
───────┴─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
───────┬─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
       │ File: main.go
───────┼─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
   1   │ package main
   2   │
   3   │ import (
   4   │     "fmt"
   5   │     "testing"
   6   │     "weak"
   7   │ )
   8   │
   9   │ func main() {
  10   │     fmt.Println("demo of https://github.com/golang/go/issues/73603")
  11   │ }
  12   │
  13   │ func ignored() {
  14   │     (&testing.T{}).Context()      // new method introduced in 1.24: https://pkg.go.dev/testing@master#T.Context
  15   │     _ = weak.Make((*string)(nil)) // new package in 1.24
  16   │ }
───────┴─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────

❯ go vet .

❯ go vet -stdversion .

❯ go vet -stdversion

❯ go vet -stdversion ./...

❯ go version
go version go1.24.2 darwin/arm64

[Groxx]

@seankhliao ^ I believe this might deserve re-opening. Maybe as a vet bug report.

[seankhliao]

It does work on tip (e.g. playground dev branch), so if there was a bug, it was fixed.