diff --git a/internal/pkgbits/decoder.go b/internal/pkgbits/decoder.go
index c299f03..f6cb37c 100644
--- a/internal/pkgbits/decoder.go
+++ b/internal/pkgbits/decoder.go
@@ -21,7 +21,7 @@
 // export data.
 type PkgDecoder struct {
 	// version is the file format version.
-	version uint32
+	version Version
 
 	// sync indicates whether the file uses sync markers.
 	sync bool
@@ -78,14 +78,15 @@
 
 	r := strings.NewReader(input)
 
-	assert(binary.Read(r, binary.LittleEndian, &pr.version) == nil)
+	var ver uint32
+	assert(binary.Read(r, binary.LittleEndian, &ver) == nil)
+	pr.version = Version(ver)
 
-	switch pr.version {
-	default:
-		panic(fmt.Errorf("cannot import %q, export data is newer version (%d) - update tool", pkgPath, pr.version))
-	case 0:
-		// no flags
-	case 1:
+	if pr.version >= numVersions {
+		panic(fmt.Errorf("cannot decode %q, export data version %d is greater than maximum supported version %d", pkgPath, pr.version, numVersions-1))
+	}
+
+	if pr.version.Has(Flags) {
 		var flags uint32
 		assert(binary.Read(r, binary.LittleEndian, &flags) == nil)
 		pr.sync = flags&flagSyncMarkers != 0
@@ -100,7 +101,9 @@
 	assert(err == nil)
 
 	pr.elemData = input[pos:]
-	assert(len(pr.elemData)-8 == int(pr.elemEnds[len(pr.elemEnds)-1]))
+
+	const fingerprintSize = 8
+	assert(len(pr.elemData)-fingerprintSize == int(pr.elemEnds[len(pr.elemEnds)-1]))
 
 	return pr
 }
@@ -134,7 +137,7 @@
 		absIdx += int(pr.elemEndsEnds[k-1])
 	}
 	if absIdx >= int(pr.elemEndsEnds[k]) {
-		errorf("%v:%v is out of bounds; %v", k, idx, pr.elemEndsEnds)
+		panicf("%v:%v is out of bounds; %v", k, idx, pr.elemEndsEnds)
 	}
 	return absIdx
 }
@@ -191,9 +194,7 @@
 		Idx:    idx,
 	}
 
-	// TODO(mdempsky) r.data.Reset(...) after #44505 is resolved.
-	r.Data = *strings.NewReader(pr.DataIdx(k, idx))
-
+	r.Data.Reset(pr.DataIdx(k, idx))
 	r.Sync(SyncRelocs)
 	r.Relocs = make([]RelocEnt, r.Len())
 	for i := range r.Relocs {
@@ -242,7 +243,7 @@
 
 func (r *Decoder) checkErr(err error) {
 	if err != nil {
-		errorf("unexpected decoding error: %w", err)
+		panicf("unexpected decoding error: %w", err)
 	}
 }
 
@@ -513,3 +514,6 @@
 
 	return path, name, tag
 }
+
+// Version reports the version of the bitstream.
+func (w *Decoder) Version() Version { return w.common.version }
diff --git a/internal/pkgbits/encoder.go b/internal/pkgbits/encoder.go
index 6482617..c17a123 100644
--- a/internal/pkgbits/encoder.go
+++ b/internal/pkgbits/encoder.go
@@ -12,18 +12,15 @@
 	"io"
 	"math/big"
 	"runtime"
+	"strings"
 )
 
-// currentVersion is the current version number.
-//
-//   - v0: initial prototype
-//
-//   - v1: adds the flags uint32 word
-const currentVersion uint32 = 1
-
 // A PkgEncoder provides methods for encoding a package's Unified IR
 // export data.
 type PkgEncoder struct {
+	// version of the bitstream.
+	version Version
+
 	// elems holds the bitstream for previously encoded elements.
 	elems [numRelocs][]string
 
@@ -47,8 +44,9 @@
 // export data files, but can help diagnosing desync errors in
 // higher-level Unified IR reader/writer code. If syncFrames is
 // negative, then sync markers are omitted entirely.
-func NewPkgEncoder(syncFrames int) PkgEncoder {
+func NewPkgEncoder(version Version, syncFrames int) PkgEncoder {
 	return PkgEncoder{
+		version:    version,
 		stringsIdx: make(map[string]Index),
 		syncFrames: syncFrames,
 	}
@@ -64,13 +62,15 @@
 		assert(binary.Write(out, binary.LittleEndian, x) == nil)
 	}
 
