diff --git a/doc/go1.17.html b/doc/go1.17.html
index be3b4e6d71..eab4e1eeeb 100644
--- a/doc/go1.17.html
+++ b/doc/go1.17.html
@@ -58,6 +58,17 @@ Do not send CLs removing the interior tags from such phrases.
   <!-- TODO(bcmills): replace the design-doc link with proper documentation. -->
 </p>
 
+<p><!-- golang.org/issue/45094 --> To facilitate the upgrade to lazy loading,
+  the <code>go</code> <code>mod</code> <code>tidy</code> subcommand now supports
+  a <code>-go</code> flag to set or change the <code>go</code> version in
+  the <code>go.mod</code> file. To enable lazy loading for an existing module
+  without changing the selected versions of its dependencies, run:
+</p>
+
+<pre>
+  go mod tidy -go=1.17
+</pre>
+
 <h4 id="module-deprecation-comments">Module deprecation comments</h4>
 
 <p><!-- golang.org/issue/40357 -->
diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index 0a12eaf4e9..052b61c03d 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -1221,7 +1221,7 @@
 //
 // Usage:
 //
-// 	go mod tidy [-e] [-v]
+// 	go mod tidy [-e] [-v] [-go=version]
 //
 // Tidy makes sure go.mod matches the source code in the module.
 // It adds any missing modules necessary to build the current module's
@@ -1235,6 +1235,12 @@
 // The -e flag causes tidy to attempt to proceed despite errors
 // encountered while loading packages.
 //
+// The -go flag causes tidy to update the 'go' directive in the go.mod
+// file to the given version, which may change which module dependencies
+// are retained as explicit requirements in the go.mod file.
+// (Go versions 1.17 and higher retain more requirements in order to
+// support lazy module loading.)
+//
 // See https://golang.org/ref/mod#go-mod-tidy for more about 'go mod tidy'.
 //
 //
diff --git a/src/cmd/go/internal/modcmd/tidy.go b/src/cmd/go/internal/modcmd/tidy.go
index a9b277817f..c72ec30a57 100644
--- a/src/cmd/go/internal/modcmd/tidy.go
+++ b/src/cmd/go/internal/modcmd/tidy.go
@@ -12,10 +12,12 @@ import (
 	"cmd/go/internal/imports"
 	"cmd/go/internal/modload"
 	"context"
+
+	"golang.org/x/mod/modfile"
 )
 
 var cmdTidy = &base.Command{
-	UsageLine: "go mod tidy [-e] [-v]",
+	UsageLine: "go mod tidy [-e] [-v] [-go=version]",
 	Short:     "add missing and remove unused modules",
 	Long: `
 Tidy makes sure go.mod matches the source code in the module.
@@ -30,16 +32,26 @@ to standard error.
 The -e flag causes tidy to attempt to proceed despite errors
 encountered while loading packages.
 
+The -go flag causes tidy to update the 'go' directive in the go.mod
+file to the given version, which may change which module dependencies
+are retained as explicit requirements in the go.mod file.
+(Go versions 1.17 and higher retain more requirements in order to
+support lazy module loading.)
+
 See https://golang.org/ref/mod#go-mod-tidy for more about 'go mod tidy'.
 	`,
 	Run: runTidy,
 }
 
-var tidyE bool // if true, report errors but proceed anyway.
+var (
+	tidyE  bool   // if true, report errors but proceed anyway.
+	tidyGo string // go version to write to the tidied go.mod file (toggles lazy loading)
+)
 
 func init() {
 	cmdTidy.Flag.BoolVar(&cfg.BuildV, "v", false, "")
 	cmdTidy.Flag.BoolVar(&tidyE, "e", false, "")
+	cmdTidy.Flag.StringVar(&tidyGo, "go", "", "")
 	base.AddModCommonFlags(&cmdTidy.Flag)
 }
 
@@ -48,6 +60,12 @@ func runTidy(ctx context.Context, cmd *base.Command, args []string) {
 		base.Fatalf("go mod tidy: no arguments allowed")
 	}
 
