[dev.typeparams] all: merge master (f22ec51) into dev.typeparams

Merge List:

+ 2021-05-25 f22ec51deb doc: add Go 1.17 release note about inlining functions with closures
+ 2021-05-25 8b462d7567 cmd/go: add a -compat flag to 'go mod tidy'
+ 2021-05-24 c89f1224a5 net: verify results from Lookup* are valid domain names
+ 2021-05-24 08a8fa9c47 misc/wasm: ensure correct stack pointer in catch clauses
+ 2021-05-24 32b73ae180 cmd/go: align checks of module path during initialization.
+ 2021-05-24 15d9d4a009 cmd/go: add tests illustrating what happens when Go 1.16 is used in a Go 1.17 main module
+ 2021-05-24 873401df5b cmd/compile: ensure equal functions don't do unaligned loads
+ 2021-05-24 b83610699a cmd/compile: record regabi status in DW_AT_producer
+ 2021-05-24 a22e317220 cmd/compile: always include underlying type for map types
+ 2021-05-24 4356e7e85f runtime: account for spill slots in Windows callback compilation
+ 2021-05-24 52d7033ff6 cmd/go/internal/modload: set the default GoVersion in a single location
+ 2021-05-24 05819bc104 cmd/go/internal/modcmd: factor out a type for flags whose arguments are Go versions
+ 2021-05-22 cca23a7373 cmd/compile: revert CL/316890
+ 2021-05-21 f87194cbd7 doc/go1.17: document changes to net/http package
+ 2021-05-21 217f5dd496 doc: document additional atomic.Value methods
+ 2021-05-21 3c656445f1 cmd/go: in TestScript/mod_replace, download an explicit module path
+ 2021-05-21 76b2d6afed os: document that StartProcess puts files into blocking mode
+ 2021-05-21 e4d7525c3e cmd/dist: display first class port status in json output
+ 2021-05-21 4fb10b2118 cmd/go: in 'go mod download' without args, don't save module zip sums
+ 2021-05-21 4fda54ce3f doc/go1.17: document database/sql changes for Go 1.17
+ 2021-05-21 8876b9bd6a doc/go1.17: document io/fs changes for Go 1.17
+ 2021-05-21 5fee772c87 doc/go1.17: document archive/zip changes for Go 1.17
+ 2021-05-21 3148694f60 cmd/go: remove warning from module deprecation notice printing
+ 2021-05-21 7e63c8b765 runtime: wait for Go runtime to initialize in Windows signal test
+ 2021-05-21 831573cd21 io/fs: added an example for io/fs.WalkDir
+ 2021-05-20 baa934d26d cmd: go get golang.org/x/tools/analysis@49064d23 && go mod vendor
+ 2021-05-20 7c692cc7ea doc/go1.17: document changes to os package
+ 2021-05-20 ce9a3b79d5 crypto/x509: add new FreeBSD 12.2+ trusted certificate folder
+ 2021-05-20 f8be906d74 test: re-enable test on riscv64 now that it supports external linking
+ 2021-05-20 def5360541 doc/go1.17: add release notes for OpenBSD ports
+ 2021-05-20 ef1f52cc38 doc/go1.17: add release note for windows/arm64 port
+ 2021-05-20 bb7495a46d doc/go1.17: document new math constants
+ 2021-05-20 f07e4dae3c syscall: document NewCallback and NewCallbackCDecl limitations
+ 2021-05-20 a8d85918b6 misc/cgo/testplugin: skip TestIssue25756pie on darwin/arm64 builder
+ 2021-05-19 6c1c055d1e cmd/internal/moddeps: use filepath.SkipDir only on directories
+ 2021-05-19 658b5e66ec net: return nil UDPAddr from ReadFromUDP
+ 2021-05-19 15a374d5c1 test: check portable error message on issue46234.go
+ 2021-05-18 eeadce2d87 go/build/constraint: fix parsing of "// +build" (with no args)
+ 2021-05-18 6d2ef2ef2a cmd/compile: don't emit inltree for closure within body of inlined func
+ 2021-05-18 048cb4ceee crypto/x509: remove duplicate import

Change-Id: Ib0442e3555493805f2aa1df26dfd6898df989a37

44 files changed, 1579 insertions, 200 deletions

diff --git a/src/cmd/compile/internal/dwarfgen/dwarf.go b/src/cmd/compile/internal/dwarfgen/dwarf.go
index 5d7dc320aa..0e22b61bc3 100644
--- a/src/cmd/compile/internal/dwarfgen/dwarf.go
+++ b/src/cmd/compile/internal/dwarfgen/dwarf.go
@@ -531,6 +531,14 @@ func RecordFlags(flags ...string) {
 		fmt.Fprintf(&cmd, " -%s=%v", f.Name, getter.Get())
 	}
 
+	// Adds flag to producer string singalling whether regabi is turned on or
+	// off.
+	// Once regabi is turned on across the board and the relative GOEXPERIMENT
+	// knobs no longer exist this code should be removed.
+	if buildcfg.Experiment.RegabiArgs {
+		cmd.Write([]byte(" regabi"))
+	}
+
 	if cmd.Len() == 0 {
 		return
 	}
diff --git a/src/cmd/compile/internal/inline/inl.go b/src/cmd/compile/internal/inline/inl.go
index bd453e40a5..263e0b310b 100644
--- a/src/cmd/compile/internal/inline/inl.go
+++ b/src/cmd/compile/internal/inline/inl.go
@@ -1124,6 +1124,10 @@ type inlsubst struct {
 	newclofn *ir.Func
 
 	fn *ir.Func // For debug -- the func that is being inlined
+
+	// If true, then don't update source positions during substitution
+	// (retain old source positions).
+	noPosUpdate bool
 }
 
 // list inlines a list of nodes.
@@ -1223,7 +1227,14 @@ func (subst *inlsubst) clovar(n *ir.Name) *ir.Name {
 // closure node.
 func (subst *inlsubst) closure(n *ir.ClosureExpr) ir.Node {
 	m := ir.Copy(n)
-	m.SetPos(subst.updatedPos(m.Pos()))
+
+	// Prior to the subst edit, set a flag in the inlsubst to
+	// indicated that we don't want to update the source positions in
+	// the new closure. If we do this, it will appear that the closure
+	// itself has things inlined into it, which is not the case. See
+	// issue #46234 for more details.
+	defer func(prev bool) { subst.noPosUpdate = prev }(subst.noPosUpdate)
+	subst.noPosUpdate = true
 	ir.EditChildren(m, subst.edit)
 
 	//fmt.Printf("Inlining func %v with closure into %v\n", subst.fn, ir.FuncName(ir.CurFunc))
@@ -1449,6 +1460,9 @@ func (subst *inlsubst) node(n ir.Node) ir.Node {
 }
 
 func (subst *inlsubst) updatedPos(xpos src.XPos) src.XPos {
+	if subst.noPosUpdate {
+		return xpos
+	}
 	pos := base.Ctxt.PosTable.Pos(xpos)
 	oldbase := pos.Base() // can be nil
 	newbase := subst.bases[oldbase]
diff --git a/src/cmd/compile/internal/reflectdata/alg.go b/src/cmd/compile/internal/reflectdata/alg.go
index d12d9ca0a7..0707e0b61c 100644
--- a/src/cmd/compile/internal/reflectdata/alg.go
+++ b/src/cmd/compile/internal/reflectdata/alg.go
@@ -6,6 +6,7 @@ package reflectdata
 
 import (
 	"fmt"
+	"math/bits"
 	"sort"
 
 	"cmd/compile/internal/base"
@@ -47,6 +48,11 @@ func eqCanPanic(t *types.Type) bool {
 func AlgType(t *types.Type) types.AlgKind {
 	a, _ := types.AlgType(t)
 	if a == types.AMEM {
+		if t.Alignment() < int64(base.Ctxt.Arch.Alignment) && t.Alignment() < t.Width {
+			// For example, we can't treat [2]int16 as an int32 if int32s require
+			// 4-byte alignment. See issue 46283.
+			return a
+		}
 		switch t.Width {
 		case 0:
 			return types.AMEM0
@@ -769,6 +775,20 @@ func memrun(t *types.Type, start int) (size int64, next int) {
 		if f := t.Field(next); f.Sym.IsBlank() || !isRegularMemory(f.Type) {
 			break
 		}
+		// For issue 46283, don't combine fields if the resulting load would
+		// require a larger alignment than the component fields.
+		if base.Ctxt.Arch.Alignment > 1 {
+			align := t.Alignment()
+			if off := t.Field(start).Offset; off&(align-1) != 0 {
+				// Offset is less aligned than the containing type.
+				// Use offset to determine alignment.
+				align = 1 << uint(bits.TrailingZeros64(uint64(off)))
+			}
+			size := t.Field(next).End() - t.Field(start).Offset
+			if size > align {
+				break
+			}
+		}
 	}
 	return t.Field(next-1).End() - t.Field(start).Offset, next
 }
diff --git a/src/cmd/compile/internal/reflectdata/reflect.go b/src/cmd/compile/internal/reflectdata/reflect.go
index f65841b33c..3576a23db9 100644
--- a/src/cmd/compile/internal/reflectdata/reflect.go
+++ b/src/cmd/compile/internal/reflectdata/reflect.go
@@ -1116,6 +1116,15 @@ func writeType(t *types.Type) *obj.LSym {
 		}
 		ot = objw.Uint32(lsym, ot, flags)
 		ot = dextratype(lsym, ot, t, 0)
+		if u := t.Underlying(); u != t {
+			// If t is a named map type, also keep the underlying map
+			// type live in the binary. This is important to make sure that
+			// a named map and that same map cast to its underlying type via
+			// reflection, use the same hash function. See issue 37716.
+			r := obj.Addrel(lsym)
+			r.Sym = writeType(u)
+			r.Type = objabi.R_KEEP
+		}
 
 	case types.TPTR:
 		if t.Elem().Kind() == types.TANY {
diff --git a/src/cmd/compile/internal/ssa/expand_calls.go b/src/cmd/compile/internal/ssa/expand_calls.go
index d37d06f8e7..7e973ab205 100644
--- a/src/cmd/compile/internal/ssa/expand_calls.go
+++ b/src/cmd/compile/internal/ssa/expand_calls.go
@@ -1717,22 +1717,6 @@ func (x *expandState) newArgToMemOrRegs(baseArg, toReplace *Value, offset int64,
 	} else {
 		w = baseArg.Block.NewValue0IA(pos, op, t, auxInt, aux)
 	}
-	// If we are creating an OpArgIntReg/OpArgFloatReg that
-	// corresponds to an in-param that fits entirely in a register,
-	// then enter it into the name/value table. The LocalSlot
-	// is somewhat fictitious, since there is no incoming live
-	// memory version of the parameter, but we need an entry in
-	// NamedValues in order for ssa debug tracking to include
-	// the value in the tracking analysis.
-	if len(pa.Registers) == 1 {
-		loc := LocalSlot{N: aux.Name, Type: t, Off: 0}
-		values, ok := x.f.NamedValues[loc]
-		if !ok {
-			ploc := x.f.localSlotAddr(loc)
-			x.f.Names = append(x.f.Names, ploc)
-		}
-		x.f.NamedValues[loc] = append(values, w)
-	}
 	x.commonArgs[key] = w
 	if toReplace != nil {
 		toReplace.copyOf(w)
diff --git a/src/cmd/compile/internal/test/align_test.go b/src/cmd/compile/internal/test/align_test.go
new file mode 100644
index 0000000000..32afc92973
--- /dev/null
+++ b/src/cmd/compile/internal/test/align_test.go
@@ -0,0 +1,96 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test to make sure that equality functions (and hash
+// functions) don't do unaligned reads on architectures
+// that can't do unaligned reads. See issue 46283.
+
+package test
+
+import "testing"
+
+type T1 struct {
+	x          float32
+	a, b, c, d int16 // memequal64
+}
+type T2 struct {
+	x          float32
+	a, b, c, d int32 // memequal128
+}
+
+type A2 [2]byte // eq uses a 2-byte load
+type A4 [4]byte // eq uses a 4-byte load
+type A8 [8]byte // eq uses an 8-byte load
+
+//go:noinline
+func cmpT1(p, q *T1) {
+	if *p != *q {
+		panic("comparison test wrong")
+	}
+}
+
+//go:noinline
+func cmpT2(p, q *T2) {
+	if *p != *q {
+		panic("comparison test wrong")
+	}
+}
+
+//go:noinline
+func cmpA2(p, q *A2) {
+	if *p != *q {
+		panic("comparison test wrong")
+	}
+}
+
+//go:noinline
+func cmpA4(p, q *A4) {
+	if *p != *q {
+		panic("comparison test wrong")
+	}
+}
+
+//go:noinline
+func cmpA8(p, q *A8) {
+	if *p != *q {
+		panic("comparison test wrong")
+	}
+}
+
+func TestAlignEqual(t *testing.T) {
+	cmpT1(&T1{}, &T1{})
+	cmpT2(&T2{}, &T2{})
+
+	m1 := map[T1]bool{}
+	m1[T1{}] = true
+	m1[T1{}] = false
+	if len(m1) != 1 {
+		t.Fatalf("len(m1)=%d, want 1", len(m1))
+	}
+	m2 := map[T2]bool{}
+	m2[T2{}] = true
+	m2[T2{}] = false
+	if len(m2) != 1 {
+		t.Fatalf("len(m2)=%d, want 1", len(m2))
+	}
+
+	type X2 struct {
+		y byte
+		z A2
+	}
+	var x2 X2
+	cmpA2(&x2.z, &A2{})
+	type X4 struct {
+		y byte
+		z A4
+	}
+	var x4 X4
+	cmpA4(&x4.z, &A4{})
+	type X8 struct {
+		y byte
+		z A8
+	}
+	var x8 X8
+	cmpA8(&x8.z, &A8{})
+}
diff --git a/src/cmd/dist/build.go b/src/cmd/dist/build.go
index 00e23ef179..1abb03bcc5 100644
--- a/src/cmd/dist/build.go
+++ b/src/cmd/dist/build.go
@@ -1607,6 +1607,18 @@ var incomplete = map[string]bool{
 	"linux/sparc64": true,
 }
 