-	writeUint32(currentVersion)
+	writeUint32(uint32(pw.version))
 
-	var flags uint32
-	if pw.SyncMarkers() {
-		flags |= flagSyncMarkers
+	if pw.version.Has(Flags) {
+		var flags uint32
+		if pw.SyncMarkers() {
+			flags |= flagSyncMarkers
+		}
+		writeUint32(flags)
 	}
-	writeUint32(flags)
 
 	// Write elemEndsEnds.
 	var sum uint32
@@ -159,7 +159,7 @@
 
 // Flush finalizes the element's bitstream and returns its Index.
 func (w *Encoder) Flush() Index {
-	var sb bytes.Buffer // TODO(mdempsky): strings.Builder after #44505 is resolved
+	var sb strings.Builder
 
 	// Backup the data so we write the relocations at the front.
 	var tmp bytes.Buffer
@@ -189,7 +189,7 @@
 
 func (w *Encoder) checkErr(err error) {
 	if err != nil {
-		errorf("unexpected encoding error: %v", err)
+		panicf("unexpected encoding error: %v", err)
 	}
 }
 
@@ -320,8 +320,14 @@
 // section (if not already present), and then writing a relocation
 // into the element bitstream.
 func (w *Encoder) String(s string) {
+	w.StringRef(w.p.StringIdx(s))
+}
+
+// StringRef writes a reference to the given index, which must be a
+// previously encoded string value.
+func (w *Encoder) StringRef(idx Index) {
 	w.Sync(SyncString)
-	w.Reloc(RelocString, w.p.StringIdx(s))
+	w.Reloc(RelocString, idx)
 }
 
 // Strings encodes and writes a variable-length slice of strings into
@@ -348,7 +354,7 @@
 func (w *Encoder) scalar(val constant.Value) {
 	switch v := constant.Val(val).(type) {
 	default:
-		errorf("unhandled %v (%v)", val, val.Kind())
+		panicf("unhandled %v (%v)", val, val.Kind())
 	case bool:
 		w.Code(ValBool)
 		w.Bool(v)
@@ -381,3 +387,6 @@
 	b := v.Append(nil, 'p', -1)
 	w.String(string(b)) // TODO: More efficient encoding.
 }
+
+// Version reports the version of the bitstream.
+func (w *Encoder) Version() Version { return w.p.version }
diff --git a/internal/pkgbits/pkgbits_test.go b/internal/pkgbits/pkgbits_test.go
new file mode 100644
index 0000000..b8f946a
--- /dev/null
+++ b/internal/pkgbits/pkgbits_test.go
@@ -0,0 +1,77 @@
+// Copyright 2024 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.
+
+package pkgbits_test
+
+import (
+	"strings"
+	"testing"
+
+	"golang.org/x/tools/internal/pkgbits"
+)
+
+func TestRoundTrip(t *testing.T) {
+	for _, version := range []pkgbits.Version{
+		pkgbits.V0,
+		pkgbits.V1,
+		pkgbits.V2,
+	} {
+		pw := pkgbits.NewPkgEncoder(version, -1)
+		w := pw.NewEncoder(pkgbits.RelocMeta, pkgbits.SyncPublic)
+		w.Flush()
+
+		var b strings.Builder
+		_ = pw.DumpTo(&b)
+		input := b.String()
+
+		pr := pkgbits.NewPkgDecoder("package_id", input)
+		r := pr.NewDecoder(pkgbits.RelocMeta, pkgbits.PublicRootIdx, pkgbits.SyncPublic)
+
+		if r.Version() != w.Version() {
+			t.Errorf("Expected reader version %q to be the writer version %q", r.Version(), w.Version())
+		}
+	}
+}
+
+// Type checker to enforce that know V* have the constant values they must have.
+var _ [0]bool = [pkgbits.V0]bool{}
+var _ [1]bool = [pkgbits.V1]bool{}
+
+func TestVersions(t *testing.T) {
+	type vfpair struct {
+		v pkgbits.Version
+		f pkgbits.Field
+	}
+
+	// has field tests
+	for _, c := range []vfpair{
+		{pkgbits.V1, pkgbits.Flags},
+		{pkgbits.V2, pkgbits.Flags},
+		{pkgbits.V0, pkgbits.HasInit},
+		{pkgbits.V1, pkgbits.HasInit},
+		{pkgbits.V0, pkgbits.DerivedFuncInstance},
+		{pkgbits.V1, pkgbits.DerivedFuncInstance},
+		{pkgbits.V0, pkgbits.DerivedInfoNeeded},
+		{pkgbits.V1, pkgbits.DerivedInfoNeeded},
+		{pkgbits.V2, pkgbits.AliasTypeParamNames},
+	} {
+		if !c.v.Has(c.f) {
+			t.Errorf("Expected version %v to have field %v", c.v, c.f)
+		}
+	}
+
+	// does not have field tests
+	for _, c := range []vfpair{
+		{pkgbits.V0, pkgbits.Flags},
+		{pkgbits.V2, pkgbits.HasInit},
+		{pkgbits.V2, pkgbits.DerivedFuncInstance},
+		{pkgbits.V2, pkgbits.DerivedInfoNeeded},
+		{pkgbits.V0, pkgbits.AliasTypeParamNames},
+		{pkgbits.V1, pkgbits.AliasTypeParamNames},
+	} {
+		if c.v.Has(c.f) {
+			t.Errorf("Expected version %v to not have field %v", c.v, c.f)
+		}
+	}
+}
diff --git a/internal/pkgbits/support.go b/internal/pkgbits/support.go
index ad26d3b..50534a2 100644
--- a/internal/pkgbits/support.go
+++ b/internal/pkgbits/support.go
@@ -12,6 +12,6 @@
 	}
 }
 