+	if tidyGo != "" {
+		if !modfile.GoVersionRE.MatchString(tidyGo) {
+			base.Fatalf(`go mod: invalid -go option %q; expecting something like "-go 1.17"`, tidyGo)
+		}
+	}
+
 	// Tidy aims to make 'go test' reproducible for any package in 'all', so we
 	// need to include test dependencies. For modules that specify go 1.15 or
 	// earlier this is a no-op (because 'all' saturates transitive test
@@ -62,6 +80,7 @@ func runTidy(ctx context.Context, cmd *base.Command, args []string) {
 	modload.RootMode = modload.NeedRoot
 
 	modload.LoadPackages(ctx, modload.PackageOpts{
+		GoVersion:                tidyGo,
 		Tags:                     imports.AnyTags(),
 		Tidy:                     true,
 		VendorModulesInGOROOTSrc: true,
diff --git a/src/cmd/go/internal/modload/build.go b/src/cmd/go/internal/modload/build.go
index 53771b2231..76e1ad589f 100644
--- a/src/cmd/go/internal/modload/build.go
+++ b/src/cmd/go/internal/modload/build.go
@@ -259,8 +259,8 @@ func moduleInfo(ctx context.Context, rs *Requirements, m module.Version, mode Li
 		if m.GoVersion == "" && checksumOk("/go.mod") {
 			// Load the go.mod file to determine the Go version, since it hasn't
 			// already been populated from rawGoVersion.
-			if summary, err := rawGoModSummary(mod); err == nil && summary.goVersionV != "" {
-				m.GoVersion = summary.goVersionV[1:]
+			if summary, err := rawGoModSummary(mod); err == nil && summary.goVersion != "" {
+				m.GoVersion = summary.goVersion
 			}
 		}
 
diff --git a/src/cmd/go/internal/modload/buildlist.go b/src/cmd/go/internal/modload/buildlist.go
index 4b83ede541..e7af892996 100644
--- a/src/cmd/go/internal/modload/buildlist.go
+++ b/src/cmd/go/internal/modload/buildlist.go
@@ -299,7 +299,7 @@ func readModGraph(ctx context.Context, depth modDepth, roots []module.Version) (
 			// sufficient to build the packages it contains. We must load its full
 			// transitive dependency graph to be sure that we see all relevant
 			// dependencies.
-			if depth == eager || summary.depth() == eager {
+			if depth == eager || summary.depth == eager {
 				for _, r := range summary.require {
 					enqueue(r, eager)
 				}
@@ -393,7 +393,7 @@ func LoadModGraph(ctx context.Context) *ModuleGraph {
 		base.Fatalf("go: %v", err)
 	}
 
-	commitRequirements(ctx, rs)
+	commitRequirements(ctx, modFileGoVersion(), rs)
 	return mg
 }
 
@@ -459,7 +459,7 @@ func EditBuildList(ctx context.Context, add, mustSelect []module.Version) (chang
 	if err != nil {
 		return false, err
 	}
-	commitRequirements(ctx, rs)
+	commitRequirements(ctx, modFileGoVersion(), rs)
 	return changed, err
 }
 
@@ -943,10 +943,40 @@ func updateEagerRoots(ctx context.Context, direct map[string]bool, rs *Requireme
 	if err != nil {
 		return rs, err
 	}
-	if reflect.DeepEqual(min, rs.rootModules) && reflect.DeepEqual(direct, rs.direct) {
-		// The root set is unchanged, so keep rs to preserve its cached ModuleGraph
-		// (if any).
+	if rs.depth == eager && reflect.DeepEqual(min, rs.rootModules) && reflect.DeepEqual(direct, rs.direct) {
+		// The root set is unchanged and rs was already eager, so keep rs to
+		// preserve its cached ModuleGraph (if any).
 		return rs, nil
 	}
-	return newRequirements(rs.depth, min, direct), nil
+	return newRequirements(eager, min, direct), nil
+}
+
+// convertDepth returns a version of rs with the given depth.
+// If rs already has the given depth, convertDepth returns rs unmodified.
+func convertDepth(ctx context.Context, rs *Requirements, depth modDepth) (*Requirements, error) {
+	if rs.depth == depth {
+		return rs, nil
+	}
+
+	if depth == eager {
+		// We are converting a lazy module to an eager one. The roots of an eager
+		// module graph are a superset of the roots of a lazy graph, so we don't
+		// need to add any new roots — we just need to prune away the ones that are
+		// redundant given eager loading, which is exactly what updateEagerRoots
+		// does.
+		return updateEagerRoots(ctx, rs.direct, rs, nil)
+	}
+
+	// We are converting an eager module to a lazy one. The module graph of an
+	// eager module includes the transitive dependencies of every module in the
+	// build list.
+	//
+	// Hey, we can express that as a lazy root set! “Include the transitive
+	// dependencies of every module in the build list” is exactly what happens in
+	// a lazy module if we promote every module in the build list to a root!
+	mg, err := rs.Graph(ctx)
+	if err != nil {
+		return rs, err
+	}
+	return newRequirements(lazy, mg.BuildList()[1:], rs.direct), nil
 }
diff --git a/src/cmd/go/internal/modload/edit.go b/src/cmd/go/internal/modload/edit.go
index 2921b38157..c350b9d1b5 100644
--- a/src/cmd/go/internal/modload/edit.go
+++ b/src/cmd/go/internal/modload/edit.go
@@ -229,7 +229,7 @@ func raiseLimitsForUpgrades(ctx context.Context, maxVersion map[string]string, d
 			if err != nil {
 				return err
 			}
-			if summary.depth() == eager {
+			if summary.depth == eager {
 				// For efficiency, we'll load all of the eager upgrades as one big
 				// graph, rather than loading the (potentially-overlapping) subgraph for
 				// each upgrade individually.
@@ -522,7 +522,7 @@ func (l *versionLimiter) check(m module.Version, depth modDepth) dqState {
 		return l.disqualify(m, dqState{err: err})
 	}
 