+// List of platforms which are first class ports. See golang.org/issue/38874.
+var firstClass = map[string]bool{
+	"darwin/amd64":  true,
+	"darwin/arm64":  true,
+	"linux/386":     true,
+	"linux/amd64":   true,
+	"linux/arm":     true,
+	"linux/arm64":   true,
+	"windows/386":   true,
+	"windows/amd64": true,
+}
+
 func needCC() bool {
 	switch os.Getenv("CGO_ENABLED") {
 	case "1":
@@ -1743,6 +1755,7 @@ func cmdlist() {
 		GOOS         string
 		GOARCH       string
 		CgoSupported bool
+		FirstClass   bool
 	}
 	var results []jsonResult
 	for _, p := range plats {
@@ -1750,7 +1763,8 @@ func cmdlist() {
 		results = append(results, jsonResult{
 			GOOS:         fields[0],
 			GOARCH:       fields[1],
-			CgoSupported: cgoEnabled[p]})
+			CgoSupported: cgoEnabled[p],
+			FirstClass:   firstClass[p]})
 	}
 	out, err := json.MarshalIndent(results, "", "\t")
 	if err != nil {
diff --git a/src/cmd/go.mod b/src/cmd/go.mod
index 88f5f2883a..1aa0320d07 100644
--- a/src/cmd/go.mod
+++ b/src/cmd/go.mod
@@ -10,6 +10,6 @@ require (
 	golang.org/x/mod v0.4.3-0.20210512182355-6088ed88cecd
 	golang.org/x/sys v0.0.0-20210511113859-b0526f3d8744 // indirect
 	golang.org/x/term v0.0.0-20210503060354-a79de5458b56
-	golang.org/x/tools v0.1.1-0.20210505014545-7cab0ef2e9a5
+	golang.org/x/tools v0.1.2-0.20210519160823-49064d2332f9
 	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 // indirect
 )
diff --git a/src/cmd/go.sum b/src/cmd/go.sum
index 73750802bc..eeb625fcf8 100644
--- a/src/cmd/go.sum
+++ b/src/cmd/go.sum
@@ -5,18 +5,41 @@ github.com/google/pprof v0.0.0-20210506205249-923b5ab0fc1a h1:jmAp/2PZAScNd62lTD
 github.com/google/pprof v0.0.0-20210506205249-923b5ab0fc1a/go.mod h1:kpwsk12EmLew5upagYY7GY0pfYCcupk39gWOCRROcvE=
 github.com/ianlancetaylor/demangle v0.0.0-20200824232613-28f6c0f3b639 h1:mV02weKRL81bEnm8A0HT1/CAelMQDBuQIfLw8n+d6xI=
 github.com/ianlancetaylor/demangle v0.0.0-20200824232613-28f6c0f3b639/go.mod h1:aSSvb/t6k1mPoxDqO4vJh6VOCGPwU4O0C2/Eqndh1Sc=
+github.com/yuin/goldmark v1.3.5/go.mod h1:mwnBkeHKe2W/ZEtQ+71ViKU8L12m81fl3OWwC1Zlc8k=
 golang.org/x/arch v0.0.0-20210502124803-cbf565b21d1e h1:pv3V0NlNSh5Q6AX/StwGLBjcLS7UN4m4Gq+V+uSecqM=
 golang.org/x/arch v0.0.0-20210502124803-cbf565b21d1e/go.mod h1:5om86z9Hs0C8fWVUuoMHwpExlXzs5Tkyp9hOrfG7pp8=
+golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
 golang.org/x/crypto v0.0.0-20210503195802-e9a32991a82e h1:8foAy0aoO5GkqCvAEJ4VC4P3zksTg4X4aJCDpZzmgQI=
 golang.org/x/crypto v0.0.0-20210503195802-e9a32991a82e/go.mod h1:P+XmwS30IXTQdn5tA2iutPOUgjI07+tq3H3K9MVA1s8=
+golang.org/x/mod v0.4.2/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/mod v0.4.3-0.20210512182355-6088ed88cecd h1:CuRnpyMrCCBulv0d/y0CswR4K0vGydgE3DZ2wYPIOo8=
 golang.org/x/mod v0.4.3-0.20210512182355-6088ed88cecd/go.mod h1:5OXOZSfqPIIbmVBIIKWRFfZjPR0E5r58TLhUjH0a2Ro=
+golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
+golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
+golang.org/x/net v0.0.0-20210226172049-e18ecbb05110/go.mod h1:m0MpNAwzfU5UDzcl9v0D8zg8gWTRqZa9RBIspLL5mdg=
+golang.org/x/net v0.0.0-20210405180319-a5a99cb37ef4/go.mod h1:p54w0d4576C0XHj96bSt6lcn1PtDYWL6XObtHCRCNQM=
+golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.0.0-20210220032951-036812b2e83c/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20191204072324-ce4227a45e2e/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20210330210617-4fbd30eecc44/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20210510120138-977fb7262007/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20210511113859-b0526f3d8744 h1:yhBbb4IRs2HS9PPlAg6DMC6mUOKexJBNsLf4Z+6En1Q=
 golang.org/x/sys v0.0.0-20210511113859-b0526f3d8744/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
 golang.org/x/term v0.0.0-20210503060354-a79de5458b56 h1:b8jxX3zqjpqb2LklXPzKSGJhzyxCOZSz8ncv8Nv+y7w=
 golang.org/x/term v0.0.0-20210503060354-a79de5458b56/go.mod h1:tfny5GFUkzUvx4ps4ajbZsCe5lw1metzhBm9T3x7oIY=
-golang.org/x/tools v0.1.1-0.20210505014545-7cab0ef2e9a5 h1:ImcI7RFHWLu2QWpFDXaReu0j+sQAHIy65vUFZImXiqY=
-golang.org/x/tools v0.1.1-0.20210505014545-7cab0ef2e9a5/go.mod h1:sH/Eidr0EddymY8HZSakBo32zU3fG5ovDq874hJLjVg=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
+golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
+golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
+golang.org/x/tools v0.1.2-0.20210519160823-49064d2332f9 h1:2XlR/j4I4xz5GQZI7zBjqTfezYyRIE2jD5IMousB2rg=
+golang.org/x/tools v0.1.2-0.20210519160823-49064d2332f9/go.mod h1:o0xws9oXOQQZyjljx8fwUC0k7L1pTE6eaCbjGeHmOkk=
+golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 h1:go1bK/D/BFZV2I8cIQd1NKEZ+0owSTG1fDTci4IqFcE=
 golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+rsc.io/pdf v0.1.1/go.mod h1:n8OzWcQ6Sp37PL01nO98y4iUCRdTGarVfzxY20ICaU4=
diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index fcc7f36335..bad2b7f16e 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -1221,7 +1221,7 @@
 //
 // Usage:
 //
-// 	go mod tidy [-e] [-v] [-go=version]
+// 	go mod tidy [-e] [-v] [-go=version] [-compat=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
@@ -1241,6 +1241,14 @@
 // (Go versions 1.17 and higher retain more requirements in order to
 // support lazy module loading.)
 //
+// The -compat flag preserves any additional checksums needed for the
+// 'go' command from the indicated major Go release to successfully load
+// the module graph, and causes tidy to error out if that version of the
+// 'go' command would load any imported package from a different module
+// version. By default, tidy acts as if the -compat flag were set to the
+// version prior to the one indicated by the 'go' directive in the go.mod
+// file.
+//
 // See https://golang.org/ref/mod#go-mod-tidy for more about 'go mod tidy'.
 //
 //
diff --git a/src/cmd/go/internal/modcmd/download.go b/src/cmd/go/internal/modcmd/download.go
index a6c6d914e1..42b06dbc95 100644
--- a/src/cmd/go/internal/modcmd/download.go
+++ b/src/cmd/go/internal/modcmd/download.go
@@ -86,9 +86,11 @@ func runDownload(ctx context.Context, cmd *base.Command, args []string) {
 	if !modload.HasModRoot() && len(args) == 0 {
 		base.Fatalf("go mod download: no modules specified (see 'go help mod download')")
 	}
-	if len(args) == 0 {
+	haveExplicitArgs := len(args) > 0
+	if !haveExplicitArgs {
 		args = []string{"all"}
-	} else if modload.HasModRoot() {
+	}
+	if modload.HasModRoot() {
 		modload.LoadModFile(ctx) // to fill Target
 		targetAtUpgrade := modload.Target.Path + "@upgrade"
 		targetAtPatch := modload.Target.Path + "@patch"
@@ -135,6 +137,18 @@ func runDownload(ctx context.Context, cmd *base.Command, args []string) {
 	type token struct{}
 	sem := make(chan token, runtime.GOMAXPROCS(0))
 	infos, infosErr := modload.ListModules(ctx, args, 0)
+	if !haveExplicitArgs {
+		// 'go mod download' is sometimes run without arguments to pre-populate
+		// the module cache. It may fetch modules that aren't needed to build
+		// packages in the main mdoule. This is usually not intended, so don't save
+		// sums for downloaded modules (golang.org/issue/45332).
+		// TODO(golang.org/issue/45551): For now, save sums needed to load the
+		// build list (same as 1.15 behavior). In the future, report an error if
+		// go.mod or go.sum need to be updated after loading the build list.
+		modload.WriteGoMod(ctx)
+		modload.DisallowWriteGoMod()
+	}
+
 	for _, info := range infos {
 		if info.Replace != nil {
 			info = info.Replace
@@ -185,8 +199,15 @@ func runDownload(ctx context.Context, cmd *base.Command, args []string) {
 		base.ExitIfErrors()
 	}
 
-	// Update go.mod and especially go.sum if needed.
-	modload.WriteGoMod(ctx)
+	// If there were explicit arguments, update go.mod and especially go.sum.
+	// 'go mod download mod@version' is a useful way to add a sum without using
+	// 'go get mod@version', which may have other side effects. We print this in
+	// some error message hints.
+	//
+	// Don't save sums for 'go mod download' without arguments; see comment above.
+	if haveExplicitArgs {
+		modload.WriteGoMod(ctx)
+	}
 
 	// If there was an error matching some of the requested packages, emit it now
 	// (after we've written the checksums for the modules that were downloaded
diff --git a/src/cmd/go/internal/modcmd/edit.go b/src/cmd/go/internal/modcmd/edit.go
index 79a93ca44b..e856e7c630 100644
--- a/src/cmd/go/internal/modcmd/edit.go
+++ b/src/cmd/go/internal/modcmd/edit.go
@@ -196,7 +196,7 @@ func runEdit(ctx context.Context, cmd *base.Command, args []string) {
 
 	if *editGo != "" {
 		if !modfile.GoVersionRE.MatchString(*editGo) {
-			base.Fatalf(`go mod: invalid -go option; expecting something like "-go 1.12"`)
+			base.Fatalf(`go mod: invalid -go option; expecting something like "-go %s"`, modload.LatestGoVersion())
 		}
 	}
 
diff --git a/src/cmd/go/internal/modcmd/tidy.go b/src/cmd/go/internal/modcmd/tidy.go
index c72ec30a57..fe25507e94 100644
--- a/src/cmd/go/internal/modcmd/tidy.go
+++ b/src/cmd/go/internal/modcmd/tidy.go
@@ -12,12 +12,14 @@ import (
 	"cmd/go/internal/imports"
 	"cmd/go/internal/modload"
 	"context"
+	"fmt"
 
 	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/semver"
 )
 
 var cmdTidy = &base.Command{
-	UsageLine: "go mod tidy [-e] [-v] [-go=version]",
+	UsageLine: "go mod tidy [-e] [-v] [-go=version] [-compat=version]",
 	Short:     "add missing and remove unused modules",
 	Long: `
 Tidy makes sure go.mod matches the source code in the module.
@@ -38,34 +40,64 @@ 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.)
 
+The -compat flag preserves any additional checksums needed for the
+'go' command from the indicated major Go release to successfully load
+the module graph, and causes tidy to error out if that version of the
+'go' command would load any imported package from a different module
+version. By default, tidy acts as if the -compat flag were set to the
+version prior to the one indicated by the 'go' directive in the go.mod
+file.
+
 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.
-	tidyGo string // go version to write to the tidied go.mod file (toggles lazy loading)
+	tidyE      bool          // if true, report errors but proceed anyway.
+	tidyGo     goVersionFlag // go version to write to the tidied go.mod file (toggles lazy loading)
+	tidyCompat goVersionFlag // go version for which the tidied go.mod and go.sum files should be “compatible”
 )
 
 func init() {
 	cmdTidy.Flag.BoolVar(&cfg.BuildV, "v", false, "")
 	cmdTidy.Flag.BoolVar(&tidyE, "e", false, "")
-	cmdTidy.Flag.StringVar(&tidyGo, "go", "", "")
+	cmdTidy.Flag.Var(&tidyGo, "go", "")
+	cmdTidy.Flag.Var(&tidyCompat, "compat", "")
 	base.AddModCommonFlags(&cmdTidy.Flag)
 }
 
+// A goVersionFlag is a flag.Value representing a supported Go version.
+//
+// (Note that the -go argument to 'go mod edit' is *not* a goVersionFlag.
+// It intentionally allows newer-than-supported versions as arguments.)
+type goVersionFlag struct {
+	v string
+}
+
+func (f *goVersionFlag) String() string   { return f.v }
+func (f *goVersionFlag) Get() interface{} { return f.v }
+
+func (f *goVersionFlag) Set(s string) error {
+	if s != "" {
+		latest := modload.LatestGoVersion()
+		if !modfile.GoVersionRE.MatchString(s) {
+			return fmt.Errorf("expecting a Go version like %q", latest)
+		}
+		if semver.Compare("v"+s, "v"+latest) > 0 {
+			return fmt.Errorf("maximum supported Go version is %s", latest)
+		}
+	}
+
+	f.v = s
+	return nil
+}
+
 func runTidy(ctx context.Context, cmd *base.Command, args []string) {
 	if len(args) > 0 {
 		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
@@ -80,9 +112,10 @@ func runTidy(ctx context.Context, cmd *base.Command, args []string) {
 	modload.RootMode = modload.NeedRoot
 
 	modload.LoadPackages(ctx, modload.PackageOpts{
-		GoVersion:                tidyGo,
+		GoVersion:                tidyGo.String(),
 		Tags:                     imports.AnyTags(),
 		Tidy:                     true,
+		TidyCompatibleVersion:    tidyCompat.String(),
 		VendorModulesInGOROOTSrc: true,
 		ResolveMissingImports:    true,
 		LoadTests:                true,
diff --git a/src/cmd/go/internal/modget/get.go b/src/cmd/go/internal/modget/get.go
index 2a7fe5226f..563f1a988f 100644
--- a/src/cmd/go/internal/modget/get.go
+++ b/src/cmd/go/internal/modget/get.go
@@ -1598,7 +1598,7 @@ func (r *resolver) checkPackageProblems(ctx context.Context, pkgPatterns []strin
 	// Report deprecations, then retractions.
 	for _, mm := range deprecations {
 		if mm.message != "" {
-			fmt.Fprintf(os.Stderr, "go: warning: module %s is deprecated: %s\n", mm.m.Path, mm.message)
+			fmt.Fprintf(os.Stderr, "go: module %s is deprecated: %s\n", mm.m.Path, mm.message)
 		}
 	}
 	var retractPath string
diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index 5cdea12cd3..86c0db3fe4 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -405,7 +405,7 @@ func loadModFile(ctx context.Context) (rs *Requirements, needCommit bool) {
 	if modRoot == "" {
 		Target = module.Version{Path: "command-line-arguments"}
 		targetPrefix = "command-line-arguments"
-		goVersion := latestGoVersion()
+		goVersion := LatestGoVersion()
 		rawGoVersion.Store(Target, goVersion)
 		requirements = newRequirements(modDepthFromGoVersion(goVersion), nil, nil)
 		return requirements, false
@@ -432,7 +432,10 @@ func loadModFile(ctx context.Context) (rs *Requirements, needCommit bool) {
 	initTarget(f.Module.Mod)
 	index = indexModFile(data, f, fixed)
 
