diff --git a/apidiff/apidiff.go b/apidiff/apidiff.go
new file mode 100644
index 0000000..dc0f0e7
--- /dev/null
+++ b/apidiff/apidiff.go
@@ -0,0 +1,216 @@
+// TODO: test swap corresponding types (e.g. u1 <-> u2 and u2 <-> u1)
+// TODO: test exported alias refers to something in another package -- does correspondence work then?
+// TODO: CODE COVERAGE
+// TODO: note that we may miss correspondences because we bail early when we compare a signature (e.g. when lengths differ; we could do up to the shorter)
+// TODO: if you add an unexported method to an exposed interface, you have to check that
+//		every exposed type that previously implemented the interface still does. Otherwise
+//		an external assignment of the exposed type to the interface type could fail.
+// TODO: check constant values: large values aren't representable by some types.
+// TODO: Document all the incompatibilities we don't check for.
+
+package apidiff
+
+import (
+	"fmt"
+	"go/constant"
+	"go/token"
+	"go/types"
+)
+
+// Changes reports on the differences between the APIs of the old and new packages.
+// It classifies each difference as either compatible or incompatible (breaking.) For
+// a detailed discussion of what constitutes an incompatible change, see the package
+// documentation.
+func Changes(old, new *types.Package) Report {
+	d := newDiffer(old, new)
+	d.checkPackage()
+	return Report{
+		Incompatible: d.incompatibles.collect(),
+		Compatible:   d.compatibles.collect(),
+	}
+}
+
+type differ struct {
+	old, new *types.Package
+	// Correspondences between named types.
+	// Even though it is the named types (*types.Named) that correspond, we use
+	// *types.TypeName as a map key because they are canonical.
+	// The values can be either named types or basic types.
+	correspondMap map[*types.TypeName]types.Type
+
+	// Messages.
+	incompatibles messageSet
+	compatibles   messageSet
+}
+
+func newDiffer(old, new *types.Package) *differ {
+	return &differ{
+		old:           old,
+		new:           new,
+		correspondMap: map[*types.TypeName]types.Type{},
+		incompatibles: messageSet{},
+		compatibles:   messageSet{},
+	}
+}
+
+func (d *differ) incompatible(obj types.Object, part, format string, args ...interface{}) {
+	addMessage(d.incompatibles, obj, part, format, args)
+}
+
+func (d *differ) compatible(obj types.Object, part, format string, args ...interface{}) {
+	addMessage(d.compatibles, obj, part, format, args)
+}
+
+func addMessage(ms messageSet, obj types.Object, part, format string, args []interface{}) {
+	ms.add(obj, part, fmt.Sprintf(format, args...))
+}
+
+func (d *differ) checkPackage() {
+	// Old changes.
+	for _, name := range d.old.Scope().Names() {
+		oldobj := d.old.Scope().Lookup(name)
+		if !oldobj.Exported() {
+			continue
+		}
+		newobj := d.new.Scope().Lookup(name)
+		if newobj == nil {
+			d.incompatible(oldobj, "", "removed")
+			continue
+		}
+		d.checkObjects(oldobj, newobj)
+	}
+	// New additions.
+	for _, name := range d.new.Scope().Names() {
+		newobj := d.new.Scope().Lookup(name)
+		if newobj.Exported() && d.old.Scope().Lookup(name) == nil {
+			d.compatible(newobj, "", "added")
+		}
+	}
+
+	// Whole-package satisfaction.
+	// For every old exposed interface oIface and its corresponding new interface nIface...
+	for otn1, nt1 := range d.correspondMap {
+		oIface, ok := otn1.Type().Underlying().(*types.Interface)
+		if !ok {
+			continue
+		}
+		nIface, ok := nt1.Underlying().(*types.Interface)
+		if !ok {
+			// If nt1 isn't an interface but otn1 is, then that's an incompatibility that
+			// we've already noticed, so there's no need to do anything here.
+			continue
+		}
+		// For every old type that implements oIface, its corresponding new type must implement
+		// nIface.
+		for otn2, nt2 := range d.correspondMap {
+			if otn1 == otn2 {
+				continue
+			}
+			if types.Implements(otn2.Type(), oIface) && !types.Implements(nt2, nIface) {
+				d.incompatible(otn2, "", "no longer implements %s", objectString(otn1))
+			}
+		}
+	}
+}
+
+func (d *differ) checkObjects(old, new types.Object) {
+	switch old := old.(type) {
+	case *types.Const:
+		if new, ok := new.(*types.Const); ok {
+			d.constChanges(old, new)
+			return
+		}
+	case *types.Var:
+		if new, ok := new.(*types.Var); ok {
+			d.checkCorrespondence(old, "", old.Type(), new.Type())
+			return
+		}
+	case *types.Func:
+		switch new := new.(type) {
+		case *types.Func:
+			d.checkCorrespondence(old, "", old.Type(), new.Type())
+			return
+		case *types.Var:
+			d.compatible(old, "", "changed from func to var")
+			d.checkCorrespondence(old, "", old.Type(), new.Type())
+			return
+
+		}
+	case *types.TypeName:
+		if new, ok := new.(*types.TypeName); ok {
+			d.checkCorrespondence(old, "", old.Type(), new.Type())
+			return
+		}
+	default:
+		panic("unexpected obj type")
+	}
+	// Here if kind of type changed.
+	d.incompatible(old, "", "changed from %s to %s",
+		objectKindString(old), objectKindString(new))
+}
+
+// Compare two constants.
+func (d *differ) constChanges(old, new *types.Const) {
+	ot := old.Type()
+	nt := new.Type()
+	// Check for change of type.
+	if !d.correspond(ot, nt) {
+		d.typeChanged(old, "", ot, nt)
+		return
+	}
+	// Check for change of value.
+	// We know the types are the same, so constant.Compare shouldn't panic.
+	if !constant.Compare(old.Val(), token.EQL, new.Val()) {
+		d.incompatible(old, "", "value changed from %s to %s", old.Val(), new.Val())
+	}
+}
+
+func objectKindString(obj types.Object) string {
+	switch obj.(type) {
+	case *types.Const:
+		return "const"
+	case *types.Var:
+		return "var"
+	case *types.Func:
+		return "func"
+	case *types.TypeName:
+		return "type"
+	default:
+		return "???"
+	}
+}
+
+func (d *differ) checkCorrespondence(obj types.Object, part string, old, new types.Type) {
+	if !d.correspond(old, new) {
+		d.typeChanged(obj, part, old, new)
+	}
+}
+
+func (d *differ) typeChanged(obj types.Object, part string, old, new types.Type) {
+	old = removeNamesFromSignature(old)
+	new = removeNamesFromSignature(new)
+	olds := types.TypeString(old, types.RelativeTo(d.old))
+	news := types.TypeString(new, types.RelativeTo(d.new))
+	d.incompatible(obj, part, "changed from %s to %s", olds, news)
+}
+
+// go/types always includes the argument and result names when formatting a signature.
+// Since these can change without affecting compatibility, we don't want users to
+// be distracted by them, so we remove them.