-	if summary.depth() == eager {
+	if summary.depth == eager {
 		depth = eager
 	}
 	for _, r := range summary.require {
diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index ef21908064..88d647d9ea 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -381,7 +381,7 @@ var errGoModDirty error = goModDirtyError{}
 func LoadModFile(ctx context.Context) *Requirements {
 	rs, needCommit := loadModFile(ctx)
 	if needCommit {
-		commitRequirements(ctx, rs)
+		commitRequirements(ctx, modFileGoVersion(), rs)
 	}
 	return rs
 }
@@ -401,8 +401,9 @@ func loadModFile(ctx context.Context) (rs *Requirements, needCommit bool) {
 	if modRoot == "" {
 		Target = module.Version{Path: "command-line-arguments"}
 		targetPrefix = "command-line-arguments"
-		rawGoVersion.Store(Target, latestGoVersion())
-		requirements = newRequirements(index.depth(), nil, nil)
+		goVersion := latestGoVersion()
+		rawGoVersion.Store(Target, goVersion)
+		requirements = newRequirements(modDepthFromGoVersion(goVersion), nil, nil)
 		return requirements, false
 	}
 
@@ -432,7 +433,7 @@ func loadModFile(ctx context.Context) (rs *Requirements, needCommit bool) {
 	}
 
 	setDefaultBuildMod() // possibly enable automatic vendoring
-	rs = requirementsFromModFile(ctx, f)
+	rs = requirementsFromModFile(ctx)
 
 	if cfg.BuildMod == "vendor" {
 		readVendorList()
@@ -440,27 +441,23 @@ func loadModFile(ctx context.Context) (rs *Requirements, needCommit bool) {
 		rs.initVendor(vendorList)
 	}
 	if index.goVersionV == "" {
-		// The main module necessarily has a go.mod file, and that file lacks a
-		// 'go' directive. The 'go' command has been adding that directive
-		// automatically since Go 1.12, so this module either dates to Go 1.11 or
-		// has been erroneously hand-edited.
-		//
-		// The semantics of the go.mod file are more-or-less the same from Go 1.11
-		// through Go 1.16, changing at 1.17 for lazy loading. So even though a
-		// go.mod file without a 'go' directive is theoretically a Go 1.11 file,
-		// scripts may assume that it ends up as a Go 1.16 module. We can't go
-		// higher than that, because we don't know which semantics the user intends.
-		//
-		// (Note that 'go mod init' always adds the latest version, so scripts that
-		// use 'go mod init' will result in current-version modules instead of Go
-		// 1.16 modules.)
-		//
-		// If we are able to modify the go.mod file, we will add a 'go' directive
-		// to at least make the situation explicit going forward.
-		if cfg.BuildMod == "mod" {
-			addGoStmt("1.16")
+		// TODO(#45551): Do something more principled instead of checking
+		// cfg.CmdName directly here.
+		if cfg.BuildMod == "mod" && cfg.CmdName != "mod graph" && cfg.CmdName != "mod why" {
+			addGoStmt(latestGoVersion())
+			if go117EnableLazyLoading {
+				// We need to add a 'go' version to the go.mod file, but we must assume
+				// that its existing contents match something between Go 1.11 and 1.16.
+				// Go 1.11 through 1.16 have eager requirements, but the latest Go
+				// version uses lazy requirements instead — so we need to cnvert the
+				// requirements to be lazy.
+				rs, err = convertDepth(ctx, rs, lazy)
+				if err != nil {
+					base.Fatalf("go: %v", err)
+				}
+			}
 		} else {
-			rawGoVersion.Store(Target, "1.16")
+			rawGoVersion.Store(Target, modFileGoVersion())
 		}
 	}
 
@@ -509,7 +506,7 @@ func CreateModFile(ctx context.Context, modPath string) {
 		base.Fatalf("go: %v", err)
 	}
 