-	if err := checkModulePathLax(f.Module.Mod.Path); err != nil {
+	if err := module.CheckImportPath(f.Module.Mod.Path); err != nil {
+		if pathErr, ok := err.(*module.InvalidPathError); ok {
+			pathErr.Kind = "module"
+		}
 		base.Fatalf("go: %v", err)
 	}
 
@@ -448,7 +451,7 @@ func loadModFile(ctx context.Context) (rs *Requirements, needCommit bool) {
 		// 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())
+			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.
@@ -492,7 +495,15 @@ func CreateModFile(ctx context.Context, modPath string) {
 		if err != nil {
 			base.Fatalf("go: %v", err)
 		}
-	} else if err := checkModulePathLax(modPath); err != nil {
+	} else if err := module.CheckImportPath(modPath); err != nil {
+		if pathErr, ok := err.(*module.InvalidPathError); ok {
+			pathErr.Kind = "module"
+			// Same as build.IsLocalPath()
+			if pathErr.Path == "." || pathErr.Path == ".." ||
+				strings.HasPrefix(pathErr.Path, "./") || strings.HasPrefix(pathErr.Path, "../") {
+				pathErr.Err = errors.New("is a local import path")
+			}
+		}
 		base.Fatalf("go: %v", err)
 	}
 
@@ -500,7 +511,7 @@ func CreateModFile(ctx context.Context, modPath string) {
 	modFile = new(modfile.File)
 	modFile.AddModuleStmt(modPath)
 	initTarget(modFile.Module.Mod)
-	addGoStmt(latestGoVersion()) // Add the go directive before converted module requirements.
+	addGoStmt(LatestGoVersion()) // Add the go directive before converted module requirements.
 
 	convertedFrom, err := convertLegacyConfig(modPath)
 	if convertedFrom != "" {
@@ -536,49 +547,6 @@ func CreateModFile(ctx context.Context, modPath string) {
 	}
 }
 
-// checkModulePathLax checks that the path meets some minimum requirements
-// to avoid confusing users or the module cache. The requirements are weaker
-// than those of module.CheckPath to allow room for weakening module path
-// requirements in the future, but strong enough to help users avoid significant
-// problems.
-func checkModulePathLax(p string) error {
-	// TODO(matloob): Replace calls of this function in this CL with calls
-	// to module.CheckImportPath once it's been laxened, if it becomes laxened.
-	// See golang.org/issue/29101 for a discussion about whether to make CheckImportPath
-	// more lax or more strict.
-
-	errorf := func(format string, args ...interface{}) error {
-		return fmt.Errorf("invalid module path %q: %s", p, fmt.Sprintf(format, args...))
-	}
-
-	// Disallow shell characters " ' * < > ? ` | to avoid triggering bugs
-	// with file systems and subcommands. Disallow file path separators : and \
-	// because path separators other than / will confuse the module cache.
-	// See fileNameOK in golang.org/x/mod/module/module.go.
-	shellChars := "`" + `"'*<>?|`
-	fsChars := `\:`
-	if i := strings.IndexAny(p, shellChars); i >= 0 {
-		return errorf("contains disallowed shell character %q", p[i])
-	}
-	if i := strings.IndexAny(p, fsChars); i >= 0 {
-		return errorf("contains disallowed path separator character %q", p[i])
-	}
-
-	// Ensure path.IsAbs and build.IsLocalImport are false, and that the path is
-	// invariant under path.Clean, also to avoid confusing the module cache.
-	if path.IsAbs(p) {
-		return errorf("is an absolute path")
-	}
-	if build.IsLocalImport(p) {
-		return errorf("is a local import path")
-	}
-	if path.Clean(p) != p {
-		return errorf("is not clean")
-	}
-
-	return nil
-}
-
 // fixVersion returns a modfile.VersionFixer implemented using the Query function.
 //
 // It resolves commit hashes and branch names to versions,
@@ -793,17 +761,39 @@ func addGoStmt(v string) {
 	rawGoVersion.Store(Target, v)
 }
 
-// latestGoVersion returns the latest version of the Go language supported by
+// LatestGoVersion returns the latest version of the Go language supported by
 // this toolchain, like "1.17".
-func latestGoVersion() string {
+func LatestGoVersion() string {
 	tags := build.Default.ReleaseTags
 	version := tags[len(tags)-1]
 	if !strings.HasPrefix(version, "go") || !modfile.GoVersionRE.MatchString(version[2:]) {
-		base.Fatalf("go: unrecognized default version %q", version)
+		base.Fatalf("go: internal error: unrecognized default version %q", version)
 	}
 	return version[2:]
 }
 
+// priorGoVersion returns the Go major release immediately preceding v,
+// or v itself if v is the first Go major release (1.0) or not a supported
+// Go version.
+func priorGoVersion(v string) string {
+	vTag := "go" + v
+	tags := build.Default.ReleaseTags
+	for i, tag := range tags {
+		if tag == vTag {
+			if i == 0 {
+				return v
+			}
+
+			version := tags[i-1]
+			if !strings.HasPrefix(version, "go") || !modfile.GoVersionRE.MatchString(version[2:]) {
+				base.Fatalf("go: internal error: unrecognized version %q", version)
+			}
+			return version[2:]
+		}
+	}
+	return v
+}
+
 var altConfigs = []string{
 	"Gopkg.lock",
 
@@ -918,14 +908,8 @@ func findModulePath(dir string) (string, error) {
 		}
 		if rel := search.InDir(dir, filepath.Join(gpdir, "src")); rel != "" && rel != "." {
 			path := filepath.ToSlash(rel)
-			// TODO(matloob): replace this with module.CheckImportPath
-			// once it's been laxened.
-			// Only checkModulePathLax here. There are some unpublishable
-			// module names that are compatible with checkModulePathLax
-			// but they already work in GOPATH so don't break users
-			// trying to do a build with modules. gorelease will alert users
-			// publishing their modules to fix their paths.
-			if err := checkModulePathLax(path); err != nil {
+			// gorelease will alert users publishing their modules to fix their paths.
+			if err := module.CheckImportPath(path); err != nil {
 				badPathErr = err
 				break
 			}
diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index 83fc7c09c3..a9d1777125 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/src/cmd/go/internal/modload/load.go
@@ -152,6 +152,13 @@ type PackageOpts struct {
 	// packages.
 	Tidy bool
 
+	// TidyCompatibleVersion is the oldest Go version that must be able to
+	// reproducibly reload the requested packages.
+	//
+	// If empty, the compatible version is the Go version immediately prior to the
+	// 'go' version listed in the go.mod file.
+	TidyCompatibleVersion string
+
 	// VendorModulesInGOROOTSrc indicates that if we are within a module in
 	// GOROOT/src, packages in the module's vendor directory should be resolved as
 	// actual module dependencies (instead of standard-library packages).
@@ -314,10 +321,6 @@ 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,
@@ -375,12 +378,31 @@ func LoadPackages(ctx context.Context, opts PackageOpts, patterns ...string) (ma
 			}
 		}
 
-		modfetch.TrimGoSum(keepSums(ctx, ld, ld.requirements, loadedZipSumsOnly))
+		keep := keepSums(ctx, ld, ld.requirements, loadedZipSumsOnly)
+		if compatDepth := modDepthFromGoVersion(ld.TidyCompatibleVersion); compatDepth != ld.requirements.depth {
+			compatRS := newRequirements(compatDepth, ld.requirements.rootModules, ld.requirements.direct)
+			ld.checkTidyCompatibility(ctx, compatRS)
+
+			for m := range keepSums(ctx, ld, compatRS, loadedZipSumsOnly) {
+				keep[m] = true
+			}
+		}
+
+		if allowWriteGoMod {
+			modfetch.TrimGoSum(keep)
+
+			// commitRequirements below will also call WriteGoSum, but the "keep" map
+			// we have here could be strictly larger: commitRequirements only commits
+			// loaded.requirements, but here we may have also loaded (and want to
+			// preserve checksums for) additional entities from compatRS, which are
+			// only needed for compatibility with ld.TidyCompatibleVersion.
+			modfetch.WriteGoSum(keep)
+		}
 	}
 
 	// Success! Update go.mod and go.sum (if needed) and return the results.
 	loaded = ld
-	commitRequirements(ctx, opts.GoVersion, loaded.requirements)
+	commitRequirements(ctx, loaded.GoVersion, loaded.requirements)
 
 	for _, pkg := range ld.pkgs {
 		if !pkg.isTest() {
@@ -605,10 +627,8 @@ 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,
@@ -620,7 +640,7 @@ func ImportFromFiles(ctx context.Context, gofiles []string) {
 			return roots
 		},
 	})
-	commitRequirements(ctx, goVersion, loaded.requirements)
+	commitRequirements(ctx, loaded.GoVersion, loaded.requirements)
 }
 
 // DirImportPath returns the effective import path for dir,
@@ -921,28 +941,38 @@ func loadFromRoots(ctx context.Context, params loaderParams) *loader {
 		work:         par.NewQueue(runtime.GOMAXPROCS(0)),
 	}
 
-	if params.GoVersion != "" {
-		goVersionV := "v" + params.GoVersion
-		if semver.Compare(goVersionV, 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
-		}
+	if ld.GoVersion == "" {
+		ld.GoVersion = modFileGoVersion()
 
-		if ld.Tidy && semver.Compare(goVersionV, "v"+latestGoVersion()) > 0 {
-			ld.errorf("go mod tidy: go.mod file indicates go %s, but maximum supported version is %s\n", params.GoVersion, latestGoVersion())
+		if ld.Tidy && semver.Compare("v"+ld.GoVersion, "v"+LatestGoVersion()) > 0 {
+			ld.errorf("go mod tidy: go.mod file indicates go %s, but maximum supported version is %s\n", ld.GoVersion, LatestGoVersion())
 			base.ExitIfErrors()
 		}
+	}
 
-		var err error
-		ld.requirements, err = convertDepth(ctx, ld.requirements, modDepthFromGoVersion(params.GoVersion))
-		if err != nil {
-			ld.errorf("go: %v\n", err)
+	if ld.Tidy {
+		if ld.TidyCompatibleVersion == "" {
+			ld.TidyCompatibleVersion = priorGoVersion(ld.GoVersion)
+		} else if semver.Compare("v"+ld.TidyCompatibleVersion, "v"+ld.GoVersion) > 0 {
+			// Each version of the Go toolchain knows how to interpret go.mod and
+			// go.sum files produced by all previous versions, so a compatibility
+			// version higher than the go.mod version adds nothing.
+			ld.TidyCompatibleVersion = ld.GoVersion
 		}
 	}
 
+	if semver.Compare("v"+ld.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.
+		ld.allClosesOverTests = true
+	}
+
+	var err error
+	ld.requirements, err = convertDepth(ctx, ld.requirements, modDepthFromGoVersion(ld.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)
@@ -1079,7 +1109,7 @@ func loadFromRoots(ctx context.Context, params loaderParams) *loader {
 			// If that is not the case, there is a bug in the loading loop above.
 			for _, m := range rs.rootModules {
 				if v, ok := ld.requirements.rootSelected(m.Path); !ok || v != m.Version {
-					ld.errorf("go: internal error: a requirement on %v is needed but was not added during package loading\n", m)
+					ld.errorf("go mod tidy: internal error: a requirement on %v is needed but was not added during package loading\n", m)
 					base.ExitIfErrors()
 				}
 			}
@@ -1750,6 +1780,219 @@ func (ld *loader) checkMultiplePaths() {
 	}
 }
 