+func removeNamesFromSignature(t types.Type) types.Type {
+	sig, ok := t.(*types.Signature)
+	if !ok {
+		return t
+	}
+
+	dename := func(p *types.Tuple) *types.Tuple {
+		var vars []*types.Var
+		for i := 0; i < p.Len(); i++ {
+			v := p.At(i)
+			vars = append(vars, types.NewVar(v.Pos(), v.Pkg(), "", v.Type()))
+		}
+		return types.NewTuple(vars...)
+	}
+
+	return types.NewSignature(sig.Recv(), dename(sig.Params()), dename(sig.Results()), sig.Variadic())
+}
diff --git a/apidiff/apidiff_test.go b/apidiff/apidiff_test.go
new file mode 100644
index 0000000..5042ba0
--- /dev/null
+++ b/apidiff/apidiff_test.go
@@ -0,0 +1,158 @@
+package apidiff
+
+import (
+	"bufio"
+	"fmt"
+	"go/types"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"reflect"
+	"sort"
+	"strings"
+	"testing"
+
+	"golang.org/x/tools/go/packages"
+)
+
+func TestChanges(t *testing.T) {
+	dir, err := ioutil.TempDir("", "apidiff_test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	dir = filepath.Join(dir, "go")
+	wanti, wantc := splitIntoPackages(t, dir)
+	defer os.RemoveAll(dir)
+	sort.Strings(wanti)
+	sort.Strings(wantc)
+
+	oldpkg, err := load("old", dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	newpkg, err := load("new", dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	report := Changes(oldpkg.Types, newpkg.Types)
+
+	if !reflect.DeepEqual(report.Incompatible, wanti) {
+		t.Errorf("incompatibles: got %v\nwant %v\n", report.Incompatible, wanti)
+	}
+	if !reflect.DeepEqual(report.Compatible, wantc) {
+		t.Errorf("compatibles: got %v\nwant %v\n", report.Compatible, wantc)
+	}
+}
+
+func splitIntoPackages(t *testing.T, dir string) (incompatibles, compatibles []string) {
+	// Read the input file line by line.
+	// Write a line into the old or new package,
+	// dependent on comments.
+	// Also collect expected messages.
+	f, err := os.Open("testdata/tests.go")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	oldd := filepath.Join(dir, "src/old")
+	newd := filepath.Join(dir, "src/new")
+	if err := os.MkdirAll(oldd, 0700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Mkdir(newd, 0700); err != nil && !os.IsExist(err) {
+		t.Fatal(err)
+	}
+
+	oldf, err := os.Create(filepath.Join(oldd, "old.go"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	newf, err := os.Create(filepath.Join(newd, "new.go"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	wl := func(f *os.File, line string) {
+		if _, err := fmt.Fprintln(f, line); err != nil {
+			t.Fatal(err)
+		}
+	}
+	writeBoth := func(line string) { wl(oldf, line); wl(newf, line) }
+	writeln := writeBoth
+	s := bufio.NewScanner(f)
+	for s.Scan() {
+		line := s.Text()
+		tl := strings.TrimSpace(line)
+		switch {
+		case tl == "// old":
+			writeln = func(line string) { wl(oldf, line) }
+		case tl == "// new":
+			writeln = func(line string) { wl(newf, line) }
+		case tl == "// both":
+			writeln = writeBoth
+		case strings.HasPrefix(tl, "// i "):
+			incompatibles = append(incompatibles, strings.TrimSpace(tl[4:]))
+		case strings.HasPrefix(tl, "// c "):
+			compatibles = append(compatibles, strings.TrimSpace(tl[4:]))
+		default:
+			writeln(line)
+		}
+	}
+	if s.Err() != nil {
+		t.Fatal(s.Err())
+	}
+	return
+}
+
+func load(importPath, goPath string) (*packages.Package, error) {
+	cfg := &packages.Config{
+		Mode: packages.LoadTypes,
+	}
+	if goPath != "" {
+		cfg.Env = append(os.Environ(), "GOPATH="+goPath)
+	}
+	pkgs, err := packages.Load(cfg, importPath)
+	if err != nil {
+		return nil, err
+	}
+	if len(pkgs[0].Errors) > 0 {
+		return nil, pkgs[0].Errors[0]
+	}
+	return pkgs[0], nil
+}
+
+func TestExportedFields(t *testing.T) {
+	pkg, err := load("golang.org/x/exp/apidiff/testdata/exported_fields", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	typeof := func(name string) types.Type {
+		return pkg.Types.Scope().Lookup(name).Type()
+	}
+
+	s := typeof("S")
+	su := s.(*types.Named).Underlying().(*types.Struct)
+
+	ef := exportedSelectableFields(su)
+	wants := []struct {
+		name string
+		typ  types.Type
+	}{
+		{"A1", typeof("A1")},
+		{"D", types.Typ[types.Bool]},
+		{"E", types.Typ[types.Int]},
+		{"F", typeof("F")},
+		{"S", types.NewPointer(s)},
+	}
+
+	if got, want := len(ef), len(wants); got != want {
+		t.Errorf("got %d fields, want %d\n%+v", got, want, ef)
+	}
+	for _, w := range wants {
+		if got := ef[w.name]; got != nil && !types.Identical(got.Type(), w.typ) {
+			t.Errorf("%s: got %v, want %v", w.name, got.Type(), w.typ)
+		}
+	}
+}
diff --git a/apidiff/compatibility.go b/apidiff/compatibility.go
new file mode 100644
index 0000000..f78da8f
--- /dev/null
+++ b/apidiff/compatibility.go
@@ -0,0 +1,361 @@
+package apidiff
+
+import (
+	"fmt"
+	"go/types"
+	"reflect"
+)
+
+func (d *differ) checkCompatible(otn *types.TypeName, old, new types.Type) {
+	switch old := old.(type) {
+	case *types.Interface:
+		if new, ok := new.(*types.Interface); ok {
+			d.checkCompatibleInterface(otn, old, new)
+			return
+		}
+
+	case *types.Struct:
+		if new, ok := new.(*types.Struct); ok {
+			d.checkCompatibleStruct(otn, old, new)
+			return
+		}
+
+	case *types.Chan:
+		if new, ok := new.(*types.Chan); ok {
+			d.checkCompatibleChan(otn, old, new)
+			return
+		}
+
+	case *types.Basic:
+		if new, ok := new.(*types.Basic); ok {
+			d.checkCompatibleBasic(otn, old, new)
+			return
+		}
+
+	case *types.Named:
+		panic("unreachable")
+
+	default:
+		d.checkCorrespondence(otn, "", old, new)
+		return
+
+	}
+	// Here if old and new are different kinds of types.
+	d.typeChanged(otn, "", old, new)
+}
+
+func (d *differ) checkCompatibleChan(otn *types.TypeName, old, new *types.Chan) {
+	d.checkCorrespondence(otn, ", element type", old.Elem(), new.Elem())
+	if old.Dir() != new.Dir() {
+		if new.Dir() == types.SendRecv {
+			d.compatible(otn, "", "removed direction")
+		} else {
+			d.incompatible(otn, "", "changed direction")
+		}
+	}
+}
+
+func (d *differ) checkCompatibleBasic(otn *types.TypeName, old, new *types.Basic) {
+	// Certain changes to numeric types are compatible. Approximately, the info must
+	// be the same, and the new values must be a superset of the old.
