cp go.mod.21 go.mod
go list -f '{{.Module.GoVersion}} {{.DefaultGODEBUG}}'
! stdout panicnil
stdout randautoseed=0

# Go 1.21 work module should NOT set panicnil=1 in Go 1.20 dependency.
cp go.mod.21 go.mod
go list -f '{{.Module.GoVersion}} {{.DefaultGODEBUG}}' q
! stdout panicnil=1
! stdout randautoseed

go mod download rsc.io/panicnil # for go.sum
go list -f '{{.Module.GoVersion}} {{.DefaultGODEBUG}}' rsc.io/panicnil

	if p.Name != "main" {
		return ""
	}
	goVersion := modload.MainModules.GoVersion()
	if modload.RootMode == modload.NoRoot && p.Module != nil {
		// This is go install pkg@version or go run pkg@version.
		// Use the Go version from the package.
		// If there isn't one, then assume Go 1.20,
		// the last version before GODEBUGs were introduced.
		goVersion = p.Module.GoVersion
		if goVersion == "" {
			goVersion = "1.20"
		}
	}

	// 1) types.Info.FileVersion, which follows the cascade:
	//   1.a) file version (ast.File.GoVersion),
	//   1.b) the package version (types.Config.GoVersion), or
	// 2) is some unknown Future version.
	//
	// File versions require a valid package version to be provided to types
	// in Config.GoVersion. Config.GoVersion is either from the package's module
	// or the toolchain (go run). This value should be provided by go/packages

package types2

import (
	"fmt"
	"go/version"
	"internal/goversion"
)

// A goVersion is a Go language version string of the form "go1.%d"
// where d is the minor version number. goVersion strings don't
// contain release numbers ("go1.20.1" is not a valid goVersion).
type goVersion string

// asGoVersion returns v as a goVersion (e.g., "go1.20.1" becomes "go1.20").

go mod edit -dropgodebug key
cmpenv go.mod go.mod.start

-- x.go --
package x

-- w/w.go --
package w

-- $WORK/go.mod.init --
module x.x/y/z

go $goversion
-- $WORK/go.mod.edit1 --
module x.x/y/z

go $goversion

require x.1 v1.0.0

exclude (
	x.1 v1.2.0
	x.1 v1.2.1
	x.1 v2.0.0+incompatible
)

replace (
	x.1 v1.3.0 => y.1 v1.4.0
	x.1 v1.4.0 => ../z
)

			s.addTooNew(e)
		}

	case interface{ Unwrap() error }:
		s.addTooNew(err.Unwrap())

	case *gover.TooNewError:
		if s.TooNew == nil ||
			gover.Compare(err.GoVersion, s.TooNew.GoVersion) > 0 ||
			gover.Compare(err.GoVersion, s.TooNew.GoVersion) == 0 && err.What < s.TooNew.What {
			s.TooNew = err
		}
	}
}

// NeedSwitch reports whether Switch would attempt to switch toolchains.

		if prog.Program == meta["Program"] && prog.Version == meta["Version"] &&
			prog.GoVersion == meta["GoVersion"] && prog.GOOS == meta["GOOS"] &&
			prog.GOARCH == meta["GOARCH"] {
			return prog
		}
	}
	prog := telemetry.ProgramReport{
		Program:   meta["Program"],
		Version:   meta["Version"],
		GoVersion: meta["GoVersion"],
		GOOS:      meta["GOOS"],
		GOARCH:    meta["GOARCH"],

	modload.ForceUseModules = true
	modload.RootMode = modload.NeedRoot

	goVersion := tidyGo.String()
	if goVersion != "" && gover.Compare(gover.Local(), goVersion) < 0 {
		toolchain.SwitchOrFatal(ctx, &gover.TooNewError{
			What:      "-go flag",
			GoVersion: goVersion,
		})
	}

	modload.LoadPackages(ctx, modload.PackageOpts{
		TidyGoVersion:            tidyGo.String(),

				mainModule,
				{Path: "go", Version: goVersion},
				{Path: "toolchain", Version: gover.LocalToolchain()},
			}
		} else {
			goVersion = gover.Local()
			pruning = pruningForGoVersion(goVersion)
			roots = []module.Version{
				{Path: "go", Version: goVersion},
				{Path: "toolchain", Version: gover.LocalToolchain()},
			}
		}
		rawGoVersion.Store(mainModule, goVersion)

}

func main() {
	telemetry.Start()
	flag.Usage = usage
	flag.Parse()
	telemetry.Inc("fix/invocations")
	telemetry.CountFlags("fix/flag:", *flag.CommandLine)

	if !version.IsValid(*goVersion) {
		report(fmt.Errorf("invalid -go=%s", *goVersion))
		os.Exit(exitCode)
	}

	sort.Sort(byDate(fixes))

	if *allowedRewrites != "" {
		allowed = make(map[string]bool)
		for _, f := range strings.Split(*allowedRewrites, ",") {