+// checkTidyCompatibility emits an error if any package would be loaded from a
+// different module under rs than under ld.requirements.
+func (ld *loader) checkTidyCompatibility(ctx context.Context, rs *Requirements) {
+	suggestUpgrade := false
+	suggestEFlag := false
+	suggestFixes := func() {
+		if ld.AllowErrors {
+			// The user is explicitly ignoring these errors, so don't bother them with
+			// other options.
+			return
+		}
+
+		// We print directly to os.Stderr because this information is advice about
+		// how to fix errors, not actually an error itself.
+		// (The actual errors should have been logged already.)
+
+		fmt.Fprintln(os.Stderr)
+
+		goFlag := ""
+		if ld.GoVersion != modFileGoVersion() {
+			goFlag = " -go=" + ld.GoVersion
+		}
+
+		compatFlag := ""
+		if ld.TidyCompatibleVersion != priorGoVersion(ld.GoVersion) {
+			compatFlag = " -compat=" + ld.TidyCompatibleVersion
+		}
+		if suggestUpgrade {
+			eDesc := ""
+			eFlag := ""
+			if suggestEFlag {
+				eDesc = ", leaving some packages unresolved"
+				eFlag = " -e"
+			}
+			fmt.Fprintf(os.Stderr, "To upgrade to the versions selected by go %s%s:\n\tgo mod tidy%s -go=%s && go mod tidy%s -go=%s%s\n", ld.TidyCompatibleVersion, eDesc, eFlag, ld.TidyCompatibleVersion, eFlag, ld.GoVersion, compatFlag)
+		} else if suggestEFlag {
+			// If some packages are missing but no package is upgraded, then we
+			// shouldn't suggest upgrading to the Go 1.16 versions explicitly — that
+			// wouldn't actually fix anything for Go 1.16 users, and *would* break
+			// something for Go 1.17 users.
+			fmt.Fprintf(os.Stderr, "To proceed despite packages unresolved in go %s:\n\tgo mod tidy -e%s%s\n", ld.TidyCompatibleVersion, goFlag, compatFlag)
+		}
+
+		fmt.Fprintf(os.Stderr, "If reproducibility with go %s is not needed:\n\tgo mod tidy%s -compat=%s\n", ld.TidyCompatibleVersion, goFlag, ld.GoVersion)
+
+		// TODO(#46141): Populate the linked wiki page.
+		fmt.Fprintf(os.Stderr, "For other options, see:\n\thttps://golang.org/wiki/PruningModules\n")
+	}
+
+	mg, err := rs.Graph(ctx)
+	if err != nil {
+		ld.errorf("go mod tidy: error loading go %s module graph: %v\n", ld.TidyCompatibleVersion, err)
+		suggestFixes()
+		return
+	}
+
+	// Re-resolve packages in parallel.
+	//
+	// We re-resolve each package — rather than just checking versions — to ensure
+	// that we have fetched module source code (and, importantly, checksums for
+	// that source code) for all modules that are necessary to ensure that imports
+	// are unambiguous. That also produces clearer diagnostics, since we can say
+	// exactly what happened to the package if it became ambiguous or disappeared
+	// entirely.
+	//
+	// We re-resolve the packages in parallel because this process involves disk
+	// I/O to check for package sources, and because the process of checking for
+	// ambiguous imports may require us to download additional modules that are
+	// otherwise pruned out in Go 1.17 — we don't want to block progress on other
+	// packages while we wait for a single new download.
+	type mismatch struct {
+		mod module.Version
+		err error
+	}
+	mismatchMu := make(chan map[*loadPkg]mismatch, 1)
+	mismatchMu <- map[*loadPkg]mismatch{}
+	for _, pkg := range ld.pkgs {
+		if pkg.mod.Path == "" && pkg.err == nil {
+			// This package is from the standard library (which does not vary based on
+			// the module graph).
+			continue
+		}
+
+		pkg := pkg
+		ld.work.Add(func() {
+			mod, _, err := importFromModules(ctx, pkg.path, rs, mg)
+			if mod != pkg.mod {
+				mismatches := <-mismatchMu
+				mismatches[pkg] = mismatch{mod: mod, err: err}
+				mismatchMu <- mismatches
+			}
+		})
+	}
+	<-ld.work.Idle()
+
+	mismatches := <-mismatchMu
+	if len(mismatches) == 0 {
+		// Since we're running as part of 'go mod tidy', the roots of the module
+		// graph should contain only modules that are relevant to some package in
+		// the package graph. We checked every package in the package graph and
+		// didn't find any mismatches, so that must mean that all of the roots of
+		// the module graph are also consistent.
+		//
+		// If we're wrong, Go 1.16 in -mod=readonly mode will error out with
+		// "updates to go.mod needed", which would be very confusing. So instead,
+		// we'll double-check that our reasoning above actually holds — if it
+		// doesn't, we'll emit an internal error and hopefully the user will report
+		// it as a bug.
+		for _, m := range ld.requirements.rootModules {
+			if v := mg.Selected(m.Path); v != m.Version {
+				fmt.Fprintln(os.Stderr)
+				base.Fatalf("go: internal error: failed to diagnose selected-version mismatch for module %s: go %s selects %s, but go %s selects %s\n\tPlease report this at https://golang.org/issue.", m.Path, ld.GoVersion, m.Version, ld.TidyCompatibleVersion, v)
+			}
+		}
+		return
+	}
+
+	// Iterate over the packages (instead of the mismatches map) to emit errors in
+	// deterministic order.
+	for _, pkg := range ld.pkgs {
+		mismatch, ok := mismatches[pkg]
+		if !ok {
+			continue
+		}
+
+		if pkg.isTest() {
+			// We already did (or will) report an error for the package itself,
+			// so don't report a duplicate (and more vebose) error for its test.
+			if _, ok := mismatches[pkg.testOf]; !ok {
+				base.Fatalf("go: internal error: mismatch recorded for test %s, but not its non-test package", pkg.path)
+			}
+			continue
+		}
+
+		switch {
+		case mismatch.err != nil:
+			// pkg resolved successfully, but errors out using the requirements in rs.
+			//
+			// This could occur because the import is provided by a single lazy root
+			// (and is thus unambiguous in lazy mode) and also one or more
+			// transitive dependencies (and is ambiguous in eager mode).
+			//
+			// It could also occur because some transitive dependency upgrades the
+			// module that previously provided the package to a version that no
+			// longer does, or to a version for which the module source code (but
+			// not the go.mod file in isolation) has a checksum error.
+			if missing := (*ImportMissingError)(nil); errors.As(mismatch.err, &missing) {
+				selected := module.Version{
+					Path:    pkg.mod.Path,
+					Version: mg.Selected(pkg.mod.Path),
+				}
+				ld.errorf("%s loaded from %v,\n\tbut go %s would fail to locate it in %s\n", pkg.stackText(), pkg.mod, ld.TidyCompatibleVersion, selected)
+			} else {
+				if ambiguous := (*AmbiguousImportError)(nil); errors.As(mismatch.err, &ambiguous) {
+					// TODO: Is this check needed?
+				}
+				ld.errorf("%s loaded from %v,\n\tbut go %s would fail to locate it:\n\t%v\n", pkg.stackText(), pkg.mod, ld.TidyCompatibleVersion, mismatch.err)
+			}
+
+			suggestEFlag = true
+
+			// Even if we press ahead with the '-e' flag, the older version will
+			// error out in readonly mode if it thinks the go.mod file contains
+			// any *explicit* dependency that is not at its selected version,
+			// even if that dependency is not relevant to any package being loaded.
+			//
+			// We check for that condition here. If all of the roots are consistent
+			// the '-e' flag suffices, but otherwise we need to suggest an upgrade.
+			if !suggestUpgrade {
+				for _, m := range ld.requirements.rootModules {
+					if v := mg.Selected(m.Path); v != m.Version {
+						suggestUpgrade = true
+						break
+					}
+				}
+			}
+
+		case pkg.err != nil:
+			// pkg had an error in lazy mode (presumably suppressed with the -e flag),
+			// but not in eager mode.
+			//
+			// This is possible, if, say, the import is unresolved in lazy mode
+			// (because the "latest" version of each candidate module either is
+			// unavailable or does not contain the package), but is resolved in
+			// eager mode due to a newer-than-latest dependency that is normally
+			// runed out of the module graph.
+			//
+			// This could also occur if the source code for the module providing the
+			// package in lazy mode has a checksum error, but eager mode upgrades
+			// that module to a version with a correct checksum.
+			//
+			// pkg.err should have already been logged elsewhere — along with a
+			// stack trace — so log only the import path and non-error info here.
+			suggestUpgrade = true
+			ld.errorf("%s failed to load from any module,\n\tbut go %s would load it from %v\n", pkg.path, ld.TidyCompatibleVersion, mismatch.mod)
+
+		case pkg.mod != mismatch.mod:
+			// The package is loaded successfully by both Go versions, but from a
+			// different module in each. This could lead to subtle (and perhaps even
+			// unnoticed!) variations in behavior between builds with different
+			// toolchains.
+			suggestUpgrade = true
+			ld.errorf("%s loaded from %v,\n\tbut go %s would select %v\n", pkg.stackText(), pkg.mod, ld.TidyCompatibleVersion, mismatch.mod.Version)
+
+		default:
+			base.Fatalf("go: internal error: mismatch recorded for package %s, but no differences found", pkg.path)
+		}
+	}
+
+	suggestFixes()
+	base.ExitIfErrors()
+}
+
 // scanDir is like imports.ScanDir but elides known magic imports from the list,
 // so that we do not go looking for packages that don't really exist.
 //
diff --git a/src/cmd/go/internal/modload/modfile.go b/src/cmd/go/internal/modload/modfile.go
index bafff3e080..a9c3a91d35 100644
--- a/src/cmd/go/internal/modload/modfile.go
+++ b/src/cmd/go/internal/modload/modfile.go
@@ -55,7 +55,7 @@ var modFile *modfile.File
 // in modFile are intepreted, or the latest Go version if modFile is nil.
 func modFileGoVersion() string {
 	if modFile == nil {
-		return latestGoVersion()
+		return LatestGoVersion()
 	}
 	if modFile.Go == nil || modFile.Go.Version == "" {
 		// The main module necessarily has a go.mod file, and that file lacks a
diff --git a/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.1.txt b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.1.txt
index 00b71bf0d5..7982cccea1 100644
--- a/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.1.txt
+++ b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.1.txt
@@ -5,7 +5,7 @@ module "rsc.io/sampler"
 
 require "golang.org/x/text" v0.0.0-20170915032832-14c0d48ead0c
 -- .info --
-{"Version":"v1.2.1","Name":"cac3af4f8a0ab40054fa6f8d423108a63a1255bb","Short":"cac3af4f8a0a","Time":"2018-02-13T18:16:22Z"}EOF
+{"Version":"v1.2.1","Name":"cac3af4f8a0ab40054fa6f8d423108a63a1255bb","Short":"cac3af4f8a0a","Time":"2018-02-13T18:16:22Z"}
 -- hello.go --
 // Copyright 2018 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
diff --git a/src/cmd/go/testdata/script/mod_deprecate_message.txt b/src/cmd/go/testdata/script/mod_deprecate_message.txt
index 4a0674b808..567027935d 100644
--- a/src/cmd/go/testdata/script/mod_deprecate_message.txt
+++ b/src/cmd/go/testdata/script/mod_deprecate_message.txt
@@ -1,26 +1,26 @@
 # When there is a short single-line message, 'go get' should print it all.
 go get -d short
-stderr '^go: warning: module short is deprecated: short$'
+stderr '^go: module short is deprecated: short$'
 go list -m -u -f '{{.Deprecated}}' short
 stdout '^short$'
 
 # When there is a multi-line message, 'go get' should print the first line.
 go get -d multiline
-stderr '^go: warning: module multiline is deprecated: first line$'
+stderr '^go: module multiline is deprecated: first line$'
 ! stderr 'second line'
 go list -m -u -f '{{.Deprecated}}' multiline
 stdout '^first line\nsecond line.$'
 
 # When there is a long message, 'go get' should print a placeholder.
 go get -d long
-stderr '^go: warning: module long is deprecated: \(message omitted: too long\)$'
+stderr '^go: module long is deprecated: \(message omitted: too long\)$'
 go list -m -u -f '{{.Deprecated}}' long
 stdout '^aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa$'
 
 # When a message contains unprintable chracters, 'go get' should say that
 # without printing the message.
 go get -d unprintable
-stderr '^go: warning: module unprintable is deprecated: \(message omitted: contains non-printable characters\)$'
+stderr '^go: module unprintable is deprecated: \(message omitted: contains non-printable characters\)$'
 go list -m -u -f '{{.Deprecated}}' unprintable
 stdout '^message contains ASCII BEL\x07$'
 
diff --git a/src/cmd/go/testdata/script/mod_download.txt b/src/cmd/go/testdata/script/mod_download.txt
index 8a9faffe4e..ad640b45de 100644
--- a/src/cmd/go/testdata/script/mod_download.txt
+++ b/src/cmd/go/testdata/script/mod_download.txt
@@ -107,13 +107,28 @@ stderr '^go mod download: skipping argument m that resolves to the main module\n
 ! go mod download m@latest
 stderr '^go mod download: m@latest: malformed module path "m": missing dot in first path element$'
 
-# download updates go.mod and populates go.sum
+# download without arguments updates go.mod and go.sum after loading the
+# build list, but does not save sums for downloaded zips.
 cd update
+cp go.mod.orig go.mod
 ! exists go.sum
 go mod download
+cmp go.mod.update go.mod
+cmp go.sum.update go.sum
+cp go.mod.orig go.mod
+rm go.sum
+
+# download with arguments (even "all") does update go.mod and go.sum.
+go mod download rsc.io/sampler
+cmp go.mod.update go.mod
 grep '^rsc.io/sampler v1.3.0 ' go.sum
-go list -m rsc.io/sampler
-stdout '^rsc.io/sampler v1.3.0$'
+cp go.mod.orig go.mod
+rm go.sum
+
+go mod download all
+cmp go.mod.update go.mod
+grep '^rsc.io/sampler v1.3.0 ' go.sum
+cd ..
 