+	if old.Kind() == new.Kind() {
+		// old and new are identical
+		return
+	}
+	if compatibleBasics[[2]types.BasicKind{old.Kind(), new.Kind()}] {
+		d.compatible(otn, "", "changed from %s to %s", old, new)
+	} else {
+		d.typeChanged(otn, "", old, new)
+	}
+}
+
+// All pairs (old, new) of compatible basic types.
+var compatibleBasics = map[[2]types.BasicKind]bool{
+	{types.Uint8, types.Uint16}:         true,
+	{types.Uint8, types.Uint32}:         true,
+	{types.Uint8, types.Uint}:           true,
+	{types.Uint8, types.Uint64}:         true,
+	{types.Uint16, types.Uint32}:        true,
+	{types.Uint16, types.Uint}:          true,
+	{types.Uint16, types.Uint64}:        true,
+	{types.Uint32, types.Uint}:          true,
+	{types.Uint32, types.Uint64}:        true,
+	{types.Uint, types.Uint64}:          true,
+	{types.Int8, types.Int16}:           true,
+	{types.Int8, types.Int32}:           true,
+	{types.Int8, types.Int}:             true,
+	{types.Int8, types.Int64}:           true,
+	{types.Int16, types.Int32}:          true,
+	{types.Int16, types.Int}:            true,
+	{types.Int16, types.Int64}:          true,
+	{types.Int32, types.Int}:            true,
+	{types.Int32, types.Int64}:          true,
+	{types.Int, types.Int64}:            true,
+	{types.Float32, types.Float64}:      true,
+	{types.Complex64, types.Complex128}: true,
+}
+
+// Interface compatibility:
+// If the old interface has an unexported method, the new interface is compatible
+// if its exported method set is a superset of the old. (Users could not implement,
+// only embed.)
+//
+// If the old interface did not have an unexported method, the new interface is
+// compatible if its exported method set is the same as the old, and it has no
+// unexported methods. (Adding an unexported method makes the interface
+// unimplementable outside the package.)
+//
+// TODO: must also check that if any methods were added or removed, every exposed
+// type in the package that implemented the interface in old still implements it in
+// new. Otherwise external assignments could fail.
+func (d *differ) checkCompatibleInterface(otn *types.TypeName, old, new *types.Interface) {
+	// Method sets are checked in checkCompatibleDefined.
+
+	// Does the old interface have an unexported method?
+	if unexportedMethod(old) != nil {
+		d.checkMethodSet(otn, old, new, additionsCompatible)
+	} else {
+		// Perform an equivalence check, but with more information.
+		d.checkMethodSet(otn, old, new, additionsIncompatible)
+		if u := unexportedMethod(new); u != nil {
+			d.incompatible(otn, u.Name(), "added unexported method")
+		}
+	}
+}
+
+// Return an unexported method from the method set of t, or nil if there are none.
+func unexportedMethod(t *types.Interface) *types.Func {
+	for i := 0; i < t.NumMethods(); i++ {
+		if m := t.Method(i); !m.Exported() {
+			return m
+		}
+	}
+	return nil
+}
+
+// We need to check three things for structs:
+// 1. The set of exported fields must be compatible. This ensures that keyed struct
+//    literals continue to compile. (There is no compatibility guarantee for unkeyed
+//    struct literals.)
+// 2. The set of exported *selectable* fields must be compatible. This includes the exported
+//    fields of all embedded structs. This ensures that selections continue to compile.
+// 3. If the old struct is comparable, so must the new one be. This ensures that equality
+//    expressions and uses of struct values as map keys continue to compile.
+//
+// An unexported embedded struct can't appear in a struct literal outside the
+// package, so it doesn't have to be present, or have the same name, in the new
+// struct.
+//
+// Field tags are ignored: they have no compile-time implications.
+func (d *differ) checkCompatibleStruct(obj types.Object, old, new *types.Struct) {
+	d.checkCompatibleObjectSets(obj, exportedFields(old), exportedFields(new))
+	d.checkCompatibleObjectSets(obj, exportedSelectableFields(old), exportedSelectableFields(new))
+	// Removing comparability from a struct is an incompatible change.
+	if types.Comparable(old) && !types.Comparable(new) {
+		d.incompatible(obj, "", "old is comparable, new is not")
+	}
+}
+
+// exportedFields collects all the immediate fields of the struct that are exported.
+// This is also the set of exported keys for keyed struct literals.
+func exportedFields(s *types.Struct) map[string]types.Object {
+	m := map[string]types.Object{}
+	for i := 0; i < s.NumFields(); i++ {
+		f := s.Field(i)
+		if f.Exported() {
+			m[f.Name()] = f
+		}
+	}
+	return m
+}
+
+// exportedSelectableFields collects all the exported fields of the struct, including
+// exported fields of embedded structs.
+//
+// We traverse the struct breadth-first, because of the rule that a lower-depth field
+// shadows one at a higher depth.
+func exportedSelectableFields(s *types.Struct) map[string]types.Object {
+	var (
+		m    = map[string]types.Object{}
+		next []*types.Struct // embedded structs at the next depth
+		seen []*types.Struct // to handle recursive embedding
+	)
+	for cur := []*types.Struct{s}; len(cur) > 0; cur, next = next, nil {
+		seen = append(seen, cur...)
+		// We only want to consider unambiguous fields. Ambiguous fields (where there
+		// is more than one field of the same name at the same level) are legal, but
+		// cannot be selected.
+		for name, f := range unambiguousFields(cur) {
+			// Record an exported field we haven't seen before. If we have seen it,
+			// it occurred a lower depth, so it shadows this field.
+			if f.Exported() && m[name] == nil {
+				m[name] = f
+			}
+			// Remember embedded structs for processing at the next depth,
+			// but only if we haven't seen the struct at this depth or above.
+			if !f.Anonymous() {
+				continue
+			}
+			t := f.Type().Underlying()
+			if p, ok := t.(*types.Pointer); ok {
+				t = p.Elem().Underlying()
+			}
+			if t, ok := t.(*types.Struct); ok && !contains(seen, t) {
+				next = append(next, t)
+			}
+		}
+	}
+	return m
+}
+
+func contains(ts []*types.Struct, t *types.Struct) bool {
+	for _, s := range ts {
+		if types.Identical(s, t) {
+			return true
+		}
+	}
+	return false
+}
+
+// Given a set of structs at the same depth, the unambiguous fields are the ones whose
+// names appear exactly once.
+func unambiguousFields(structs []*types.Struct) map[string]*types.Var {
+	fields := map[string]*types.Var{}
+	seen := map[string]bool{}
+	for _, s := range structs {
+		for i := 0; i < s.NumFields(); i++ {
+			f := s.Field(i)
+			name := f.Name()
+			if seen[name] {
+				delete(fields, name)
+			} else {
+				seen[name] = true
+				fields[name] = f
+			}
+		}
+	}
+	return fields
+}
+
+// Anything removed or change from the old set is an incompatible change.
+// Anything added to the new set is a compatible change.
+func (d *differ) checkCompatibleObjectSets(obj types.Object, old, new map[string]types.Object) {
+	for name, oldo := range old {
+		newo := new[name]
+		if newo == nil {
+			d.incompatible(obj, name, "removed")
+		} else {
+			d.checkCorrespondence(obj, name, oldo.Type(), newo.Type())
+		}
+	}
+	for name := range new {
+		if old[name] == nil {
+			d.compatible(obj, name, "added")
+		}
+	}
+}
+
+func (d *differ) checkCompatibleDefined(otn *types.TypeName, old *types.Named, new types.Type) {
+	// We've already checked that old and new correspond.