-func errorf(format string, args ...interface{}) {
+func panicf(format string, args ...any) {
 	panic(fmt.Errorf(format, args...))
 }
diff --git a/internal/pkgbits/sync.go b/internal/pkgbits/sync.go
index 5bd51ef..a17a008 100644
--- a/internal/pkgbits/sync.go
+++ b/internal/pkgbits/sync.go
@@ -110,4 +110,8 @@
 	SyncStmtsEnd
 	SyncLabel
 	SyncOptLabel
+
+	SyncMultiExpr
+	SyncRType
+	SyncConvRTTI
 )
diff --git a/internal/pkgbits/syncmarker_string.go b/internal/pkgbits/syncmarker_string.go
index 4a5b0ca..582ad56 100644
--- a/internal/pkgbits/syncmarker_string.go
+++ b/internal/pkgbits/syncmarker_string.go
@@ -74,11 +74,14 @@
 	_ = x[SyncStmtsEnd-64]
 	_ = x[SyncLabel-65]
 	_ = x[SyncOptLabel-66]
+	_ = x[SyncMultiExpr-67]
+	_ = x[SyncRType-68]
+	_ = x[SyncConvRTTI-69]
 }
 
-const _SyncMarker_name = "EOFBoolInt64Uint64StringValueValRelocsRelocUseRelocPublicPosPosBaseObjectObject1PkgPkgDefMethodTypeTypeIdxTypeParamNamesSignatureParamsParamCodeObjSymLocalIdentSelectorPrivateFuncExtVarExtTypeExtPragmaExprListExprsExprExprTypeAssignOpFuncLitCompLitDeclFuncBodyOpenScopeCloseScopeCloseAnotherScopeDeclNamesDeclNameStmtsBlockStmtIfStmtForStmtSwitchStmtRangeStmtCaseClauseCommClauseSelectStmtDeclsLabeledStmtUseObjLocalAddLocalLinknameStmt1StmtsEndLabelOptLabel"
+const _SyncMarker_name = "EOFBoolInt64Uint64StringValueValRelocsRelocUseRelocPublicPosPosBaseObjectObject1PkgPkgDefMethodTypeTypeIdxTypeParamNamesSignatureParamsParamCodeObjSymLocalIdentSelectorPrivateFuncExtVarExtTypeExtPragmaExprListExprsExprExprTypeAssignOpFuncLitCompLitDeclFuncBodyOpenScopeCloseScopeCloseAnotherScopeDeclNamesDeclNameStmtsBlockStmtIfStmtForStmtSwitchStmtRangeStmtCaseClauseCommClauseSelectStmtDeclsLabeledStmtUseObjLocalAddLocalLinknameStmt1StmtsEndLabelOptLabelMultiExprRTypeConvRTTI"
 