 # allow go mod download without go.mod
 env GO111MODULE=auto
@@ -131,7 +146,7 @@ stderr 'get '$GOPROXY
 -- go.mod --
 module m
 
--- update/go.mod --
+-- update/go.mod.orig --
 module m
 
 go 1.16
@@ -140,3 +155,17 @@ require (
 	rsc.io/quote v1.5.2
 	rsc.io/sampler v1.2.1 // older version than in build list
 )
+-- update/go.mod.update --
+module m
+
+go 1.16
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/sampler v1.3.0 // older version than in build list
+)
+-- update/go.sum.update --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.2.1/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
diff --git a/src/cmd/go/testdata/script/mod_get_deprecated.txt b/src/cmd/go/testdata/script/mod_get_deprecated.txt
index 4633009f69..7bdd7a58a8 100644
--- a/src/cmd/go/testdata/script/mod_get_deprecated.txt
+++ b/src/cmd/go/testdata/script/mod_get_deprecated.txt
@@ -4,14 +4,14 @@ go get -d ./use/nothing
 
 # 'go get pkg' should show a deprecation message for the module providing pkg.
 go get -d example.com/deprecated/a
-stderr '^go: warning: module example.com/deprecated/a is deprecated: in example.com/deprecated/a@v1.9.0$'
+stderr '^go: module example.com/deprecated/a is deprecated: in example.com/deprecated/a@v1.9.0$'
 go get -d example.com/deprecated/a@v1.0.0
-stderr '^go: warning: module example.com/deprecated/a is deprecated: in example.com/deprecated/a@v1.9.0$'
+stderr '^go: module example.com/deprecated/a is deprecated: in example.com/deprecated/a@v1.9.0$'
 
 # 'go get pkg' should show a deprecation message for a module providing
 # packages directly imported by pkg.
 go get -d ./use/a
-stderr '^go: warning: module example.com/deprecated/a is deprecated: in example.com/deprecated/a@v1.9.0$'
+stderr '^go: module example.com/deprecated/a is deprecated: in example.com/deprecated/a@v1.9.0$'
 
 # 'go get pkg' may show a deprecation message for an indirectly required module
 # if it provides a package named on the command line.
@@ -20,7 +20,7 @@ go get -d ./use/b
 go get -d local/use
 ! stderr 'module.*is deprecated'
 go get -d example.com/deprecated/b
-stderr '^go: warning: module example.com/deprecated/b is deprecated: in example.com/deprecated/b@v1.9.0$'
+stderr '^go: module example.com/deprecated/b is deprecated: in example.com/deprecated/b@v1.9.0$'
 
 # 'go get pkg' does not show a deprecation message for a module providing a
 # directly imported package if the module is no longer deprecated in its
diff --git a/src/cmd/go/testdata/script/mod_get_trailing_slash.txt b/src/cmd/go/testdata/script/mod_get_trailing_slash.txt
index 3b38d8ba7d..c536693537 100644
--- a/src/cmd/go/testdata/script/mod_get_trailing_slash.txt
+++ b/src/cmd/go/testdata/script/mod_get_trailing_slash.txt
@@ -1,6 +1,3 @@
-# Populate go.sum
-go mod download
-
 # go list should succeed to load a package ending with ".go" if the path does
 # not correspond to an existing local file. Listing a pattern ending with
 # ".go/" should try to list a package regardless of whether a file exists at the
@@ -31,3 +28,10 @@ module m
 go 1.13
 
 require example.com/dotgo.go v1.0.0
+-- go.sum --
+example.com/dotgo.go v1.0.0 h1:XKJfs0V8x2PvY2tX8bJBCEbCDLnt15ma2onwhVpew/I=
+example.com/dotgo.go v1.0.0/go.mod h1:Qi6z/X3AC5vHiuMt6HF2ICx3KhIBGrMdrA7YoPDKqR0=
+-- use.go --
+package use
+
+import _ "example.com/dotgo.go"
diff --git a/src/cmd/go/testdata/script/mod_init_path.txt b/src/cmd/go/testdata/script/mod_init_path.txt
index ccdfc92317..e5fd4ddbcb 100644
--- a/src/cmd/go/testdata/script/mod_init_path.txt
+++ b/src/cmd/go/testdata/script/mod_init_path.txt
@@ -1,7 +1,7 @@
 env GO111MODULE=on
 
 ! go mod init .
-stderr '^go: invalid module path "\.": is a local import path$'
+stderr '^go: malformed module path ".": is a local import path$'
 
 cd x
 go mod init example.com/x
diff --git a/src/cmd/go/testdata/script/mod_invalid_path.txt b/src/cmd/go/testdata/script/mod_invalid_path.txt
index c8c075daae..333a3ffa35 100644
--- a/src/cmd/go/testdata/script/mod_invalid_path.txt
+++ b/src/cmd/go/testdata/script/mod_invalid_path.txt
@@ -8,11 +8,8 @@ stderr '^go: no module declaration in go.mod. To specify the module path:\n\tgo
 # Test that go mod init in GOPATH doesn't add a module declaration
 # with a path that can't possibly be a module path, because
 # it isn't even a valid import path.
-# The single quote and backtick are the only characters  we don't allow
-# in checkModulePathLax, but is allowed in a Windows file name.
-# TODO(matloob): choose a different character once
-# module.CheckImportPath is laxened and replaces
-# checkModulePathLax.
+# The single quote and backtick are the only characters which are not allowed
+# but are a valid Windows file name.
 cd $WORK/'gopath/src/m''d'
 ! go mod init
 stderr 'cannot determine module path'
@@ -21,7 +18,7 @@ stderr 'cannot determine module path'
 # possibly be a module path, because it isn't even a valid import path
 cd $WORK/gopath/src/badname
 ! go list .
-stderr 'invalid module path'
+stderr 'malformed module path'
 
 # Test that an import path containing an element with a leading dot is valid,
 # but such a module path is not.
diff --git a/src/cmd/go/testdata/script/mod_query.txt b/src/cmd/go/testdata/script/mod_query.txt
index e10185709d..a75f86ed7c 100644
--- a/src/cmd/go/testdata/script/mod_query.txt
+++ b/src/cmd/go/testdata/script/mod_query.txt
@@ -1,9 +1,7 @@
 env GO111MODULE=on
 
-# Populate go.sum.
 # TODO(golang.org/issue/41297): we shouldn't need go.sum. None of the commands
 # below depend on the build list.
-go mod download
 
 go list -m -versions rsc.io/quote
 stdout '^rsc.io/quote v1.0.0 v1.1.0 v1.2.0 v1.2.1 v1.3.0 v1.4.0 v1.5.0 v1.5.1 v1.5.2 v1.5.3-pre1$'
@@ -36,6 +34,9 @@ stdout 'no matching versions for query ">v1.5.3"'
 module x
 require rsc.io/quote v1.0.0
 
+-- go.sum --
+rsc.io/quote v1.0.0 h1:kQ3IZQzPTiDJxSZI98YaWgxFEhlNdYASHvh+MplbViw=
+rsc.io/quote v1.0.0/go.mod h1:v83Ri/njykPcgJltBc/gEkJTmjTsNgtO1Y7vyIK1CQA=
 -- use.go --
 package use
 
diff --git a/src/cmd/go/testdata/script/mod_replace.txt b/src/cmd/go/testdata/script/mod_replace.txt
index dc9667f1d0..a0a367fb1d 100644
--- a/src/cmd/go/testdata/script/mod_replace.txt
+++ b/src/cmd/go/testdata/script/mod_replace.txt
@@ -48,7 +48,7 @@ stderr 'module rsc.io/quote/v3@upgrade found \(v3.0.0, replaced by ./local/rsc.i
 # The reported Dir and GoMod for a replaced module should be accurate.
 cp go.mod.orig go.mod
 go mod edit -replace=rsc.io/quote/v3=not-rsc.io/quote@v0.1.0-nomod
-go mod download
+go mod download rsc.io/quote/v3
 go list -m -f '{{.Path}} {{.Version}} {{.Dir}} {{.GoMod}}{{with .Replace}} => {{.Path}} {{.Version}} {{.Dir}} {{.GoMod}}{{end}}' rsc.io/quote/v3
 stdout '^rsc.io/quote/v3 v3.0.0 '$GOPATH'[/\\]pkg[/\\]mod[/\\]not-rsc.io[/\\]quote@v0.1.0-nomod '$GOPATH'[/\\]pkg[/\\]mod[/\\]cache[/\\]download[/\\]not-rsc.io[/\\]quote[/\\]@v[/\\]v0.1.0-nomod.mod => not-rsc.io/quote v0.1.0-nomod '$GOPATH'[/\\]pkg[/\\]mod[/\\]not-rsc.io[/\\]quote@v0.1.0-nomod '$GOPATH'[/\\]pkg[/\\]mod[/\\]cache[/\\]download[/\\]not-rsc.io[/\\]quote[/\\]@v[/\\]v0.1.0-nomod.mod$'
 
diff --git a/src/cmd/go/testdata/script/mod_retract.txt b/src/cmd/go/testdata/script/mod_retract.txt
index a52e05bc72..4f95ece8d7 100644
--- a/src/cmd/go/testdata/script/mod_retract.txt
+++ b/src/cmd/go/testdata/script/mod_retract.txt
@@ -1,8 +1,5 @@
 cp go.mod go.mod.orig
 
-# Populate go.sum.
-go mod download
-
 # 'go list pkg' does not report an error when a retracted version is used.
 go list -e -f '{{if .Error}}{{.Error}}{{end}}' ./use
 ! stdout .
@@ -32,6 +29,11 @@ go 1.15
 
 require example.com/retract v1.0.0-bad
 
+-- go.sum --
+example.com/retract v1.0.0-bad h1:liAW69rbtjY67x2CcNzat668L/w+YGgNX3lhJsWIJis=
+example.com/retract v1.0.0-bad/go.mod h1:0DvGGofJ9hr1q63cBrOY/jSY52OwhRGA0K47NE80I5Y=
+example.com/retract/self/prev v1.1.0 h1:0/8I/GTG+1eJTFeDQ/fUbgrMsVHHyKhh3Z8DSZp1fuA=
+example.com/retract/self/prev v1.1.0/go.mod h1:xl2EcklWuZZHVtHWcpzfSJQmnzAGpKZYpA/Wto7SZN4=
 -- use/use.go --
 package use
 