+	d.checkCompatible(otn, old.Underlying(), new.Underlying())
+	// If there are different kinds of types (e.g. struct and interface), don't bother checking
+	// the method sets.
+	if reflect.TypeOf(old.Underlying()) != reflect.TypeOf(new.Underlying()) {
+		return
+	}
+	// Interface method sets are checked in checkCompatibleInterface.
+	if _, ok := old.Underlying().(*types.Interface); ok {
+		return
+	}
+
+	// A new method set is compatible with an old if the new exported methods are a superset of the old.
+	d.checkMethodSet(otn, old, new, additionsCompatible)
+	d.checkMethodSet(otn, types.NewPointer(old), types.NewPointer(new), additionsCompatible)
+}
+
+const (
+	additionsCompatible   = true
+	additionsIncompatible = false
+)
+
+func (d *differ) checkMethodSet(otn *types.TypeName, oldt, newt types.Type, addcompat bool) {
+	// TODO: find a way to use checkCompatibleObjectSets for this.
+	oldMethodSet := exportedMethods(oldt)
+	newMethodSet := exportedMethods(newt)
+	msname := otn.Name()
+	if _, ok := oldt.(*types.Pointer); ok {
+		msname = "*" + msname
+	}
+	for name, oldMethod := range oldMethodSet {
+		newMethod := newMethodSet[name]
+		if newMethod == nil {
+			var part string
+			// Due to embedding, it's possible that the method's receiver type is not
+			// the same as the defined type whose method set we're looking at. So for
+			// a type T with removed method M that is embedded in some other type U,
+			// we will generate two "removed" messages for T.M, one for its own type
+			// T and one for the embedded type U. We want both messages to appear,
+			// but the messageSet dedup logic will allow only one message for a given
+			// object. So use the part string to distinguish them.
+			if receiverNamedType(oldMethod).Obj() != otn {
+				part = fmt.Sprintf(", method set of %s", msname)
+			}
+			d.incompatible(oldMethod, part, "removed")
+		} else {
+			obj := oldMethod
+			// If a value method is changed to a pointer method and has a signature
+			// change, then we can get two messages for the same method definition: one
+			// for the value method set that says it's removed, and another for the
+			// pointer method set that says it changed. To keep both messages (since
+			// messageSet dedups), use newMethod for the second. (Slight hack.)
+			if !hasPointerReceiver(oldMethod) && hasPointerReceiver(newMethod) {
+				obj = newMethod
+			}
+			d.checkCorrespondence(obj, "", oldMethod.Type(), newMethod.Type())
+		}
+	}
+
+	// Check for added methods.
+	for name, newMethod := range newMethodSet {
+		if oldMethodSet[name] == nil {
+			if addcompat {
+				d.compatible(newMethod, "", "added")
+			} else {
+				d.incompatible(newMethod, "", "added")
+			}
+		}
+	}
+}
+
+// exportedMethods collects all the exported methods of type's method set.
+func exportedMethods(t types.Type) map[string]types.Object {
+	m := map[string]types.Object{}
+	ms := types.NewMethodSet(t)
+	for i := 0; i < ms.Len(); i++ {
+		obj := ms.At(i).Obj()
+		if obj.Exported() {
+			m[obj.Name()] = obj
+		}
+	}
+	return m
+}
+
+func receiverType(method types.Object) types.Type {
+	return method.Type().(*types.Signature).Recv().Type()
+}
+
+func receiverNamedType(method types.Object) *types.Named {
+	switch t := receiverType(method).(type) {
+	case *types.Pointer:
+		return t.Elem().(*types.Named)
+	case *types.Named:
+		return t
+	default:
+		panic("unreachable")
+	}
+}
+
+func hasPointerReceiver(method types.Object) bool {
+	_, ok := receiverType(method).(*types.Pointer)
+	return ok
+}
diff --git a/apidiff/correspondence.go b/apidiff/correspondence.go
new file mode 100644
index 0000000..bd14c09
--- /dev/null
+++ b/apidiff/correspondence.go
@@ -0,0 +1,219 @@
+package apidiff
+
+import (
+	"go/types"
+	"sort"
+)
+
+// Two types are correspond if they are identical except for defined types,
+// which must correspond.
+//
+// Two defined types correspond if they can be interchanged in the old and new APIs,
+// possibly after a renaming.
+//
+// This is not a pure function. If we come across named types while traversing,
+// we establish correspondence.
+func (d *differ) correspond(old, new types.Type) bool {
+	return d.corr(old, new, nil)
+}
+
+// corr determines whether old and new correspond. The argument p is a list of
+// known interface identities, to avoid infinite recursion.
+//
+// corr calls itself recursively as much as possible, to establish more
+// correspondences and so check more of the API. E.g. if the new function has more
+// parameters than the old, compare all the old ones before returning false.
+//
+// Compare this to the implementation of go/types.Identical.
+func (d *differ) corr(old, new types.Type, p *ifacePair) bool {
+	// Structure copied from types.Identical.
+	switch old := old.(type) {
+	case *types.Basic:
+		return types.Identical(old, new)
+
+	case *types.Array:
+		if new, ok := new.(*types.Array); ok {
+			return d.corr(old.Elem(), new.Elem(), p) && old.Len() == new.Len()
+		}
+
+	case *types.Slice:
+		if new, ok := new.(*types.Slice); ok {
+			return d.corr(old.Elem(), new.Elem(), p)
+		}
+
+	case *types.Map:
+		if new, ok := new.(*types.Map); ok {
+			return d.corr(old.Key(), new.Key(), p) && d.corr(old.Elem(), new.Elem(), p)
+		}
+
+	case *types.Chan:
+		if new, ok := new.(*types.Chan); ok {
+			return d.corr(old.Elem(), new.Elem(), p) && old.Dir() == new.Dir()
+		}
+
+	case *types.Pointer:
+		if new, ok := new.(*types.Pointer); ok {
+			return d.corr(old.Elem(), new.Elem(), p)
+		}
+
+	case *types.Signature:
+		if new, ok := new.(*types.Signature); ok {
+			pe := d.corr(old.Params(), new.Params(), p)
+			re := d.corr(old.Results(), new.Results(), p)
+			return old.Variadic() == new.Variadic() && pe && re
+		}
+
+	case *types.Tuple:
+		if new, ok := new.(*types.Tuple); ok {
+			for i := 0; i < old.Len(); i++ {
+				if i >= new.Len() || !d.corr(old.At(i).Type(), new.At(i).Type(), p) {
+					return false
+				}
+			}
+			return old.Len() == new.Len()
+		}
+
+	case *types.Struct:
+		if new, ok := new.(*types.Struct); ok {
+			for i := 0; i < old.NumFields(); i++ {
+				if i >= new.NumFields() {
+					return false
+				}
+				of := old.Field(i)
+				nf := new.Field(i)
+				if of.Anonymous() != nf.Anonymous() ||
+					old.Tag(i) != new.Tag(i) ||
+					!d.corr(of.Type(), nf.Type(), p) ||
+					!d.corrFieldNames(of, nf) {
+					return false
+				}
+			}
+			return old.NumFields() == new.NumFields()
+		}
+
+	case *types.Interface:
+		if new, ok := new.(*types.Interface); ok {
+			// Deal with circularity. See the comment in types.Identical.