-	commitRequirements(ctx, requirementsFromModFile(ctx, modFile))
+	commitRequirements(ctx, modFileGoVersion(), requirementsFromModFile(ctx))
 
 	// Suggest running 'go mod tidy' unless the project is empty. Even if we
 	// imported all the correct requirements above, we're probably missing
@@ -661,12 +658,13 @@ func initTarget(m module.Version) {
 	}
 }
 
-// requirementsFromModFile returns the set of non-excluded requirements from f.
-func requirementsFromModFile(ctx context.Context, f *modfile.File) *Requirements {
-	roots := make([]module.Version, 0, len(f.Require))
+// requirementsFromModFile returns the set of non-excluded requirements from
+// the global modFile.
+func requirementsFromModFile(ctx context.Context) *Requirements {
+	roots := make([]module.Version, 0, len(modFile.Require))
 	mPathCount := map[string]int{Target.Path: 1}
 	direct := map[string]bool{}
-	for _, r := range f.Require {
+	for _, r := range modFile.Require {
 		if index != nil && index.exclude[r.Mod] {
 			if cfg.BuildMod == "mod" {
 				fmt.Fprintf(os.Stderr, "go: dropping requirement on excluded version %s %s\n", r.Mod.Path, r.Mod.Version)
@@ -683,7 +681,7 @@ func requirementsFromModFile(ctx context.Context, f *modfile.File) *Requirements
 		}
 	}
 	module.Sort(roots)
-	rs := newRequirements(index.depth(), roots, direct)
+	rs := newRequirements(modDepthFromGoVersion(modFileGoVersion()), roots, direct)
 
 	// If any module path appears more than once in the roots, we know that the
 	// go.mod file needs to be updated even though we have not yet loaded any
@@ -988,12 +986,12 @@ func WriteGoMod(ctx context.Context) {
 	if !allowWriteGoMod {
 		panic("WriteGoMod called while disallowed")
 	}
-	commitRequirements(ctx, LoadModFile(ctx))
+	commitRequirements(ctx, modFileGoVersion(), LoadModFile(ctx))
 }
 
 // commitRequirements writes sets the global requirements variable to rs and
 // writes its contents back to the go.mod file on disk.
-func commitRequirements(ctx context.Context, rs *Requirements) {
+func commitRequirements(ctx context.Context, goVersion string, rs *Requirements) {
 	requirements = rs
 
 	if !allowWriteGoMod {
@@ -1014,6 +1012,9 @@ func commitRequirements(ctx context.Context, rs *Requirements) {
 		})
 	}
 	modFile.SetRequire(list)
+	if goVersion != "" {
+		modFile.AddGoStmt(goVersion)
+	}
 	modFile.Cleanup()
 
 	dirty := index.modFileIsDirty(modFile)
diff --git a/src/cmd/go/internal/modload/list.go b/src/cmd/go/internal/modload/list.go
index 344b2aa2c7..ccdeb9b1d1 100644
--- a/src/cmd/go/internal/modload/list.go
+++ b/src/cmd/go/internal/modload/list.go
@@ -72,7 +72,7 @@ func ListModules(ctx context.Context, args []string, mode ListMode) ([]*modinfo.
 	}
 
 	if err == nil {
-		commitRequirements(ctx, rs)
+		commitRequirements(ctx, modFileGoVersion(), rs)
 	}
 	return mods, err
 }
diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index ddacf49ead..f434b399d8 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/src/cmd/go/internal/modload/load.go
@@ -49,7 +49,7 @@ package modload
 // Because "go mod vendor" prunes out the tests of vendored packages, the
 // behavior of the "all" pattern with -mod=vendor in Go 1.11–1.15 is the same
 // as the "all" pattern (regardless of the -mod flag) in 1.16+.
-// The allClosesOverTests parameter to the loader indicates whether the "all"
+// The loader uses the GoVersion parameter to determine whether the "all"
 // pattern should close over tests (as in Go 1.11–1.15) or stop at only those
 // packages transitively imported by the packages and tests in the main module
 // ("all" in Go 1.16+ and "go mod vendor" in Go 1.11+).
@@ -121,6 +121,7 @@ import (
 	"cmd/go/internal/str"
 
 	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
 )
 
 // loaded is the most recently-used package loader.
@@ -133,6 +134,14 @@ var loaded *loader
 
 // PackageOpts control the behavior of the LoadPackages function.
 type PackageOpts struct {
+	// GoVersion is the Go version to which the go.mod file should be updated
+	// after packages have been loaded.
+	//
+	// An empty GoVersion means to use the Go version already specified in the
+	// main module's go.mod file, or the latest Go version if there is no main
+	// module.
+	GoVersion string
+
 	// Tags are the build tags in effect (as interpreted by the
 	// cmd/go/internal/imports package).
 	// If nil, treated as equivalent to imports.Tags().
@@ -305,12 +314,15 @@ func LoadPackages(ctx context.Context, opts PackageOpts, patterns ...string) (ma
 
 	initialRS, _ := loadModFile(ctx) // Ignore needCommit — we're going to commit at the end regardless.
 