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat.txt b/src/cmd/go/testdata/script/mod_tidy_compat.txt
new file mode 100644
index 0000000000..e6edef5ee3
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat.txt
@@ -0,0 +1,95 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+
+# This module has the same module dependency graph in Go 1.16 as in Go 1.17,
+# but in 1.16 requires (checksums for) additional (irrelevant) go.mod files.
+#
+# The module graph under both versions looks like:
+#
+# m ---- example.com/version v1.1.0
+# |
+# + ---- example.net/lazy v0.1.0 ---- example.com/version v1.0.1
+#
+# Go 1.17 avoids loading the go.mod file for example.com/version v1.0.1
+# (because it is lower than the verison explicitly required by m,
+# and the module that requires it — m — specifies 'go 1.17').
+#
+# That go.mod file happens not to affect the final 1.16 module graph anyway,
+# so the pruned graph is equivalent to the unpruned one.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+go list -m all
+cmp stdout m_all.txt
+
+go mod edit -go=1.16
+go list -m all
+cmp stdout m_all.txt
+
+
+# If we explicitly drop compatibility with 1.16, we retain fewer checksums,
+# which gives a cleaner go.sum file but causes 1.16 to fail in readonly mode.
+
+cp go.mod.orig go.mod
+go mod tidy -compat=1.17
+cmp go.mod go.mod.orig
+
+go list -m all
+cmp stdout m_all.txt
+
+go mod edit -go=1.16
+! go list -m all
+stderr '^go list -m: example.net/lazy@v0.1.0 requires\n\texample.com/version@v1.0.1: missing go.sum entry; to add it:\n\tgo mod download example.com/version$'
+
+
+-- go.mod --
+// Module m happens to have the exact same build list as what would be
+// selected under Go 1.16, but computes that build list without looking at
+// as many go.mod files.
+module example.com/m
+
+go 1.17
+
+replace example.net/lazy v0.1.0 => ./lazy
+
+require (
+	example.com/version v1.1.0
+	example.net/lazy v0.1.0
+)
+-- m_all.txt --
+example.com/m
+example.com/version v1.1.0
+example.net/lazy v0.1.0 => ./lazy
+-- compatible.go --
+package compatible
+
+import (
+	_ "example.com/version"
+	_ "example.net/lazy"
+)
+-- lazy/go.mod --
+// Module lazy requires example.com/version v1.0.1.
+//
+// However, since this module is lazy, its dependents
+// should not need checksums for that version of the module
+// unless they actually import packages from it.
+module example.net/lazy
+
+go 1.17
+
+require example.com/version v1.0.1
+-- lazy/lazy.go --
+package lazy
+
+import _ "example.com/version"
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat_added.txt b/src/cmd/go/testdata/script/mod_tidy_compat_added.txt
new file mode 100644
index 0000000000..94fa79bc9f
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat_added.txt
@@ -0,0 +1,105 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+
+# For this module, Go 1.17 produces an error for one module, and Go 1.16
+# produces a different error for a different module.
+
+cp go.mod go.mod.orig
+
+! go mod tidy
+
+stderr '^example\.com/m imports\n\texample\.net/added: module example\.net/added@latest found \(v0\.3\.0, replaced by \./a1\), but does not contain package example\.net/added$'
+
+cmp go.mod go.mod.orig
+
+
+# When we run 'go mod tidy -e', we should proceed past the first error and follow
+# it with a second error describing the version descrepancy.
+#
+# We should not provide advice on how to push past the version descrepancy,
+# because the '-e' flag should already do that, writing out an otherwise-tidied
+# go.mod file.
+
+go mod tidy -e
+
+stderr '^example\.com/m imports\n\texample\.net/added: module example\.net/added@latest found \(v0\.3\.0, replaced by \./a1\), but does not contain package example\.net/added\nexample\.net/added failed to load from any module,\n\tbut go 1\.16 would load it from example\.net/added@v0\.2\.0$'
+
+! stderr '\n\tgo mod tidy'
+
+cmp go.mod go.mod.tidy
+
+
+-- go.mod --
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/added v0.1.0 => ./a1
+	example.net/added v0.2.0 => ./a2
+	example.net/added v0.3.0 => ./a1
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/pruned v0.1.0 => ./pruned
+)
+
+require (
+	example.net/added v0.1.0
+	example.net/lazy v0.1.0
+)
+-- go.mod.tidy --
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/added v0.1.0 => ./a1
+	example.net/added v0.2.0 => ./a2
+	example.net/added v0.3.0 => ./a1
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/pruned v0.1.0 => ./pruned
+)
+
+require example.net/lazy v0.1.0
+-- m.go --
+package m
+
+import (
+	_ "example.net/added"
+	_ "example.net/lazy"
+)
+
+-- a1/go.mod --
+module example.net/added
+
+go 1.17
+-- a2/go.mod --
+module example.net/added
+
+go 1.17
+-- a2/added.go --
+package added
+
+-- lazy/go.mod --
+module example.net/lazy
+
+go 1.17
+
+require example.net/pruned v0.1.0
+-- lazy/lazy.go --
+package lazy
+
+-- pruned/go.mod --
+module example.net/pruned
+
+go 1.17
+
+require example.net/added v0.2.0
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat_ambiguous.txt b/src/cmd/go/testdata/script/mod_tidy_compat_ambiguous.txt
new file mode 100644
index 0000000000..ed1dd53eff
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat_ambiguous.txt
@@ -0,0 +1,98 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+# For this module, the dependency providing package
+# example.net/ambiguous/nested/pkg is unambiguous in Go 1.17 (because only one
+# root of the module graph contains the package), whereas it is ambiguous in
+# Go 1.16 (because two different modules contain plausible packages and Go 1.16
+# does not privilege roots above other dependencies).
+#
+# However, the overall build list is identical for both versions.
+
+cp go.mod go.mod.orig
+
+! go mod tidy
+
+stderr '^example\.com/m imports\n\texample\.net/indirect imports\n\texample\.net/ambiguous/nested/pkg loaded from example\.net/ambiguous/nested@v0\.1\.0,\n\tbut go 1.16 would fail to locate it:\n\tambiguous import: found package example\.net/ambiguous/nested/pkg in multiple modules:\n\texample\.net/ambiguous v0.1.0 \(.*\)\n\texample\.net/ambiguous/nested v0.1.0 \(.*\)\n\n'
+
+stderr '\n\nTo proceed despite packages unresolved in go 1\.16:\n\tgo mod tidy -e\nIf reproducibility with go 1.16 is not needed:\n\tgo mod tidy -compat=1\.17\nFor other options, see:\n\thttps://golang\.org/wiki/PruningModules\n'
+
+cmp go.mod go.mod.orig
+
+
+# If we run 'go mod tidy -e', we should still save enough checksums to run
+# 'go list -m all' reproducibly with go 1.16, even though we can't list
+# the specific package.
+
+go mod tidy -e
+! stderr '\n\tgo mod tidy'
+cmp go.mod go.mod.orig
+
+go list -m all
+cmp stdout all-m.txt
+
+go list -f $MODFMT example.net/ambiguous/nested/pkg
+stdout '^example.net/ambiguous/nested v0\.1\.0$'
+! stderr .
+
+go mod edit -go=1.16
+go list -m all
+cmp stdout all-m.txt
+
+! go list -f $MODFMT example.net/ambiguous/nested/pkg
+stderr '^ambiguous import: found package example\.net/ambiguous/nested/pkg in multiple modules:\n\texample\.net/ambiguous v0\.1\.0 \(.*\)\n\texample\.net/ambiguous/nested v0\.1\.0 \(.*\)\n'
+
+
+# On the other hand, if we use -compat=1.17, 1.16 can't even load
+# the build list (due to missing checksums).
+
+cp go.mod.orig go.mod
+go mod tidy -compat=1.17
+! stderr .
+go list -m all
+cmp stdout all-m.txt
+
+go mod edit -go=1.16
+! go list -m all
+stderr '^go list -m: example\.net/indirect@v0\.1\.0 requires\n\texample\.net/ambiguous@v0\.1\.0: missing go\.sum entry; to add it:\n\tgo mod download example\.net/ambiguous\n'
+
+
+-- go.mod --
+module example.com/m
+
+go 1.17
+
+replace example.net/indirect v0.1.0 => ./indirect
+
+require (
+	example.net/ambiguous/nested v0.1.0 // indirect
+	example.net/indirect v0.1.0
+)
+-- all-m.txt --
+example.com/m
+example.net/ambiguous v0.1.0
+example.net/ambiguous/nested v0.1.0
+example.net/indirect v0.1.0 => ./indirect
+-- m.go --
+package m
+
+import _ "example.net/indirect"
+
+-- indirect/go.mod --
+module example.net/indirect
+
+go 1.17
+
+require example.net/ambiguous v0.1.0
+-- indirect/indirect.go --
+package indirect
+
+import _ "example.net/ambiguous/nested/pkg"
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat_deleted.txt b/src/cmd/go/testdata/script/mod_tidy_compat_deleted.txt
new file mode 100644
index 0000000000..3aacde2025
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat_deleted.txt
@@ -0,0 +1,128 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+
+# For this module, the "deleted" dependency contains an imported package, but
+# Go 1.16 selects a higher version (in which that package has been deleted).
+
+cp go.mod go.mod.orig
+
+! go mod tidy
+
+stderr '^example\.com/m imports\n\texample\.net/deleted loaded from example\.net/deleted@v0\.1\.0,\n\tbut go 1\.16 would fail to locate it in example\.net/deleted@v0\.2\.0\n\n'
+
+stderr '\n\nTo upgrade to the versions selected by go 1.16, leaving some packages unresolved:\n\tgo mod tidy -e -go=1\.16 && go mod tidy -e -go=1\.17\nIf reproducibility with go 1.16 is not needed:\n\tgo mod tidy -compat=1\.17\nFor other options, see:\n\thttps://golang\.org/wiki/PruningModules\n'
+
+
+# The suggested 'go mod tidy -e' command should proceed anyway.
+
+go mod tidy -e
+cmp go.mod go.mod.tidy
+
+
+# In 'go 1.16' mode we should error out in the way we claimed.
+
+cd 116-outside
+! go list -deps -f $MODFMT example.com/m
+stderr '^\.\.[/\\]m\.go:4:2: no required module provides package example\.net/deleted; to add it:\n\tgo get example\.net/deleted$'
+cd ..
+
+go mod edit -go=1.16
+! go list -deps -f $MODFMT example.com/m
+stderr '^go: updates to go\.mod needed; to update it:\n\tgo mod tidy$'
+
+! go mod tidy
+stderr '^example\.com/m imports\n\texample\.net/deleted: module example\.net/deleted@latest found \(v0\.2\.0, replaced by \./d2\), but does not contain package example\.net/deleted$'
+
+
+-- go.mod --
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/deleted v0.1.0 => ./d1
+	example.net/deleted v0.2.0 => ./d2
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/pruned v0.1.0 => ./pruned
+)
+
+require (
+	example.net/deleted v0.1.0
+	example.net/deleted v0.1.0 // redundant
+	example.net/lazy v0.1.0
+)
+-- go.mod.tidy --
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/deleted v0.1.0 => ./d1
+	example.net/deleted v0.2.0 => ./d2
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/pruned v0.1.0 => ./pruned
+)
+
+require (
+	example.net/deleted v0.1.0
+	example.net/lazy v0.1.0
+)
+-- 116-outside/go.mod --
+module outside
+
+go 1.16
+
+replace (
+	example.com/m => ../
+	example.net/deleted v0.1.0 => ../d1
+	example.net/deleted v0.2.0 => ../d2
+	example.net/lazy v0.1.0 => ../lazy
+	example.net/pruned v0.1.0 => ../pruned
+)
+
+require example.com/m v0.1.0
+-- m.go --
+package m
+
+import (
+	_ "example.net/deleted"
+	_ "example.net/lazy"
+)
+
+-- d1/go.mod --
+module example.net/deleted
+
+go 1.17
+-- d1/deleted.go --
+package deleted
+-- d2/go.mod --
+module example.net/deleted
+
+go 1.17
+-- d2/README --
+There is no longer a Go package here.
+
+-- lazy/go.mod --
+module example.net/lazy
+
+go 1.17
+
+require example.net/pruned v0.1.0
+-- lazy/lazy.go --
+package lazy
+
+-- pruned/go.mod --
+module example.net/pruned
+
+go 1.17
+
+require example.net/deleted v0.2.0
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat_implicit.txt b/src/cmd/go/testdata/script/mod_tidy_compat_implicit.txt
new file mode 100644
index 0000000000..e00aea930e
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat_implicit.txt
@@ -0,0 +1,129 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+
+# For this module, Go 1.16 selects the same versions of all explicit dependencies
+# as Go 1.17 does. However, Go 1.16 selects a higher version of an *implicit*
+# dependency, imported by a test of one of the (external) imported packages.
+# As a result, Go 1.16 also needs checksums for the module sources for that higher
+# version.
+#
+# The Go 1.16 module graph looks like:
+#
+# m ---- lazy v0.1.0 ---- incompatible v1.0.0
+#         |
+#         + ------------- requireincompatible v0.1.0 ---- incompatible v2.0.0+incompatible
+#
+# The Go 1.17 module graph is the same except that the dependencies of
+# requireincompatible are pruned out (because the module that requires
+# it — lazy v0.1.0 — specifies 'go 1.17', and it is not otherwise relevant to
+# the main module).
+
+# 'go mod tidy' should by default diagnose the difference in dependencies as an
+# error, with useful suggestions about how to resolve it.
+
+cp go.mod go.mod.orig
+! go mod tidy
+stderr '^example\.com/m imports\n\texample\.net/lazy tested by\n\texample\.net/lazy.test imports\n\texample\.com/retract/incompatible loaded from example\.com/retract/incompatible@v1\.0\.0,\n\tbut go 1\.16 would select v2\.0\.0\+incompatible\n\n'
+stderr '\n\nTo upgrade to the versions selected by go 1\.16:\n\tgo mod tidy -go=1\.16 && go mod tidy -go=1\.17\nIf reproducibility with go 1.16 is not needed:\n\tgo mod tidy -compat=1.17\nFor other options, see:\n\thttps://golang\.org/wiki/PruningModules\n'
+
+cmp go.mod go.mod.orig
+
+# The suggested '-compat' flag to ignore differences should silence the error
+# and leave go.mod unchanged, resulting in checksum errors when Go 1.16 tries
+# to load a module pruned out by Go 1.17.
+
+go mod tidy -compat=1.17
+! stderr .
+cmp go.mod go.mod.orig
+
+go list -deps -test -f $MODFMT all
+stdout '^example\.com/retract/incompatible v1\.0\.0$'
+
+go mod edit -go=1.16
+! go list -deps -test -f $MODFMT all
+
+	# TODO(#46160): -count=1 instead of -count=2.
+stderr -count=2 '^go: example\.net/lazy@v0\.1\.0 requires\n\texample\.com/retract/incompatible@v1\.0\.0: missing go\.sum entry; to add it:\n\tgo mod download example\.com/retract/incompatible$'
+
+
+# If we combine a Go 1.16 go.sum file...
+go mod tidy -go=1.16
+
+# ...with a Go 1.17 go.mod file...
+cp go.mod.orig go.mod
+
+# ...then Go 1.17 no longer works. 😞
+! go list -deps -test -f $MODFMT all
+stderr -count=1 '^can''t load test package: lazy[/\\]lazy_test.go:3:8: missing go\.sum entry for module providing package example\.com/retract/incompatible \(imported by example\.net/lazy\); to add:\n\tgo get -t example.net/lazy@v0\.1\.0$'
+
+
+# However, if we take the union of the go.sum files...
+go list -mod=mod -deps -test all
+cmp go.mod go.mod.orig
+
+# ...then Go 1.17 continues to work...
+go list -deps -test -f $MODFMT all
+stdout '^example\.com/retract/incompatible v1\.0\.0$'
+
+# ...and 1.16 also works(‽), but selects a different version for the
+# external-test dependency.
+go mod edit -go=1.16
+go list -deps -test -f $MODFMT all
+stdout '^example\.com/retract/incompatible v2\.0\.0\+incompatible$'
+
+
+-- go.mod --
+// Module m imports packages from the same versions under Go 1.17
+// as under Go 1.16, but under 1.16 its (implicit) external test dependencies
+// are higher.
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/requireincompatible v0.1.0 => ./requireincompatible
+)
+
+require example.net/lazy v0.1.0
+-- implicit.go --
+package implicit
+
+import _ "example.net/lazy"
+-- lazy/go.mod --
+// Module lazy requires example.com/retract/incompatible v1.0.0.
+//
+// When viewed from the outside it also has a transitive dependency
+// on v2.0.0+incompatible, but in lazy mode that transitive dependency
+// is pruned out.
+module example.net/lazy
+
+go 1.17
+
+exclude example.com/retract/incompatible v2.0.0+incompatible
+
+require (
+	example.com/retract/incompatible v1.0.0
+	example.net/requireincompatible v0.1.0
+)
+-- lazy/lazy.go --
+package lazy
+-- lazy/lazy_test.go --
+package lazy_test
+
+import _ "example.com/retract/incompatible"
+-- requireincompatible/go.mod --
+module example.net/requireincompatible
+
+go 1.15
+
+require example.com/retract/incompatible v2.0.0+incompatible
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat_incompatible.txt b/src/cmd/go/testdata/script/mod_tidy_compat_incompatible.txt
new file mode 100644
index 0000000000..2d8726544a
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat_incompatible.txt
@@ -0,0 +1,135 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+
+# For this module, Go 1.17 prunes out a (transitive and otherwise-irrelevant)
+# requirement on a retracted higher version of a dependency.
+# However, when Go 1.16 reads the same requirements from the go.mod file,
+# it does not prune out that requirement, and selects the retracted version.
+#
+# The Go 1.16 module graph looks like:
+#
+# m ---- lazy v0.1.0 ---- requireincompatible v0.1.0 ---- incompatible v2.0.0+incompatible
+# |        |
+# + -------+------------- incompatible v1.0.0
+#
+# The Go 1.17 module graph is the same except that the dependencies of
+# requireincompatible are pruned out (because the module that requires
+# it — lazy v0.1.0 — specifies 'go 1.17', and it is not otherwise relevant to
+# the main module).
+
+
+# 'go mod tidy' should by default diagnose the difference in dependencies as an
+# error, with useful suggestions about how to resolve it.
+
+cp go.mod go.mod.orig
+! go mod tidy
+stderr '^example\.com/m imports\n\texample\.net/lazy imports\n\texample\.com/retract/incompatible loaded from example\.com/retract/incompatible@v1\.0\.0,\n\tbut go 1\.16 would select v2\.0\.0\+incompatible\n\n'
+stderr '\n\nTo upgrade to the versions selected by go 1\.16:\n\tgo mod tidy -go=1\.16 && go mod tidy -go=1\.17\nIf reproducibility with go 1\.16 is not needed:\n\tgo mod tidy -compat=1.17\nFor other options, see:\n\thttps://golang\.org/wiki/PruningModules\n'
+
+cmp go.mod go.mod.orig
+
+
+# The suggested '-compat' flag to ignore differences should silence the error
+# and leave go.mod unchanged, resulting in checksum errors when Go 1.16 tries
+# to load a module pruned out by Go 1.17.
+
+go mod tidy -compat=1.17
+! stderr .
+cmp go.mod go.mod.orig
+
+go mod edit -go=1.16
+! go list -f $MODFMT -deps ./...
+	# TODO(#46160): -count=1 instead of -count=2.
+stderr -count=2 '^go: example\.net/lazy@v0\.1\.0 requires\n\texample\.net/requireincompatible@v0\.1\.0 requires\n\texample\.com/retract/incompatible@v2\.0\.0\+incompatible: missing go.sum entry; to add it:\n\tgo mod download example.com/retract/incompatible$'
+
+
+# There are two ways for the module author to bring the two into alignment.
+# One is to *explicitly* 'exclude' the version that is already *implicitly*
+# pruned out under 1.17.
+
+go mod edit -exclude=example.com/retract/incompatible@v2.0.0+incompatible
+go list -f $MODFMT -deps ./...
+stdout '^example.com/retract/incompatible v1\.0\.0$'
+! stdout 'v2\.0\.0'
+
+
+# The other is to explicitly upgrade the version required under Go 1.17
+# to match the version selected by Go 1.16. The commands suggested by
+# 'go mod tidy' should do exactly that.
+
+cp go.mod.orig go.mod
+
+go mod tidy -go=1.16
+go list -f $MODFMT -deps ./...
+stdout '^example.com/retract/incompatible v2\.0\.0\+incompatible$'
+! stdout 'v1\.0\.0'
+
+go mod tidy -go=1.17
+go list -f $MODFMT -deps ./...
+stdout '^example.com/retract/incompatible v2\.0\.0\+incompatible$'
+! stdout 'v1\.0\.0'
+
+go mod edit -go=1.16
+go list -f $MODFMT -deps ./...
+stdout '^example.com/retract/incompatible v2\.0\.0\+incompatible$'
+! stdout 'v1\.0\.0'
+
+
+-- go.mod --
+// Module m indirectly imports a package from
+// example.com/retract/incompatible. Its selected version of
+// that module is lower under Go 1.17 semantics than under Go 1.16.
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/requireincompatible v0.1.0 => ./requireincompatible
+)
+
+require (
+	example.com/retract/incompatible v1.0.0 // indirect
+	example.net/lazy v0.1.0
+)
+-- incompatible.go --
+package incompatible
+
+import _ "example.net/lazy"
+
+-- lazy/go.mod --
+// Module lazy requires example.com/retract/incompatible v1.0.0.
+//
+// When viewed from the outside it also has a transitive dependency
+// on v2.0.0+incompatible, but in lazy mode that transitive dependency
+// is pruned out.
+module example.net/lazy
+
+go 1.17
+
+exclude example.com/retract/incompatible v2.0.0+incompatible
+
+require (
+	example.com/retract/incompatible v1.0.0
+	example.net/requireincompatible v0.1.0
+)
+-- lazy/lazy.go --
+package lazy
+
+import _ "example.com/retract/incompatible"
+
+-- requireincompatible/go.mod --
+module example.net/requireincompatible
+
+go 1.15
+
+require example.com/retract/incompatible v2.0.0+incompatible
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat_irrelevant.txt b/src/cmd/go/testdata/script/mod_tidy_compat_irrelevant.txt
new file mode 100644
index 0000000000..7c22fca6c0
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat_irrelevant.txt
@@ -0,0 +1,99 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+
+# This module selects the same versions in Go 1.16 and 1.17 for all modules
+# that provide packages (or test dependencies of packages) imported by the
+# main module. However, in Go 1.16 it selects a higher version of a
+# transitive module dependency that is not otherwise relevant to the main module.
+# As a result, Go 1.16 needs an additional checksum for the go.mod file of
+# that irrelevant dependency.
+#
+# The Go 1.16 module graph looks like:
+#
+# m ---- lazy v0.1.0 ---- incompatible v1.0.0
+#         |
+#         + ------------- requireincompatible v0.1.0 ---- incompatible v2.0.0+incompatible
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+go list -deps -test -f $MODFMT all
+cp stdout out-117.txt
+
+go mod edit -go=1.16
+go list -deps -test -f $MODFMT all
+cmp stdout out-117.txt
+
+
+# If we explicitly drop compatibility with 1.16, we retain fewer checksums,
+# which gives a cleaner go.sum file but causes 1.16 to fail in readonly mode.
+
+cp go.mod.orig go.mod
+go mod tidy -compat=1.17
+cmp go.mod go.mod.orig
+
+go list -deps -test -f $MODFMT all
+cmp stdout out-117.txt
+
+go mod edit -go=1.16
+! go list -deps -test -f $MODFMT all
+	# TODO(#46160): -count=1 instead of -count=2.
+stderr -count=2 '^go: example.net/lazy@v0.1.0 requires\n\texample.com/retract/incompatible@v1.0.0: missing go.sum entry; to add it:\n\tgo mod download example.com/retract/incompatible$'
+
+
+-- go.mod --
+// Module m imports packages from the same versions under Go 1.17
+// as under Go 1.16, but under 1.16 its (implicit) external test dependencies
+// are higher.
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/requireincompatible v0.1.0 => ./requireincompatible
+)
+
+require example.net/lazy v0.1.0
+-- m.go --
+package m
+
+import _ "example.net/lazy"
+-- lazy/go.mod --
+// Module lazy requires example.com/retract/incompatible v1.0.0.
+//
+// When viewed from the outside it also has a transitive dependency
+// on v2.0.0+incompatible, but in lazy mode that transitive dependency
+// is pruned out.
+module example.net/lazy
+
+go 1.17
+
+exclude example.com/retract/incompatible v2.0.0+incompatible
+
+require (
+	example.com/retract/incompatible v1.0.0
+	example.net/requireincompatible v0.1.0
+)
+-- lazy/lazy.go --
+package lazy
+-- lazy/unimported/unimported.go --
+package unimported
+
+import _ "example.com/retract/incompatible"
+-- requireincompatible/go.mod --
+module example.net/requireincompatible
+
+go 1.15
+
+require example.com/retract/incompatible v2.0.0+incompatible
diff --git a/src/cmd/go/testdata/script/mod_tidy_oldgo.txt b/src/cmd/go/testdata/script/mod_tidy_oldgo.txt
new file mode 100644
index 0000000000..0e88b068a7
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_oldgo.txt
@@ -0,0 +1,21 @@
+# Modules were introduced in Go 1.11, but for various reasons users may
+# decide to declare a (much!) older go version in their go.mod file.
+# Modules with very old versions should not be rejected, and should have
+# the same module-graph semantics as in Go 1.11.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+-- go.mod --
+module example.com/legacy/go1
+
+go 1.0
+
+require golang.org/x/text v0.3.0
+-- main.go --
+package main
+
+import _ "golang.org/x/text/language"
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/mod_tidy_version.txt b/src/cmd/go/testdata/script/mod_tidy_version.txt
index 5441d9cc06..eaa6ee7b0d 100644
--- a/src/cmd/go/testdata/script/mod_tidy_version.txt
+++ b/src/cmd/go/testdata/script/mod_tidy_version.txt
@@ -32,12 +32,22 @@
 