+			q := &ifacePair{old, new, p}
+			for p != nil {
+				if p.identical(q) {
+					return true // same pair was compared before
+				}
+				p = p.prev
+			}
+			oldms := d.sortedMethods(old)
+			newms := d.sortedMethods(new)
+			for i, om := range oldms {
+				if i >= len(newms) {
+					return false
+				}
+				nm := newms[i]
+				if d.methodID(om) != d.methodID(nm) || !d.corr(om.Type(), nm.Type(), q) {
+					return false
+				}
+			}
+			return old.NumMethods() == new.NumMethods()
+		}
+
+	case *types.Named:
+		if new, ok := new.(*types.Named); ok {
+			return d.establishCorrespondence(old, new)
+		}
+		if new, ok := new.(*types.Basic); ok {
+			// Basic types are defined types, too, so we have to support them.
+
+			return d.establishCorrespondence(old, new)
+		}
+
+	default:
+		panic("unknown type kind")
+	}
+	return false
+}
+
+// Compare old and new field names. We are determining correspondence across packages,
+// so just compare names, not packages. For an unexported, embedded field of named
+// type (non-named embedded fields are possible with aliases), we check that the type
+// names correspond. We check the types for correspondence before this is called, so
+// we've established correspondence.
+func (d *differ) corrFieldNames(of, nf *types.Var) bool {
+	if of.Anonymous() && nf.Anonymous() && !of.Exported() && !nf.Exported() {
+		if on, ok := of.Type().(*types.Named); ok {
+			nn := nf.Type().(*types.Named)
+			return d.establishCorrespondence(on, nn)
+		}
+	}
+	return of.Name() == nf.Name()
+}
+
+// Establish that old corresponds with new if it does not already
+// correspond to something else.
+func (d *differ) establishCorrespondence(old *types.Named, new types.Type) bool {
+	oldname := old.Obj()
+	oldc := d.correspondMap[oldname]
+	if oldc == nil {
+		// For now, assume the types don't correspond unless they are from the old
+		// and new packages, respectively.
+		//
+		// This is too conservative. For instance,
+		//    [old] type A = q.B; [new] type A q.C
+		// could be OK if in package q, B is an alias for C.
+		// Or, using p as the name of the current old/new packages:
+		//    [old] type A = q.B; [new] type A int
+		// could be OK if in q,
+		//    [old] type B int; [new] type B = p.A
+		// In this case, p.A and q.B name the same type in both old and new worlds.
+		// Note that this case doesn't imply circular package imports: it's possible
+		// that in the old world, p imports q, but in the new, q imports p.
+		//
+		// However, if we didn't do something here, then we'd incorrectly allow cases
+		// like the first one above in which q.B is not an alias for q.C
+		//
+		// What we should do is check that the old type, in the new world's package
+		// of the same path, doesn't correspond to something other than the new type.
+		// That is a bit hard, because there is no easy way to find a new package
+		// matching an old one.
+		if newn, ok := new.(*types.Named); ok {
+			if old.Obj().Pkg() != d.old || newn.Obj().Pkg() != d.new {
+				return old.Obj().Id() == newn.Obj().Id()
+			}
+		}
+		// If there is no correspondence, create one.
+		d.correspondMap[oldname] = new
+		// Check that the corresponding types are compatible.
+		d.checkCompatibleDefined(oldname, old, new)
+		return true
+	}
+	return types.Identical(oldc, new)
+}
+
+func (d *differ) sortedMethods(iface *types.Interface) []*types.Func {
+	ms := make([]*types.Func, iface.NumMethods())
+	for i := 0; i < iface.NumMethods(); i++ {
+		ms[i] = iface.Method(i)
+	}
+	sort.Slice(ms, func(i, j int) bool { return d.methodID(ms[i]) < d.methodID(ms[j]) })
+	return ms
+}
+
+func (d *differ) methodID(m *types.Func) string {
+	// If the method belongs to one of the two packages being compared, use
+	// just its name even if it's unexported. That lets us treat unexported names
+	// from the old and new packages as equal.
+	if m.Pkg() == d.old || m.Pkg() == d.new {
+		return m.Name()
+	}
+	return m.Id()
+}
+
+// Copied from the go/types package:
+
+// An ifacePair is a node in a stack of interface type pairs compared for identity.
+type ifacePair struct {
+	x, y *types.Interface
+	prev *ifacePair
+}
+
+func (p *ifacePair) identical(q *ifacePair) bool {
+	return p.x == q.x && p.y == q.y || p.x == q.y && p.y == q.x
+}
diff --git a/apidiff/messageset.go b/apidiff/messageset.go
new file mode 100644
index 0000000..1354790
--- /dev/null
+++ b/apidiff/messageset.go
@@ -0,0 +1,79 @@
+// TODO: show that two-non-empty dotjoin can happen, by using an anon struct as a field type
+// TODO: don't report removed/changed methods for both value and pointer method sets?
+
+package apidiff
+
+import (
+	"fmt"
+	"go/types"
+	"sort"
+	"strings"
+)
+
+// There can be at most one message for each object or part thereof.
+// Parts include interface methods and struct fields.
+//
+// The part thing is necessary. Method (Func) objects have sufficient info, but field
+// Vars do not: they just have a field name and a type, without the enclosing struct.
+type messageSet map[types.Object]map[string]string
+
+// Add a message for obj and part, overwriting a previous message
+// (shouldn't happen).
+// obj is required but part can be empty.
+func (m messageSet) add(obj types.Object, part, msg string) {
+	s := m[obj]
+	if s == nil {
+		s = map[string]string{}
+		m[obj] = s
+	}
+	if f, ok := s[part]; ok && f != msg {
+		fmt.Printf("! second, different message for obj %s, part %q\n", obj, part)
+		fmt.Printf("  first:  %s\n", f)
+		fmt.Printf("  second: %s\n", msg)
+	}
+	s[part] = msg
+}
+
+func (m messageSet) collect() []string {
+	var s []string
+	for obj, parts := range m {
+		// Format each object name relative to its own package.
+		objstring := objectString(obj)
+		for part, msg := range parts {
+			var p string
+
+			if strings.HasPrefix(part, ",") {
+				p = objstring + part
+			} else {
+				p = dotjoin(objstring, part)
+			}
+			s = append(s, p+": "+msg)
+		}
+	}
+	sort.Strings(s)
+	return s
+}
+
+func objectString(obj types.Object) string {
+	if f, ok := obj.(*types.Func); ok {
+		sig := f.Type().(*types.Signature)
+		if recv := sig.Recv(); recv != nil {
+			tn := types.TypeString(recv.Type(), types.RelativeTo(obj.Pkg()))
+			if tn[0] == '*' {
+				tn = "(" + tn + ")"
+			}
+			return fmt.Sprintf("%s.%s", tn, obj.Name())
+		}
+	}
+	return obj.Name()
+}
+
+func dotjoin(s1, s2 string) string {
+	if s1 == "" {
+		return s2
+	}
+	if s2 == "" {
+		return s1
+	}
+	return s1 + "." + s2
+}
diff --git a/apidiff/report.go b/apidiff/report.go
new file mode 100644
index 0000000..2917d98
--- /dev/null
+++ b/apidiff/report.go
@@ -0,0 +1,51 @@
+package apidiff
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+)
+
+// Report describes the changes detected by Changes.