+	if opts.GoVersion == "" {
+		opts.GoVersion = modFileGoVersion()
+	}
+
 	ld := loadFromRoots(ctx, loaderParams{
 		PackageOpts:  opts,
 		requirements: initialRS,
 
-		allClosesOverTests: index.allPatternClosesOverTests() && !opts.UseVendorAll,
-		allPatternIsRoot:   allPatternIsRoot,
+		allPatternIsRoot: allPatternIsRoot,
 
 		listRoots: func(rs *Requirements) (roots []string) {
 			updateMatches(rs, nil)
@@ -368,7 +380,7 @@ func LoadPackages(ctx context.Context, opts PackageOpts, patterns ...string) (ma
 
 	// Success! Update go.mod and go.sum (if needed) and return the results.
 	loaded = ld
-	commitRequirements(ctx, loaded.requirements)
+	commitRequirements(ctx, opts.GoVersion, loaded.requirements)
 
 	for _, pkg := range ld.pkgs {
 		if !pkg.isTest() {
@@ -593,21 +605,22 @@ func ImportFromFiles(ctx context.Context, gofiles []string) {
 		base.Fatalf("go: %v", err)
 	}
 
+	goVersion := modFileGoVersion()
 	loaded = loadFromRoots(ctx, loaderParams{
 		PackageOpts: PackageOpts{
+			GoVersion:             goVersion,
 			Tags:                  tags,
 			ResolveMissingImports: true,
 			SilencePackageErrors:  true,
 		},
-		requirements:       rs,
-		allClosesOverTests: index.allPatternClosesOverTests(),
+		requirements: rs,
 		listRoots: func(*Requirements) (roots []string) {
 			roots = append(roots, imports...)
 			roots = append(roots, testImports...)
 			return roots
 		},
 	})
-	commitRequirements(ctx, loaded.requirements)
+	commitRequirements(ctx, goVersion, loaded.requirements)
 }
 
 // DirImportPath returns the effective import path for dir,
@@ -743,6 +756,12 @@ func Lookup(parentPath string, parentIsStd bool, path string) (dir, realPath str
 type loader struct {
 	loaderParams
 
+	// allClosesOverTests indicates whether the "all" pattern includes
+	// dependencies of tests outside the main module (as in Go 1.11–1.15).
+	// (Otherwise — as in Go 1.16+ — the "all" pattern includes only the packages
+	// transitively *imported by* the packages and tests in the main module.)
+	allClosesOverTests bool
+
 	work *par.Queue
 
 	// reset on each iteration
@@ -757,8 +776,7 @@ type loaderParams struct {
 	PackageOpts
 	requirements *Requirements
 
-	allClosesOverTests bool // Does the "all" pattern include the transitive closure of tests of packages in "all"?
-	allPatternIsRoot   bool // Is the "all" pattern an additional root?
+	allPatternIsRoot bool // Is the "all" pattern an additional root?
 
 	listRoots func(rs *Requirements) []string
 }
@@ -903,6 +921,22 @@ func loadFromRoots(ctx context.Context, params loaderParams) *loader {
 		work:         par.NewQueue(runtime.GOMAXPROCS(0)),
 	}
 
+	if params.GoVersion != "" {
+		if semver.Compare("v"+params.GoVersion, narrowAllVersionV) < 0 && !ld.UseVendorAll {
+			// The module's go version explicitly predates the change in "all" for lazy
+			// loading, so continue to use the older interpretation.
+			// (If params.GoVersion is empty, we are probably not in any module at all
+			// and should use the latest semantics.)
+			ld.allClosesOverTests = true
+		}
+
+		var err error
+		ld.requirements, err = convertDepth(ctx, ld.requirements, modDepthFromGoVersion(params.GoVersion))
+		if err != nil {
+			ld.errorf("go: %v\n", err)
+		}
+	}
+
 	if ld.requirements.depth == eager {
 		var err error
 		ld.requirements, _, err = expandGraph(ctx, ld.requirements)
diff --git a/src/cmd/go/internal/modload/modfile.go b/src/cmd/go/internal/modload/modfile.go
index cd08fa5859..7595db7755 100644
--- a/src/cmd/go/internal/modload/modfile.go
+++ b/src/cmd/go/internal/modload/modfile.go
@@ -51,6 +51,27 @@ const (
 