 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"$'
+stderr '^invalid value "bananas" for flag -go: expecting a Go version like "'$goversion'"$'
 cmp go.mod go.mod.orig
 
+! go mod tidy -go=0.9
+stderr '^invalid value "0.9" for flag -go: expecting a Go version like "'$goversion'"$'
+
+! go mod tidy -go=2000.0
+stderr '^invalid value "2000.0" for flag -go: maximum supported Go version is '$goversion'$'
+
+
+# Supported versions should change the go.mod file to be tidy according to the
+# indicated version.
 
 go mod tidy -go=1.15
 cmp go.mod go.mod.115
diff --git a/src/cmd/internal/moddeps/moddeps_test.go b/src/cmd/internal/moddeps/moddeps_test.go
index ba574f4004..7723250468 100644
--- a/src/cmd/internal/moddeps/moddeps_test.go
+++ b/src/cmd/internal/moddeps/moddeps_test.go
@@ -227,7 +227,7 @@ func makeGOROOTCopy(t *testing.T) string {
 		if err != nil {
 			return err
 		}
-		if src == filepath.Join(runtime.GOROOT(), ".git") {
+		if info.IsDir() && src == filepath.Join(runtime.GOROOT(), ".git") {
 			return filepath.SkipDir
 		}
 
@@ -237,9 +237,8 @@ func makeGOROOTCopy(t *testing.T) string {
 		}
 		dst := filepath.Join(gorootCopyDir, rel)
 
-		switch src {
-		case filepath.Join(runtime.GOROOT(), "bin"),
-			filepath.Join(runtime.GOROOT(), "pkg"):
+		if info.IsDir() && (src == filepath.Join(runtime.GOROOT(), "bin") ||
+			src == filepath.Join(runtime.GOROOT(), "pkg")) {
 			// If the OS supports symlinks, use them instead
 			// of copying the bin and pkg directories.
 			if err := os.Symlink(src, dst); err == nil {
@@ -414,7 +413,7 @@ func findGorootModules(t *testing.T) []gorootModule {
 			if info.IsDir() && (info.Name() == "vendor" || info.Name() == "testdata") {
 				return filepath.SkipDir
 			}
-			if path == filepath.Join(runtime.GOROOT(), "pkg") {
+			if info.IsDir() && path == filepath.Join(runtime.GOROOT(), "pkg") {
 				// GOROOT/pkg contains generated artifacts, not source code.
 				//
 				// In https://golang.org/issue/37929 it was observed to somehow contain
@@ -422,7 +421,7 @@ func findGorootModules(t *testing.T) []gorootModule {
 				// running time of this test anyway.)
 				return filepath.SkipDir
 			}
-			if strings.HasPrefix(info.Name(), "_") || strings.HasPrefix(info.Name(), ".") {
+			if info.IsDir() && (strings.HasPrefix(info.Name(), "_") || strings.HasPrefix(info.Name(), ".")) {
 				// _ and . prefixed directories can be used for internal modules
 				// without a vendor directory that don't contribute to the build
 				// but might be used for example as code generators.
@@ -457,8 +456,31 @@ func findGorootModules(t *testing.T) []gorootModule {
 			goroot.modules = append(goroot.modules, m)
 			return nil
 		})
-	})
+		if goroot.err != nil {
+			return
+		}
 
+		// knownGOROOTModules is a hard-coded list of modules that are known to exist in GOROOT.
+		// If findGorootModules doesn't find a module, it won't be covered by tests at all,
+		// so make sure at least these modules are found. See issue 46254. If this list
+		// becomes a nuisance to update, can be replaced with len(goroot.modules) check.
+		knownGOROOTModules := [...]string{
+			"std",
+			"cmd",
+			"misc",
+			"test/bench/go1",
+		}
+		var seen = make(map[string]bool) // Key is module path.
+		for _, m := range goroot.modules {
+			seen[m.Path] = true
+		}
+		for _, m := range knownGOROOTModules {
+			if !seen[m] {
+				goroot.err = fmt.Errorf("findGorootModules didn't find the well-known module %q", m)
+				break
+			}
+		}
+	})
 	if goroot.err != nil {
 		t.Fatal(goroot.err)
 	}
diff --git a/src/cmd/internal/objabi/reloctype.go b/src/cmd/internal/objabi/reloctype.go
index ea55fa3b0a..52827a6dee 100644
--- a/src/cmd/internal/objabi/reloctype.go
+++ b/src/cmd/internal/objabi/reloctype.go
@@ -101,6 +101,9 @@ const (
 	// *rtype, and may be set to zero by the linker if it determines the method
 	// text is unreachable by the linked program.
 	R_METHODOFF
+	// R_KEEP tells the linker to keep the referred-to symbol in the final binary
+	// if the symbol containing the R_KEEP relocation is in the final binary.
+	R_KEEP
 	R_POWER_TOC
 	R_GOTPCREL
 	// R_JMPMIPS (only used on mips64) resolves to non-PC-relative target address
diff --git a/src/cmd/internal/objabi/reloctype_string.go b/src/cmd/internal/objabi/reloctype_string.go
index 8882d19f88..4638ef14d9 100644
--- a/src/cmd/internal/objabi/reloctype_string.go
+++ b/src/cmd/internal/objabi/reloctype_string.go
@@ -34,44 +34,45 @@ func _() {
 	_ = x[R_USEIFACE-24]
 	_ = x[R_USEIFACEMETHOD-25]
 	_ = x[R_METHODOFF-26]
-	_ = x[R_POWER_TOC-27]
-	_ = x[R_GOTPCREL-28]
-	_ = x[R_JMPMIPS-29]
-	_ = x[R_DWARFSECREF-30]
-	_ = x[R_DWARFFILEREF-31]
-	_ = x[R_ARM64_TLS_LE-32]
-	_ = x[R_ARM64_TLS_IE-33]
-	_ = x[R_ARM64_GOTPCREL-34]
-	_ = x[R_ARM64_GOT-35]
-	_ = x[R_ARM64_PCREL-36]
-	_ = x[R_ARM64_LDST8-37]
-	_ = x[R_ARM64_LDST16-38]
-	_ = x[R_ARM64_LDST32-39]
-	_ = x[R_ARM64_LDST64-40]
-	_ = x[R_ARM64_LDST128-41]
-	_ = x[R_POWER_TLS_LE-42]
-	_ = x[R_POWER_TLS_IE-43]
-	_ = x[R_POWER_TLS-44]
-	_ = x[R_ADDRPOWER_DS-45]
-	_ = x[R_ADDRPOWER_GOT-46]
-	_ = x[R_ADDRPOWER_PCREL-47]
-	_ = x[R_ADDRPOWER_TOCREL-48]
-	_ = x[R_ADDRPOWER_TOCREL_DS-49]
-	_ = x[R_RISCV_PCREL_ITYPE-50]
-	_ = x[R_RISCV_PCREL_STYPE-51]
-	_ = x[R_RISCV_TLS_IE_ITYPE-52]
-	_ = x[R_RISCV_TLS_IE_STYPE-53]
-	_ = x[R_PCRELDBL-54]
-	_ = x[R_ADDRMIPSU-55]
-	_ = x[R_ADDRMIPSTLS-56]
-	_ = x[R_ADDRCUOFF-57]
-	_ = x[R_WASMIMPORT-58]
-	_ = x[R_XCOFFREF-59]
+	_ = x[R_KEEP-27]
+	_ = x[R_POWER_TOC-28]
+	_ = x[R_GOTPCREL-29]
+	_ = x[R_JMPMIPS-30]
+	_ = x[R_DWARFSECREF-31]
+	_ = x[R_DWARFFILEREF-32]
+	_ = x[R_ARM64_TLS_LE-33]
+	_ = x[R_ARM64_TLS_IE-34]
+	_ = x[R_ARM64_GOTPCREL-35]
+	_ = x[R_ARM64_GOT-36]
+	_ = x[R_ARM64_PCREL-37]
+	_ = x[R_ARM64_LDST8-38]
+	_ = x[R_ARM64_LDST16-39]
+	_ = x[R_ARM64_LDST32-40]
+	_ = x[R_ARM64_LDST64-41]
+	_ = x[R_ARM64_LDST128-42]
+	_ = x[R_POWER_TLS_LE-43]
+	_ = x[R_POWER_TLS_IE-44]
+	_ = x[R_POWER_TLS-45]
+	_ = x[R_ADDRPOWER_DS-46]
+	_ = x[R_ADDRPOWER_GOT-47]
+	_ = x[R_ADDRPOWER_PCREL-48]
+	_ = x[R_ADDRPOWER_TOCREL-49]
+	_ = x[R_ADDRPOWER_TOCREL_DS-50]
+	_ = x[R_RISCV_PCREL_ITYPE-51]
+	_ = x[R_RISCV_PCREL_STYPE-52]
+	_ = x[R_RISCV_TLS_IE_ITYPE-53]
+	_ = x[R_RISCV_TLS_IE_STYPE-54]
+	_ = x[R_PCRELDBL-55]
+	_ = x[R_ADDRMIPSU-56]
+	_ = x[R_ADDRMIPSTLS-57]
+	_ = x[R_ADDRCUOFF-58]
+	_ = x[R_WASMIMPORT-59]
+	_ = x[R_XCOFFREF-60]
 }
 