-var _SyncMarker_index = [...]uint16{0, 3, 7, 12, 18, 24, 29, 32, 38, 43, 51, 57, 60, 67, 73, 80, 83, 89, 95, 99, 106, 120, 129, 135, 140, 147, 150, 160, 168, 175, 182, 188, 195, 201, 209, 214, 218, 226, 232, 234, 241, 248, 252, 260, 269, 279, 296, 305, 313, 318, 327, 333, 340, 350, 359, 369, 379, 389, 394, 405, 416, 424, 432, 437, 445, 450, 458}
+var _SyncMarker_index = [...]uint16{0, 3, 7, 12, 18, 24, 29, 32, 38, 43, 51, 57, 60, 67, 73, 80, 83, 89, 95, 99, 106, 120, 129, 135, 140, 147, 150, 160, 168, 175, 182, 188, 195, 201, 209, 214, 218, 226, 232, 234, 241, 248, 252, 260, 269, 279, 296, 305, 313, 318, 327, 333, 340, 350, 359, 369, 379, 389, 394, 405, 416, 424, 432, 437, 445, 450, 458, 467, 472, 480}
 
 func (i SyncMarker) String() string {
 	i -= 1
diff --git a/internal/pkgbits/version.go b/internal/pkgbits/version.go
new file mode 100644
index 0000000..53af9df
--- /dev/null
+++ b/internal/pkgbits/version.go
@@ -0,0 +1,85 @@
+// Copyright 2024 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.
+
+package pkgbits
+
+// Version indicates a version of a unified IR bitstream.
+// Each Version indicates the addition, removal, or change of
+// new data in the bitstream.
+//
+// These are serialized to disk and the interpretation remains fixed.
+type Version uint32
+
+const (
+	// V0: initial prototype.
+	//
+	// All data that is not assigned a Field is in version V0
+	// and has not been deprecated.
+	V0 Version = iota
+
+	// V1: adds the Flags uint32 word
+	V1
+
+	// V2: removes unused legacy fields and supports type parameters for aliases.
+	// - remove the legacy "has init" bool from the public root
+	// - remove obj's "derived func instance" bool
+	// - add a TypeParamNames field to ObjAlias
+	// - remove derived info "needed" bool
+	V2
+
+	numVersions = iota
+)
+
+// Field denotes a unit of data in the serialized unified IR bitstream.
+// It is conceptually a like field in a structure.
+//
+// We only really need Fields when the data may or may not be present
+// in a stream based on the Version of the bitstream.
+//
+// Unlike much of pkgbits, Fields are not serialized and
+// can change values as needed.
+type Field int
+
+const (
+	// Flags in a uint32 in the header of a bitstream
+	// that is used to indicate whether optional features are enabled.
+	Flags Field = iota
+
+	// Deprecated: HasInit was a bool indicating whether a package
+	// has any init functions.
+	HasInit
+
+	// Deprecated: DerivedFuncInstance was a bool indicating
+	// whether an object was a function instance.
+	DerivedFuncInstance
+
+	// ObjAlias has a list of TypeParamNames.
+	AliasTypeParamNames
+
+	// Deprecated: DerivedInfoNeeded was a bool indicating
+	// whether a type was a derived type.
+	DerivedInfoNeeded
+
+	numFields = iota
+)
+
+// introduced is the version a field was added.
+var introduced = [numFields]Version{
+	Flags:               V1,
+	AliasTypeParamNames: V2,
+}
+
+// removed is the version a field was removed in or 0 for fields
+// that have not yet been deprecated.
+// (So removed[f]-1 is the last version it is included in.)
+var removed = [numFields]Version{
+	HasInit:             V2,
+	DerivedFuncInstance: V2,
+	DerivedInfoNeeded:   V2,
+}
+
+// Has reports whether field f is present in a bitstream at version v.
+func (v Version) Has(f Field) bool {
+	return introduced[f] <= v && (v < removed[f] || removed[f] == V0)
+}