 var modFile *modfile.File
 
+// modFileGoVersion returns the (non-empty) Go version at which the requirements
+// in modFile are intepreted, or the latest Go version if modFile is nil.
+func modFileGoVersion() string {
+	if modFile == nil {
+		return latestGoVersion()
+	}
+	if modFile.Go == nil || modFile.Go.Version == "" {
+		// The main module necessarily has a go.mod file, and that file lacks a
+		// 'go' directive. The 'go' command has been adding that directive
+		// automatically since Go 1.12, so this module either dates to Go 1.11 or
+		// has been erroneously hand-edited.
+		//
+		// The semantics of the go.mod file are more-or-less the same from Go 1.11
+		// through Go 1.16, changing at 1.17 for lazy loading. So even though a
+		// go.mod file without a 'go' directive is theoretically a Go 1.11 file,
+		// scripts may assume that it ends up as a Go 1.16 module.
+		return "1.16"
+	}
+	return modFile.Go.Version
+}
+
 // A modFileIndex is an index of data corresponding to a modFile
 // at a specific point in time.
 type modFileIndex struct {
@@ -79,6 +100,16 @@ const (
 	eager                 // load all transitive dependencies eagerly
 )
 
+func modDepthFromGoVersion(goVersion string) modDepth {
+	if !go117EnableLazyLoading {
+		return eager
+	}
+	if semver.Compare("v"+goVersion, lazyLoadingVersionV) < 0 {
+		return eager
+	}
+	return lazy
+}
+
 // CheckAllowed returns an error equivalent to ErrDisallowed if m is excluded by
 // the main module's go.mod or retracted by its author. Most version queries use
 // this to filter out versions that should not be used.
@@ -344,32 +375,6 @@ func indexModFile(data []byte, modFile *modfile.File, needsFix bool) *modFileInd
 	return i
 }
 
-// allPatternClosesOverTests reports whether the "all" pattern includes
-// dependencies of tests outside the main module (as in Go 1.11–1.15).
-// (Otherwise — as in Go 1.16+ — the "all" pattern includes only the packages
-// transitively *imported by* the packages and tests in the main module.)
-func (i *modFileIndex) allPatternClosesOverTests() bool {
-	if i != nil && i.goVersionV != "" && semver.Compare(i.goVersionV, narrowAllVersionV) < 0 {
-		// The module explicitly predates the change in "all" for lazy loading, so
-		// continue to use the older interpretation. (If i == nil, we not in any
-		// module at all and should use the latest semantics.)
-		return true
-	}
-	return false
-}
-
-// depth reports the modDepth indicated by the indexed go.mod file,
-// or lazy if the go.mod file has not been indexed.
-func (i *modFileIndex) depth() modDepth {
-	if !go117EnableLazyLoading {
-		return eager
-	}
-	if i != nil && semver.Compare(i.goVersionV, lazyLoadingVersionV) < 0 {
-		return eager
-	}
-	return lazy
-}
-
 // modFileIsDirty reports whether the go.mod file differs meaningfully
 // from what was indexed.
 // If modFile has been changed (even cosmetically) since it was first read,
@@ -396,7 +401,7 @@ func (i *modFileIndex) modFileIsDirty(modFile *modfile.File) bool {
 			return true
 		}
 	} else if "v"+modFile.Go.Version != i.goVersionV {
-		if i.goVersionV == "" && cfg.BuildMod == "readonly" {
+		if i.goVersionV == "" && cfg.BuildMod != "mod" {
 			// go.mod files did not always require a 'go' version, so do not error out
 			// if one is missing — we may be inside an older module in the module
 			// cache, and should bias toward providing useful behavior.
@@ -452,7 +457,8 @@ var rawGoVersion sync.Map // map[module.Version]string
 // module.
 type modFileSummary struct {
 	module     module.Version
-	goVersionV string // GoVersion with "v" prefix
+	goVersion  string
+	depth      modDepth
 	require    []module.Version
 	retract    []retraction
 	deprecated string
@@ -465,20 +471,6 @@ type retraction struct {
 	Rationale string
 }
 