+type Report struct {
+	Incompatible, Compatible []string
+}
+
+func (r Report) String() string {
+	var buf bytes.Buffer
+	if err := r.Text(&buf); err != nil {
+		return fmt.Sprintf("!!%v", err)
+	}
+	return buf.String()
+}
+
+func (r Report) Text(w io.Writer) error {
+	var err error
+
+	write := func(s string) {
+		if err == nil {
+			_, err = io.WriteString(w, s)
+		}
+	}
+
+	writeslice := func(ss []string) {
+		for _, s := range ss {
+			write("- ")
+			write(s)
+			write("\n")
+		}
+	}
+
+	if len(r.Incompatible) > 0 {
+		write("Incompatible changes:\n")
+		writeslice(r.Incompatible)
+	}
+	if len(r.Compatible) > 0 {
+		if len(r.Incompatible) > 0 {
+			write("\n")
+		}
+		write("Compatible changes:\n")
+		writeslice(r.Compatible)
+	}
+	return err
+}
diff --git a/apidiff/testdata/exported_fields/ef.go b/apidiff/testdata/exported_fields/ef.go
new file mode 100644
index 0000000..19da716
--- /dev/null
+++ b/apidiff/testdata/exported_fields/ef.go
@@ -0,0 +1,37 @@
+package exported_fields
+
+// Used for testing exportedFields.
+// Its exported fields are:
+//   A1 [1]int
+//   D bool
+//   E int
+//   F F
+//   S *S
+type (
+	S struct {
+		int
+		*embed2
+		embed
+		E int // shadows embed.E
+		alias
+		A1
+		*S
+	}
+
+	A1 [1]int
+
+	embed struct {
+		E string
+	}
+
+	embed2 struct {
+		embed3
+		F // shadows embed3.F
+	}
+	embed3 struct {
+		F bool
+	}
+	alias = struct{ D bool }
+
+	F int
+)
diff --git a/apidiff/testdata/tests.go b/apidiff/testdata/tests.go
new file mode 100644
index 0000000..014e813
--- /dev/null
+++ b/apidiff/testdata/tests.go
@@ -0,0 +1,924 @@
+// This file is split into two packages, old and new.
+// It is syntactically valid Go so that gofmt can process it.
+//
+// If a comment begins with:   Then:
+//  old                        write subsequent lines to the "old" package
+//  new                        write subsequent lines to the "new" package
+//  both                       write subsequent lines to both packages
+//  c                          expect a compatible error with the following text
+//  i                          expect an incompatible error with the following text
+package ignore
+
+// both
+import "io"
+
+//////////////// Basics
+
+//// Same type in both: OK.
+// both
+type A int
+
+//// Changing the type is an incompatible change.
+// old
+type B int
+
+// new
+// i B: changed from int to string
+type B string
+
+//// Adding a new type, whether alias or not, is a compatible change.
+// new
+// c AA: added
+type AA = A
+
+// c B1: added
+type B1 bool
+
+//// Change of type for an unexported name doesn't matter...
+// old
+type t int
+
+// new
+type t string // OK: t isn't part of the API
+
+//// ...unless it is exposed.
+// both
+var V2 u
+
+// old
+type u string
+
+// new
+// i u: changed from string to int
+type u int
+
+//// An exposed, unexported type can be renamed.
+// both
+type u2 int
+
+// old
+type u1 int
+
+var V5 u1
+
+// new
+var V5 u2 // OK: V5 has changed type, but old u1 corresopnds to new u2
+
+//// Splitting a single type into two is an incompatible change.
+// both
+type u3 int
+
+// old
+type (
+	Split1 = u1
+	Split2 = u1
+)
+
+// new
+type (
+	Split1 = u2 // OK, since old u1 corresponds to new u2
+
+	// This tries to make u1 correspond to u3
+	// i Split2: changed from u1 to u3
+	Split2 = u3
+)
+
+//// Merging two types into one is OK.
+// old
+type (
+	GoodMerge1 = u2
+	GoodMerge2 = u3
+)
+
+// new
+type (
+	GoodMerge1 = u3
+	GoodMerge2 = u3
+)
+
+//// Merging isn't OK here because a method is lost.
+// both
+type u4 int
+
+func (u4) M() {}
+
+// old
+type (
+	BadMerge1 = u3
+	BadMerge2 = u4
+)
+
+// new
+type (
+	BadMerge1 = u3
+	// i u4.M: removed
+	// What's really happening here is that old u4 corresponds to new u3,
+	// and new u3's method set is not a superset of old u4's.
+	BadMerge2 = u3
+)
+
+// old
+type Rem int
+
+// new
+// i Rem: removed
+
+//////////////// Constants
+
+//// type changes
+// old
+const (
+	C1     = 1
+	C2 int = 2
+	C3     = 3
+	C4 u1  = 4
+)
+
+var V8 int
+
+// new
+const (
+	// i C1: changed from untyped int to untyped string
+	C1 = "1"
+	// i C2: changed from int to untyped int
+	C2 = -1
+	// i C3: changed from untyped int to int
+	C3 int = 3
+	// i V8: changed from var to const
+	V8 int = 1
+	C4 u2  = 4 // OK: u1 corresponds to u2
+)
+
+// value change
+// old
+const (
+	Cr1 = 1
+	Cr2 = "2"
+	Cr3 = 3.5
+	Cr4 = complex(0, 4.1)
+)
+
+// new
+const (
+	// i Cr1: value changed from 1 to -1
+	Cr1 = -1
+	// i Cr2: value changed from "2" to "3"
+	Cr2 = "3"
+	// i Cr3: value changed from 3.5 to 3.8
+	Cr3 = 3.8
+	// i Cr4: value changed from (0 + 4.1i) to (4.1 + 0i)
+	Cr4 = complex(4.1, 0)
+)
+
+//////////////// Variables
+
+//// simple type changes
+// old
+var (
+	V1 string
+	V3 A
+	V7 <-chan int
+)
+
+// new
+var (
+	// i V1: changed from string to []string
+	V1 []string
+	V3 A // OK: same
+	// i V7: changed from <-chan int to chan int
+	V7 chan int
+)
+
+//// interface type  changes
+// old
+var (
+	V9  interface{ M() }
+	V10 interface{ M() }
+	V11 interface{ M() }
+)
+
+// new
+var (
+	// i V9: changed from interface{M()} to interface{}
+	V9 interface{}
+	// i V10: changed from interface{M()} to interface{M(); M2()}
+	V10 interface {
+		M2()
+		M()
+	}
+	// i V11: changed from interface{M()} to interface{M(int)}
+	V11 interface{ M(int) }
+)
+
+//// struct type changes
+// old
+var (
+	VS1 struct{ A, B int }
+	VS2 struct{ A, B int }
+	VS3 struct{ A, B int }
+	VS4 struct {
+		A int
+		u1
+	}
+)
+
+// new
+var (
+	// i VS1: changed from struct{A int; B int} to struct{B int; A int}
+	VS1 struct{ B, A int }
+	// i VS2: changed from struct{A int; B int} to struct{A int}
+	VS2 struct{ A int }
+	// i VS3: changed from struct{A int; B int} to struct{A int; B int; C int}
+	VS3 struct{ A, B, C int }
+	VS4 struct {
+		A int
+		u2
+	}
+)
+
+//////////////// Types
+
+// old
+const C5 = 3
+
+type (
+	A1 [1]int
+	A2 [2]int
+	A3 [C5]int
+)
+
+// new
+// i C5: value changed from 3 to 4
+const C5 = 4
+
+type (
+	A1 [1]int
+	// i A2: changed from [2]int to [2]bool
+	A2 [2]bool
+	// i A3: changed from [3]int to [4]int
+	A3 [C5]int
+)
+
+// old
+type (
+	Sl []int
+	P1 *int
+	P2 *u1
+)
+
+// new
+type (
+	// i Sl: changed from []int to []string
+	Sl []string
