cmd/go: go tool panic when GOEXPERIMENT=jsonv2 #74862
Description
What version of Go are you using (go version)?
$ go version go version go1.25rc2 darwin/arm64
Does this issue reproduce with the latest release?
What operating system and processor architecture are you using (go env)?
go env Output
$ go env 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/bmizerany/Library/Caches/go-build' GOCACHEPROG='' GODEBUG='' GOENV='/Users/bmizerany/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/db/svmm3t1x3yn4d1skpbq3ddv00000gn/T/go-build3403534159=/tmp/go-build -gno-record-gcc-switches -fno-common' GOHOSTARCH='arm64' GOHOSTOS='darwin' GOINSECURE='' GOMOD='/Users/bmizerany/src/foo/go.mod' GOMODCACHE='/Users/bmizerany/go/pkg/mod' GONOPROXY='github.com/ollama/*' GONOSUMDB='github.com/ollama/*' GOOS='darwin' GOPATH='/Users/bmizerany/go' GOPRIVATE='github.com/ollama/*' GOPROXY='https://proxy.golang.org,direct' GOROOT='/Users/bmizerany/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25rc2.darwin-arm64' GOSUMDB='sum.golang.org' GOTELEMETRY='on' GOTELEMETRYDIR='/Users/bmizerany/Library/Application Support/go/telemetry' GOTMPDIR='' GOTOOLCHAIN='auto' GOTOOLDIR='/Users/bmizerany/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25rc2.darwin-arm64/pkg/tool/darwin_arm64' GOVCS='' GOVERSION='go1.25rc2' GOWORK='' PKG_CONFIG='pkg-config' GOROOT/bin/go version: go version go1.25rc2 darwin/arm64 GOROOT/bin/go tool compile -V: compile version go1.25rc2 uname -v: Darwin Kernel Version 24.5.0: Tue Apr 22 19:52:00 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6031 ProductName: macOS ProductVersion: 15.5 BuildVersion: 24F74 lldb --version: lldb-1700.0.9.502 Apple Swift version 6.1.2 (swiftlang-6.1.2.1.2 clang-1700.0.13.5)
What did you do?
; go env -w GOEXPERIMENT=jsonv2
; go version
What did you expect to see?
The go version printed to stdout.
What did you see instead?
Given this package:
-- go.mod --
module example
go 1.25rc2
-- example.go --
package main
func main() { println("hello world") }
The panic is reproduced like:
; go env -w GOEXPERIMENT=jsonv2
; go version
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x3 pc=0x103246bf0]
goroutine 1 [running]:
cmd/go/internal/fips140.Init()
cmd/go/internal/fips140/fips140.go:117 +0x120
cmd/go/internal/modload.Init()
cmd/go/internal/modload/init.go:419 +0x38
cmd/go/internal/toolchain.Exec({0x14000196690, 0x9})
cmd/go/internal/toolchain/select.go:356 +0x46c
cmd/go/internal/toolchain.Select()
cmd/go/internal/toolchain/select.go:284 +0xf28
main.main()
cmd/go/main.go:107 +0x58
NOTE: This also happens for go env, go build, etc..
If I unset the env var (using the toolchain of a previous version of Go), it fixes the issue:
; go1.23.4 env -w GOEXPERIMENT=
; go version
go version go1.25rc2 darwin/arm64
If I move outside of the package directory (say ~), I see, unsurprisingly:
; go version
go: unknown GOEXPERIMENT jsonv2