-func (s *modFileSummary) depth() modDepth {
-	if !go117EnableLazyLoading {
-		return eager
-	}
-	// The 'go' command fills in the 'go' directive automatically, so an empty
-	// goVersionV in a dependency implies either Go 1.11 (eager loading) or no
-	// explicit go.mod file at all (no difference between eager and lazy because
-	// the module doesn't specify any requirements at all).
-	if s.goVersionV == "" || semver.Compare(s.goVersionV, lazyLoadingVersionV) < 0 {
-		return eager
-	}
-	return lazy
-}
-
 // goModSummary returns a summary of the go.mod file for module m,
 // taking into account any replacements for m, exclusions of its dependencies,
 // and/or vendoring.
@@ -638,7 +630,10 @@ func rawGoModSummary(m module.Version) (*modFileSummary, error) {
 		}
 		if f.Go != nil && f.Go.Version != "" {
 			rawGoVersion.LoadOrStore(m, f.Go.Version)
-			summary.goVersionV = "v" + f.Go.Version
+			summary.goVersion = f.Go.Version
+			summary.depth = modDepthFromGoVersion(f.Go.Version)
+		} else {
+			summary.depth = eager
 		}
 		if len(f.Require) > 0 {
 			summary.require = make([]module.Version, 0, len(f.Require))
diff --git a/src/cmd/go/testdata/script/mod_go_version_missing.txt b/src/cmd/go/testdata/script/mod_go_version_missing.txt
index ea5050ca3d..aca36a0450 100644
--- a/src/cmd/go/testdata/script/mod_go_version_missing.txt
+++ b/src/cmd/go/testdata/script/mod_go_version_missing.txt
@@ -30,12 +30,26 @@ cmp go.mod go.mod.orig
 stderr 'cannot find package "\." in:\n\t.*[/\\]vendor[/\\]example.com[/\\]badedit$'
 
 # When we set -mod=mod, the go version should be updated immediately,
-# to Go 1.16 (not the current version).
+# to the current version, converting the requirements from eager to lazy.
+#
+# Since we don't know which requirements are actually relevant to the main
+# module, all requirements are added as roots, making the requirements untidy.
 
 go list -mod=mod all
 ! stdout '^example.com/testdep$'
 cmp stdout list-1.txt
-cmp go.mod go.mod.want
+cmpenv go.mod go.mod.untidy
+
+go mod tidy
+cmpenv go.mod go.mod.tidy
+
+# On the other hand, if we jump straight to 'go mod tidy',
+# the requirements remain tidy from the start.
+
+cp go.mod.orig go.mod
+go mod tidy
+cmpenv go.mod go.mod.tidy
+
 
 # The updated version should have been written back to go.mod, so now the 'go'
 # directive is explicit. -mod=vendor should trigger by default, and the stronger
@@ -54,10 +68,24 @@ replace (
 	example.com/dep v0.1.0 => ./dep
 	example.com/testdep v0.1.0 => ./testdep
 )
--- go.mod.want --
+-- go.mod.untidy --
 module example.com/m
 
-go 1.16
+go $goversion
+
+require (
+	example.com/dep v0.1.0
+	example.com/testdep v0.1.0 // indirect
+)
+
+replace (
+	example.com/dep v0.1.0 => ./dep
+	example.com/testdep v0.1.0 => ./testdep
+)
+-- go.mod.tidy --
+module example.com/m
+
+go $goversion
 
 require example.com/dep v0.1.0
 
diff --git a/src/cmd/go/testdata/script/mod_load_replace_mismatch.txt b/src/cmd/go/testdata/script/mod_load_replace_mismatch.txt
index dd386f1628..2ca8b3cace 100644
--- a/src/cmd/go/testdata/script/mod_load_replace_mismatch.txt
+++ b/src/cmd/go/testdata/script/mod_load_replace_mismatch.txt
@@ -18,6 +18,6 @@ package use
 import _ "rsc.io/quote"
 
 -- want --
-go mod download: rsc.io/quote@v1.5.2 (replaced by example.com/quote@v1.5.2): parsing go.mod:
+go: rsc.io/quote@v1.5.2 (replaced by example.com/quote@v1.5.2): parsing go.mod:
 	module declares its path as: rsc.io/Quote
 	        but was required as: rsc.io/quote
diff --git a/src/cmd/go/testdata/script/mod_retention.txt b/src/cmd/go/testdata/script/mod_retention.txt
index 711d28b10f..0e639db551 100644
--- a/src/cmd/go/testdata/script/mod_retention.txt
+++ b/src/cmd/go/testdata/script/mod_retention.txt
@@ -62,6 +62,7 @@ cmp go.mod go.mod.tidy
 
 # A missing "go" version directive should be added.
 # However, that should not remove other redundant requirements.
+# In fact, it may *add* redundant requirements due to activating lazy loading.
 cp go.mod.nogo go.mod
 go list -mod=mod all
 cmpenv go.mod go.mod.addedgo
@@ -136,9 +137,10 @@ require (
 -- go.mod.addedgo --
 module m
 