+	// i P1: changed from *int to **bool
+	P1 **bool
+	P2 *u2 // OK: u1 corresponds to u2
+)
+
+// old
+type Bc1 int32
+type Bc2 uint
+type Bc3 float32
+type Bc4 complex64
+
+// new
+// c Bc1: changed from int32 to int
+type Bc1 int
+
+// c Bc2: changed from uint to uint64
+type Bc2 uint64
+
+// c Bc3: changed from float32 to float64
+type Bc3 float64
+
+// c Bc4: changed from complex64 to complex128
+type Bc4 complex128
+
+// old
+type Bi1 int32
+type Bi2 uint
+type Bi3 float64
+type Bi4 complex128
+
+// new
+// i Bi1: changed from int32 to int16
+type Bi1 int16
+
+// i Bi2: changed from uint to uint32
+type Bi2 uint32
+
+// i Bi3: changed from float64 to float32
+type Bi3 float32
+
+// i Bi4: changed from complex128 to complex64
+type Bi4 complex64
+
+// old
+type (
+	M1 map[string]int
+	M2 map[string]int
+	M3 map[string]int
+)
+
+// new
+type (
+	M1 map[string]int
+	// i M2: changed from map[string]int to map[int]int
+	M2 map[int]int
+	// i M3: changed from map[string]int to map[string]string
+	M3 map[string]string
+)
+
+// old
+type (
+	Ch1 chan int
+	Ch2 <-chan int
+	Ch3 chan int
+	Ch4 <-chan int
+)
+
+// new
+type (
+	// i Ch1, element type: changed from int to bool
+	Ch1 chan bool
+	// i Ch2: changed direction
+	Ch2 chan<- int
+	// i Ch3: changed direction
+	Ch3 <-chan int
+	// c Ch4: removed direction
+	Ch4 chan int
+)
+
+// old
+type I1 interface {
+	M1()
+	M2()
+}
+
+// new
+type I1 interface {
+	// M1()
+	// i I1.M1: removed
+	M2(int)
+	// i I1.M2: changed from func() to func(int)
+	M3()
+	// i I1.M3: added
+	m()
+	// i I1.m: added unexported method
+}
+
+// old
+type I2 interface {
+	M1()
+	m()
+}
+
+// new
+type I2 interface {
+	M1()
+	// m() Removing an unexported method is OK.
+	m2() // OK, because old already had an unexported method
+	// c I2.M2: added
+	M2()
+}
+
+// old
+type I3 interface {
+	io.Reader
+	M()
+}
+
+// new
+// OK: what matters is the method set; the name of the embedded
+// interface isn't important.
+type I3 interface {
+	M()
+	Read([]byte) (int, error)
+}
+
+// old
+type I4 io.Writer
+
+// new
+// OK: in both, I4 is a distinct type from io.Writer, and
+// the old and new I4s have the same method set.
+type I4 interface {
+	Write([]byte) (int, error)
+}
+
+// old
+type I5 = io.Writer
+
+// new
+// i I5: changed from io.Writer to I5
+// In old, I5 and io.Writer are the same type; in new,
+// they are different. That can break something like:
+//   var _ func(io.Writer) = func(pkg.I6) {}
+type I5 io.Writer
+
+// old
+type I6 interface{ Write([]byte) (int, error) }
+
+// new
+// i I6: changed from I6 to io.Writer
+// Similar to the above.
+type I6 = io.Writer
+
+//// correspondence with a basic type
+// Basic types are technically defined types, but they aren't
+// represented that way in go/types, so the cases below are special.
+
+// both
+type T1 int
+
+// old
+var VT1 T1
+
+// new
+// i VT1: changed from T1 to int
+// This fails because old T1 corresponds to both int and new T1.
+var VT1 int
+
+// old
+type t2 int
+
+var VT2 t2
+
+// new
+// OK: t2 corresponds to int. It's fine that old t2
+// doesn't exist in new.
+var VT2 int
+
+// both
+type t3 int
+
+func (t3) M() {}
+
+// old
+var VT3 t3
+
+// new
+// i t3.M: removed
+// Here the change from t3 to int is incompatible
+// because old t3 has an exported method.
+var VT3 int
+
+// old
+var VT4 int
+
+// new
+type t4 int
+
+// i VT4: changed from int to t4
+// This is incompatible because of code like
+//    VT4 + int(1)
+// which works in old but fails in new.
+// The difference from the above cases is that
+// in those, we were merging two types into one;
+// here, we are splitting int into t4 and int.
+var VT4 t4
+
+//////////////// Functions
+
+// old
+func F1(a int, b string) map[u1]A { return nil }
+func F2(int)                      {}
+func F3(int)                      {}
+func F4(int) int                  { return 0 }
+func F5(int) int                  { return 0 }
+func F6(int)                      {}
+func F7(interface{})              {}
+
+// new
+func F1(c int, d string) map[u2]AA { return nil } //OK: same (since u1 corresponds to u2)
+
+// i F2: changed from func(int) to func(int) bool
+func F2(int) bool { return true }
+
+// i F3: changed from func(int) to func(int, int)
+func F3(int, int) {}
+
+// i F4: changed from func(int) int to func(bool) int
+func F4(bool) int { return 0 }
+
+// i F5: changed from func(int) int to func(int) string
+func F5(int) string { return "" }
+
+// i F6: changed from func(int) to func(...int)
+func F6(...int) {}
+
+// i F7: changed from func(interface{}) to func(interface{x()})
+func F7(a interface{ x() }) {}
+
+// old
+func F8(bool) {}
+
+// new
+// c F8: changed from func to var
+var F8 func(bool)
+
+// old
+var F9 func(int)
+
+// new
+// i F9: changed from var to func
+func F9(int) {}
+
+// both
+// OK, even though new S1 is incompatible with old S1 (see below)
+func F10(S1) {}
+
+//////////////// Structs
+
+// old
+type S1 struct {
+	A int
+	B string
+	C bool
+	d float32
+}
+
+// new
+type S1 = s1
+
+type s1 struct {
+	C chan int
+	// i S1.C: changed from bool to chan int
+	A int
+	// i S1.B: removed
+	// i S1: old is comparable, new is not
+	x []int
+	d float32
+	E bool
+	// c S1.E: added
+}
+
+// old
+type embed struct {
+	E string
+}
+
+type S2 struct {
+	A int
+	embed
+}
+
+// new
+type embedx struct {
+	E string
+}
+
+type S2 struct {
+	embedx // OK: the unexported embedded field changed names, but the exported field didn't
+	A      int
+}
+
+// both
+type F int
+
+// old
+type S3 struct {
+	A int
+	embed
+}
+
+// new
+type embed struct{ F int }
+
+type S3 struct {
+	// i S3.E: removed
+	embed
+	// c S3.F: added
+	A int
+}
+
+// old
+type embed2 struct {
+	embed3
+	F // shadows embed3.F
+}
+
+type embed3 struct {
+	F bool
+}
+
+type alias = struct{ D bool }
+
+type S4 struct {
+	int
+	*embed2
+	embed
+	E int // shadows embed.E
+	alias
+	A1
+	*S4
+}
+
+// new
+type S4 struct {
+	// OK: removed unexported fields
+	// D and F marked as added because they are now part of the immediate fields
+	D bool
+	// c S4.D: added
+	E int // OK: same as in old
+	F F
+	// c S4.F: added
+	A1  // OK: same
+	*S4 // OK: same (recursive embedding)
+}
+
+//// Difference between exported selectable fields and exported immediate fields.