-const _RelocType_name = "R_ADDRR_ADDRPOWERR_ADDRARM64R_ADDRMIPSR_ADDROFFR_SIZER_CALLR_CALLARMR_CALLARM64R_CALLINDR_CALLPOWERR_CALLMIPSR_CALLRISCVR_CONSTR_PCRELR_TLS_LER_TLS_IER_GOTOFFR_PLT0R_PLT1R_PLT2R_USEFIELDR_USETYPER_USEIFACER_USEIFACEMETHODR_METHODOFFR_POWER_TOCR_GOTPCRELR_JMPMIPSR_DWARFSECREFR_DWARFFILEREFR_ARM64_TLS_LER_ARM64_TLS_IER_ARM64_GOTPCRELR_ARM64_GOTR_ARM64_PCRELR_ARM64_LDST8R_ARM64_LDST16R_ARM64_LDST32R_ARM64_LDST64R_ARM64_LDST128R_POWER_TLS_LER_POWER_TLS_IER_POWER_TLSR_ADDRPOWER_DSR_ADDRPOWER_GOTR_ADDRPOWER_PCRELR_ADDRPOWER_TOCRELR_ADDRPOWER_TOCREL_DSR_RISCV_PCREL_ITYPER_RISCV_PCREL_STYPER_RISCV_TLS_IE_ITYPER_RISCV_TLS_IE_STYPER_PCRELDBLR_ADDRMIPSUR_ADDRMIPSTLSR_ADDRCUOFFR_WASMIMPORTR_XCOFFREF"
+const _RelocType_name = "R_ADDRR_ADDRPOWERR_ADDRARM64R_ADDRMIPSR_ADDROFFR_SIZER_CALLR_CALLARMR_CALLARM64R_CALLINDR_CALLPOWERR_CALLMIPSR_CALLRISCVR_CONSTR_PCRELR_TLS_LER_TLS_IER_GOTOFFR_PLT0R_PLT1R_PLT2R_USEFIELDR_USETYPER_USEIFACER_USEIFACEMETHODR_METHODOFFR_KEEPR_POWER_TOCR_GOTPCRELR_JMPMIPSR_DWARFSECREFR_DWARFFILEREFR_ARM64_TLS_LER_ARM64_TLS_IER_ARM64_GOTPCRELR_ARM64_GOTR_ARM64_PCRELR_ARM64_LDST8R_ARM64_LDST16R_ARM64_LDST32R_ARM64_LDST64R_ARM64_LDST128R_POWER_TLS_LER_POWER_TLS_IER_POWER_TLSR_ADDRPOWER_DSR_ADDRPOWER_GOTR_ADDRPOWER_PCRELR_ADDRPOWER_TOCRELR_ADDRPOWER_TOCREL_DSR_RISCV_PCREL_ITYPER_RISCV_PCREL_STYPER_RISCV_TLS_IE_ITYPER_RISCV_TLS_IE_STYPER_PCRELDBLR_ADDRMIPSUR_ADDRMIPSTLSR_ADDRCUOFFR_WASMIMPORTR_XCOFFREF"
 
-var _RelocType_index = [...]uint16{0, 6, 17, 28, 38, 47, 53, 59, 68, 79, 88, 99, 109, 120, 127, 134, 142, 150, 158, 164, 170, 176, 186, 195, 205, 221, 232, 243, 253, 262, 275, 289, 303, 317, 333, 344, 357, 370, 384, 398, 412, 427, 441, 455, 466, 480, 495, 512, 530, 551, 570, 589, 609, 629, 639, 650, 663, 674, 686, 696}
+var _RelocType_index = [...]uint16{0, 6, 17, 28, 38, 47, 53, 59, 68, 79, 88, 99, 109, 120, 127, 134, 142, 150, 158, 164, 170, 176, 186, 195, 205, 221, 232, 238, 249, 259, 268, 281, 295, 309, 323, 339, 350, 363, 376, 390, 404, 418, 433, 447, 461, 472, 486, 501, 518, 536, 557, 576, 595, 615, 635, 645, 656, 669, 680, 692, 702}
 
 func (i RelocType) String() string {
 	i -= 1
diff --git a/src/cmd/internal/sys/arch.go b/src/cmd/internal/sys/arch.go
index e8687363de..a3e39768b6 100644
--- a/src/cmd/internal/sys/arch.go
+++ b/src/cmd/internal/sys/arch.go
@@ -40,6 +40,12 @@ type Arch struct {
 
 	// MinLC is the minimum length of an instruction code.
 	MinLC int
+
+	// Alignment is maximum alignment required by the architecture
+	// for any (compiler-generated) load or store instruction.
+	// Loads or stores smaller than Alignment must be naturally aligned.
+	// Loads or stores larger than Alignment need only be Alignment-aligned.
+	Alignment int8
 }
 
 // InFamily reports whether a is a member of any of the specified
@@ -60,6 +66,7 @@ var Arch386 = &Arch{
 	PtrSize:   4,
 	RegSize:   4,
 	MinLC:     1,
+	Alignment: 1,
 }
 
 var ArchAMD64 = &Arch{
@@ -69,6 +76,7 @@ var ArchAMD64 = &Arch{
 	PtrSize:   8,
 	RegSize:   8,
 	MinLC:     1,
+	Alignment: 1,
 }
 
 var ArchARM = &Arch{
@@ -78,6 +86,7 @@ var ArchARM = &Arch{
 	PtrSize:   4,
 	RegSize:   4,
 	MinLC:     4,
+	Alignment: 4, // TODO: just for arm5?
 }
 
 var ArchARM64 = &Arch{
@@ -87,6 +96,7 @@ var ArchARM64 = &Arch{
 	PtrSize:   8,
 	RegSize:   8,
 	MinLC:     4,
+	Alignment: 1,
 }
 
 var ArchMIPS = &Arch{
@@ -96,6 +106,7 @@ var ArchMIPS = &Arch{
 	PtrSize:   4,
 	RegSize:   4,
 	MinLC:     4,
+	Alignment: 4,
 }
 
 var ArchMIPSLE = &Arch{
@@ -105,6 +116,7 @@ var ArchMIPSLE = &Arch{
 	PtrSize:   4,
 	RegSize:   4,
 	MinLC:     4,
+	Alignment: 4,
 }
 
 var ArchMIPS64 = &Arch{
@@ -114,6 +126,7 @@ var ArchMIPS64 = &Arch{
 	PtrSize:   8,
 	RegSize:   8,
 	MinLC:     4,
+	Alignment: 8,
 }
 
 var ArchMIPS64LE = &Arch{
@@ -123,6 +136,7 @@ var ArchMIPS64LE = &Arch{
 	PtrSize:   8,
 	RegSize:   8,
 	MinLC:     4,
+	Alignment: 8,
 }
 
 var ArchPPC64 = &Arch{
@@ -132,6 +146,7 @@ var ArchPPC64 = &Arch{
 	PtrSize:   8,
 	RegSize:   8,
 	MinLC:     4,
+	Alignment: 1,
 }
 
 var ArchPPC64LE = &Arch{
@@ -141,6 +156,7 @@ var ArchPPC64LE = &Arch{
 	PtrSize:   8,
 	RegSize:   8,
 	MinLC:     4,
+	Alignment: 1,
 }
 
 var ArchRISCV64 = &Arch{
@@ -150,6 +166,7 @@ var ArchRISCV64 = &Arch{
 	PtrSize:   8,
 	RegSize:   8,
 	MinLC:     4,
+	Alignment: 8, // riscv unaligned loads work, but are really slow (trap + simulated by OS)
 }
 
 var ArchS390X = &Arch{
@@ -159,6 +176,7 @@ var ArchS390X = &Arch{
 	PtrSize:   8,
 	RegSize:   8,
 	MinLC:     2,
+	Alignment: 1,
 }
 
 var ArchWasm = &Arch{
@@ -168,6 +186,7 @@ var ArchWasm = &Arch{
 	PtrSize:   8,
 	RegSize:   8,
 	MinLC:     1,
+	Alignment: 1,
 }
 
 var Archs = [...]*Arch{
diff --git a/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/printf/printf.go b/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/printf/printf.go
index ddad4c796c..822820f06e 100644
--- a/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/printf/printf.go
+++ b/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/printf/printf.go
@@ -590,7 +590,7 @@ func checkPrintf(pass *analysis.Pass, kind Kind, call *ast.CallExpr, fn *types.F
 		}
 		if state.verb == 'w' {
 			if kind != KindErrorf {
-				pass.Reportf(call.Pos(), "%s call has error-wrapping directive %%w", state.name)
+				pass.Reportf(call.Pos(), "%s call has error-wrapping directive %%w, which is only supported by Errorf", state.name)
 				return
 			}
 			if anyW {
diff --git a/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/sigchanyzer/sigchanyzer.go b/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/sigchanyzer/sigchanyzer.go
index b00aa7e144..0d6c8ebf16 100644
--- a/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/sigchanyzer/sigchanyzer.go
+++ b/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/sigchanyzer/sigchanyzer.go
@@ -59,12 +59,19 @@ func run(pass *analysis.Pass) (interface{}, error) {
 		if chanDecl == nil || len(chanDecl.Args) != 1 {
 			return
 		}
-		chanDecl.Args = append(chanDecl.Args, &ast.BasicLit{
+
+		// Make a copy of the channel's declaration to avoid
+		// mutating the AST. See https://golang.org/issue/46129.
+		chanDeclCopy := &ast.CallExpr{}
+		*chanDeclCopy = *chanDecl
+		chanDeclCopy.Args = append([]ast.Expr(nil), chanDecl.Args...)
+		chanDeclCopy.Args = append(chanDeclCopy.Args, &ast.BasicLit{
 			Kind:  token.INT,
 			Value: "1",
 		})
+
 		var buf bytes.Buffer
-		if err := format.Node(&buf, token.NewFileSet(), chanDecl); err != nil {
+		if err := format.Node(&buf, token.NewFileSet(), chanDeclCopy); err != nil {
 			return
 		}
 		pass.Report(analysis.Diagnostic{
diff --git a/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/stdmethods/stdmethods.go b/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/stdmethods/stdmethods.go
index 856c6ae0d8..64a28ac0b9 100644
--- a/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/stdmethods/stdmethods.go
+++ b/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/stdmethods/stdmethods.go
@@ -61,10 +61,12 @@ var Analyzer = &analysis.Analyzer{
 // we let it go. But if it does have a fmt.ScanState, then the
 // rest has to match.
 var canonicalMethods = map[string]struct{ args, results []string }{
+	"As": {[]string{"interface{}"}, []string{"bool"}}, // errors.As
 	// "Flush": {{}, {"error"}}, // http.Flusher and jpeg.writer conflict
 	"Format":        {[]string{"=fmt.State", "rune"}, []string{}},                      // fmt.Formatter
 	"GobDecode":     {[]string{"[]byte"}, []string{"error"}},                           // gob.GobDecoder
 	"GobEncode":     {[]string{}, []string{"[]byte", "error"}},                         // gob.GobEncoder
+	"Is":            {[]string{"error"}, []string{"bool"}},                             // errors.Is
 	"MarshalJSON":   {[]string{}, []string{"[]byte", "error"}},                         // json.Marshaler
 	"MarshalXML":    {[]string{"*xml.Encoder", "xml.StartElement"}, []string{"error"}}, // xml.Marshaler
 	"ReadByte":      {[]string{}, []string{"byte", "error"}},                           // io.ByteReader
@@ -76,6 +78,7 @@ var canonicalMethods = map[string]struct{ args, results []string }{
 	"UnmarshalXML":  {[]string{"*xml.Decoder", "xml.StartElement"}, []string{"error"}}, // xml.Unmarshaler
 	"UnreadByte":    {[]string{}, []string{"error"}},
 	"UnreadRune":    {[]string{}, []string{"error"}},
+	"Unwrap":        {[]string{}, []string{"error"}},                      // errors.Unwrap
 	"WriteByte":     {[]string{"byte"}, []string{"error"}},                // jpeg.writer (matching bufio.Writer)
 	"WriteTo":       {[]string{"=io.Writer"}, []string{"int64", "error"}}, // io.WriterTo
 }
@@ -123,6 +126,14 @@ func canonicalMethod(pass *analysis.Pass, id *ast.Ident) {
 		return
 	}
 
+	// Special case: Is, As and Unwrap only apply when type
+	// implements error.
+	if id.Name == "Is" || id.Name == "As" || id.Name == "Unwrap" {
+		if recv := sign.Recv(); recv == nil || !implementsError(recv.Type()) {
+			return
+		}
+	}
+
 	// Do the =s (if any) all match?
 	if !matchParams(pass, expect.args, args, "=") || !matchParams(pass, expect.results, results, "=") {
 		return
@@ -185,3 +196,9 @@ func matchParamType(expect string, actual types.Type) bool {
 	// Overkill but easy.
 	return typeString(actual) == expect
 }
+
+var errorType = types.Universe.Lookup("error").Type().Underlying().(*types.Interface)
+
+func implementsError(actual types.Type) bool {
+	return types.Implements(actual, errorType)
+}
diff --git a/src/cmd/vendor/modules.txt b/src/cmd/vendor/modules.txt
index 016ec011a9..9a1723d32c 100644
--- a/src/cmd/vendor/modules.txt
+++ b/src/cmd/vendor/modules.txt
@@ -48,7 +48,7 @@ golang.org/x/sys/windows
 # golang.org/x/term v0.0.0-20210503060354-a79de5458b56
 ## explicit; go 1.17
 golang.org/x/term
-# golang.org/x/tools v0.1.1-0.20210505014545-7cab0ef2e9a5
+# golang.org/x/tools v0.1.2-0.20210519160823-49064d2332f9
 ## explicit; go 1.17
 golang.org/x/tools/cover
 golang.org/x/tools/go/analysis