-go 1.16
+go $goversion
 
 require (
+	golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c // indirect
 	rsc.io/quote v1.5.2
 	rsc.io/sampler v1.3.0 // indirect
 	rsc.io/testonly v1.0.0 // indirect
diff --git a/src/cmd/go/testdata/script/mod_tidy_version.txt b/src/cmd/go/testdata/script/mod_tidy_version.txt
new file mode 100644
index 0000000000..5441d9cc06
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_version.txt
@@ -0,0 +1,248 @@
+# https://golang.org/issue/45094: 'go mod tidy' now accepts a '-go' flag
+# to change the language version in use.
+#
+# The package import graph used in this test looks like:
+#
+# m --- a --- b
+#             |
+#             b_test --- c
+#                        |
+#                        c_test --- d
+#
+# The module diagram looks like:
+#
+# m --- a --- b
+# |
+# + --- c
+# |
+# + --- d
+#
+# Module b omits its dependency on c, and module c omits its dependency on d.
+#
+# In go 1.15, the tidy main module must require a (because it is direct),
+# c (because it is a missing test dependency of an imported package),
+# and d (because it is a missing transitive test dependency).
+#
+# In go 1.16, the tidy main module can omit d because it is no longer
+# included in "all".
+#
+# In go 1.17, the main module must explicitly require b
+# (because it is transitively imported by the main module).
+
+
+cp go.mod go.mod.orig
+
+# An invalid argument should be rejected.
+
+! go mod tidy -go=bananas
+stderr '^go mod: invalid -go option "bananas"; expecting something like "-go 1.17"$'
+cmp go.mod go.mod.orig
+
+
+go mod tidy -go=1.15
+cmp go.mod go.mod.115
+
+go mod tidy
+cmp go.mod go.mod.115
+
+
+go mod tidy -go=1.16
+cmp go.mod go.mod.116
+
+go mod tidy
+cmp go.mod go.mod.116
+
+
+go mod tidy -go=1.17
+cmp go.mod go.mod.117
+
+go mod tidy
+cmp go.mod go.mod.117
+
+
+# If we downgrade back to 1.15, we should re-resolve d to v0.2.0 instead
+# of the original v0.1.0 (because the original requirement is lost).
+
+go mod tidy -go=1.15
+cmp go.mod go.mod.115-2
+
+
+# -go= (with an empty argument) maintains the existing version or adds the
+#  default version (just like omitting the flag).
+
+go mod tidy -go=''
+cmp go.mod go.mod.115-2
+
+cp go.mod.orig go.mod
+go mod tidy -go=''
+cmpenv go.mod go.mod.latest
+
+
+
+-- go.mod --
+module example.com/m
+
+require (
+	example.net/a v0.1.0
+	example.net/c v0.1.0 // indirect
+	example.net/d v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- m.go --
+package m
+
+import _ "example.net/a"
+
+-- go.mod.115 --
+module example.com/m
+
+go 1.15
+
+require (
+	example.net/a v0.1.0
+	example.net/c v0.1.0 // indirect
+	example.net/d v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- go.mod.115-2 --
+module example.com/m
+
+go 1.15
+
+require (
+	example.net/a v0.1.0
+	example.net/c v0.1.0 // indirect
+	example.net/d v0.2.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- go.mod.116 --
+module example.com/m
+
+go 1.16
+
+require (
+	example.net/a v0.1.0
+	example.net/c v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- go.mod.117 --
+module example.com/m
+
+go 1.17
+
+require (
+	example.net/a v0.1.0
+	example.net/b v0.1.0 // indirect
+	example.net/c v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- go.mod.latest --
+module example.com/m
+
+go $goversion
+
+require (
+	example.net/a v0.1.0
+	example.net/b v0.1.0 // indirect
+	example.net/c v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- a/go.mod --
+module example.net/a
+
+go 1.15
+
+require example.net/b v0.1.0
+-- a/a.go --
+package a
+
+import _ "example.net/b"
+
+-- b/go.mod --
+module example.net/b
+
+go 1.15
+-- b/b.go --
+package b
+-- b/b_test.go --
+package b_test
+
+import _ "example.net/c"
+
+-- c/go.mod --
+module example.net/c
+
+go 1.15
+-- c/c.go --
+package c
+-- c/c_test.go --
+package c_test
+
+import _ "example.net/d"
+
+-- d/go.mod --
+module example.net/d
+
+go 1.15
+-- d/d.go --
+package d