+// both
+type S5 struct{ A int }
+
+// old
+// Exported immediate fields: A, S5
+// Exported selectable fields: A int, S5 S5
+type S6 struct {
+	S5 S5
+	A  int
+}
+
+// new
+// Exported immediate fields: S5
+// Exported selectable fields: A int, S5 S5.
+
+// i S6.A: removed
+type S6 struct {
+	S5
+}
+
+//// Ambiguous fields can exist; they just can't be selected.
+// both
+type (
+	embed7a struct{ E int }
+	embed7b struct{ E bool }
+)
+
+// old
+type S7 struct { // legal, but no selectable fields
+	embed7a
+	embed7b
+}
+
+// new
+type S7 struct {
+	embed7a
+	embed7b
+	// c S7.E: added
+	E string
+}
+
+//////////////// Method sets
+
+// old
+type SM struct {
+	embedm
+	Embedm
+}
+
+func (SM) V1() {}
+func (SM) V2() {}
+func (SM) V3() {}
+func (SM) V4() {}
+func (SM) v()  {}
+
+func (*SM) P1() {}
+func (*SM) P2() {}
+func (*SM) P3() {}
+func (*SM) P4() {}
+func (*SM) p()  {}
+
+type embedm int
+
+func (embedm) EV1()  {}
+func (embedm) EV2()  {}
+func (embedm) EV3()  {}
+func (*embedm) EP1() {}
+func (*embedm) EP2() {}
+func (*embedm) EP3() {}
+
+type Embedm struct {
+	A int
+}
+
+func (Embedm) FV()  {}
+func (*Embedm) FP() {}
+
+type RepeatEmbedm struct {
+	Embedm
+}
+
+// new
+type SM struct {
+	embedm2
+	embedm3
+	Embedm
+	// i SM.A: changed from int to bool
+}
+
+// c SMa: added
+type SMa = SM
+
+func (SM) V1() {} // OK: same
+
+// func (SM) V2() {}
+// i SM.V2: removed
+
+// i SM.V3: changed from func() to func(int)
+func (SM) V3(int) {}
+
+// c SM.V5: added
+func (SM) V5() {}
+
+func (SM) v(int) {} // OK: unexported method change
+func (SM) v2()   {} // OK: unexported method added
+
+func (*SM) P1() {} // OK: same
+//func (*SM) P2() {}
+// i (*SM).P2: removed
+
+// i (*SM).P3: changed from func() to func(int)
+func (*SMa) P3(int) {}
+
+// c (*SM).P5: added
+func (*SM) P5() {}
+
+// func (*SM) p() {}  // OK: unexported method removed
+
+// Changing from a value to a pointer receiver or vice versa
+// just looks like adding and removing a method.
+
+// i SM.V4: removed
+// i (*SM).V4: changed from func() to func(int)
+func (*SM) V4(int) {}
+
+// c SM.P4: added
+// P4 is not removed from (*SM) because value methods
+// are in the pointer method set.
+func (SM) P4() {}
+
+type embedm2 int
+
+// i embedm.EV1: changed from func() to func(int)
+func (embedm2) EV1(int) {}
+
+// i embedm.EV2, method set of SM: removed
+// i embedm.EV2, method set of *SM: removed
+
+// i (*embedm).EP2, method set of *SM: removed
+func (*embedm2) EP1() {}
+
+type embedm3 int
+
+func (embedm3) EV3()  {} // OK: compatible with old embedm.EV3
+func (*embedm3) EP3() {} // OK: compatible with old (*embedm).EP3
+
+type Embedm struct {
+	// i Embedm.A: changed from int to bool
+	A bool
+}
+
+// i Embedm.FV: changed from func() to func(int)
+func (Embedm) FV(int) {}
+func (*Embedm) FP()   {}
+
+type RepeatEmbedm struct {
+	// i RepeatEmbedm.A: changed from int to bool
+	Embedm
+}
+
+//////////////// Whole-package interface satisfaction
+
+// old
+type WI1 interface {
+	M1()
+	m1()
+}
+
+type WI2 interface {
+	M2()
+	m2()
+}
+
+type WS1 int
+
+func (WS1) M1() {}
+func (WS1) m1() {}
+
+type WS2 int
+
+func (WS2) M2() {}
+func (WS2) m2() {}
+
+// new
+type WI1 interface {
+	M1()
+	m()
+}
+
+type WS1 int
+
+func (WS1) M1() {}
+
+// i WS1: no longer implements WI1
+//func (WS1) m1() {}
+
+type WI2 interface {
+	M2()
+	m2()
+	// i WS2: no longer implements WI2
+	m3()
+}
+
+type WS2 int
+
+func (WS2) M2() {}
+func (WS2) m2() {}
+
+//////////////// Miscellany
+
+// This verifies that the code works even through
+// multiple levels of unexported typed.
+
+// old
+var Z w
+
+type w []x
+type x []z
+type z int
+
+// new
+var Z w
+
+type w []x
+type x []z
+
+// i z: changed from int to bool
+type z bool
+
+// old
+type H struct{}
+
+func (H) M() {}
+
+// new
+// i H: changed from struct{} to interface{M()}
+type H interface {
+	M()
+}
+
+//// Splitting types
+
+//// OK: in both old and new, {J1, K1, L1} name the same type.
+// old
+type (
+	J1 = K1
+	K1 = L1
+	L1 int
+)
+
+// new
+type (
+	J1 = K1
+	K1 int
+	L1 = J1
+)
+
+//// Old has one type, K2; new has J2 and K2.
+// both
+type K2 int
+
+// old
+type J2 = K2
+
+// new
+// i K2: changed from K2 to K2
+type J2 K2 // old K2 corresponds with new J2
+// old K2 also corresponds with new K2: problem
+
+// both
+type k3 int
+
+var Vj3 j3 // expose j3
+
+// old
+type j3 = k3
+
+// new
+// OK: k3 isn't exposed
+type j3 k3
+
+// both
+type k4 int
+
+var Vj4 j4 // expose j4
+var VK4 k4 // expose k4
+
+// old
+type j4 = k4
+
+// new
+// i Vj4: changed from k4 to j4
+// e.g. p.Vj4 = p.Vk4
+type j4 k4