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

$ go version
go version go1.21.0 darwin/arm64

Does this issue reproduce with the latest release?

Yes.

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

$ go env
GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/gopher/Library/Caches/go-build'
GOENV='/Users/gopher/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/gopher/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/gopher/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.21.0'
GCCGO='gccgo'
AR='ar'
CC='clang'
CXX='clang++'
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/s_/5sjqzr0j6xggz_xtzmq_8r4m00jwcj/T/go-build4187513592=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

I tried building a Go program, while using GOTOOLCHAIN to select an older version of the Go toolchain (1.20.7 instead of 1.21.0), that should fail because it imports a package that doesn't exist in the Go standard library at that version:

package p

import _ "pkgnotinstd"

$ GOTOOLCHAIN=go1.20.7 go build

(The real thing I was doing was using GOTOOLCHAIN=<older toolchain> go test ./... to check that my change that starts to use Go 1.21's new "slices" package in a build-constrained .go file didn't accidentally break with older supported Go releases too.)

What did you expect to see?

An error message that mentions the name of the package. For example, something like:

p.go:3:8: package pkgnotinstd is not in std (/usr/local/go/src/pkgnotinstd)

Or more realistically, the less-improved error message as implemented in Go 1.20.7:

p.go:3:8: package pkgnotinstd is not in GOROOT (/usr/local/go/src/pkgnotinstd)

What did you see instead?

An error message that says "" instead of the package name or path:

p.go:3:8: import "": import of unknown directory

CC @bcmills, @matloob, @rsc.