proto: wire decoding support

Add proto.Unmarshal.

Test cases all produce identical results to the v1 unmarshaller.

Change-Id: I42259266018a14e88a650c5d83a043cb17a3a15d
Reviewed-on: https://go-review.googlesource.com/c/153918
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/proto/decode.go b/proto/decode.go
new file mode 100644
index 0000000..4af5f16
--- /dev/null
+++ b/proto/decode.go
@@ -0,0 +1,191 @@
+// Copyright 2018 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 proto
+
+import (
+	"errors"
+
+	"github.com/golang/protobuf/v2/internal/encoding/wire"
+	"github.com/golang/protobuf/v2/internal/pragma"
+	"github.com/golang/protobuf/v2/reflect/protoreflect"
+)
+
+// UnmarshalOptions configures the unmarshaler.
+//
+// Example usage:
+//   err := UnmarshalOptions{DiscardUnknown: true}.Unmarshal(b, m)
+type UnmarshalOptions struct {
+	// If DiscardUnknown is set, unknown fields are ignored.
+	DiscardUnknown bool
+
+	pragma.NoUnkeyedLiterals
+}
+
+// Unmarshal parses the wire-format message in b and places the result in m.
+func Unmarshal(b []byte, m Message) error {
+	return UnmarshalOptions{}.Unmarshal(b, m)
+}
+
+// Unmarshal parses the wire-format message in b and places the result in m.
+func (o UnmarshalOptions) Unmarshal(b []byte, m Message) error {
+	// TODO: Reset m?
+	return o.unmarshalMessage(b, m.ProtoReflect())
+}
+
+func (o UnmarshalOptions) unmarshalMessage(b []byte, m protoreflect.Message) error {
+	messageType := m.Type()
+	fieldTypes := messageType.Fields()
+	knownFields := m.KnownFields()
+	unknownFields := m.UnknownFields()
+	for len(b) > 0 {
+		// Parse the tag (field number and wire type).
+		num, wtyp, tagLen := wire.ConsumeTag(b)
+		if tagLen < 0 {
+			return wire.ParseError(tagLen)
+		}
+
+		// Parse the field value.
+		fieldType := fieldTypes.ByNumber(num)
+		var err error
+		var valLen int
+		switch {
+		case fieldType == nil:
+			err = errUnknown
+		case fieldType.Cardinality() != protoreflect.Repeated:
+			valLen, err = o.unmarshalScalarField(b[tagLen:], wtyp, num, knownFields, fieldType)
+		case !fieldType.IsMap():
+			valLen, err = o.unmarshalList(b[tagLen:], wtyp, num, knownFields.Get(num).List(), fieldType.Kind())
+		default:
+			valLen, err = o.unmarshalMap(b[tagLen:], wtyp, num, knownFields.Get(num).Map(), fieldType)
+		}
+		if err == errUnknown {
+			valLen = wire.ConsumeFieldValue(num, wtyp, b[tagLen:])
+			if valLen < 0 {
+				return wire.ParseError(valLen)
+			}
+			unknownFields.Set(num, append(unknownFields.Get(num), b[:tagLen+valLen]...))
+		} else if err != nil {
+			return err
+		}
+		b = b[tagLen+valLen:]
+	}
+	// TODO: required field checks
+	return nil
+}
+
+func (o UnmarshalOptions) unmarshalScalarField(b []byte, wtyp wire.Type, num wire.Number, knownFields protoreflect.KnownFields, field protoreflect.FieldDescriptor) (n int, err error) {
+	v, n, err := o.unmarshalScalar(b, wtyp, num, field.Kind())
+	if err != nil {
+		return 0, err
+	}
+	switch field.Kind() {
+	case protoreflect.GroupKind, protoreflect.MessageKind:
+		// Messages are merged with any existing message value,
+		// unless the message is part of a oneof.
+		//
+		// TODO: C++ merges into oneofs, while v1 does not.
+		// Evaluate which behavior to pick.
+		var m protoreflect.Message
+		if knownFields.Has(num) && field.OneofType() == nil {
+			m = knownFields.Get(num).Message()
+		} else {
+			m = knownFields.NewMessage(num).ProtoReflect()
+			knownFields.Set(num, protoreflect.ValueOf(m))
+		}
+		if err := o.unmarshalMessage(v.Bytes(), m); err != nil {
+			return 0, err
+		}
+	default:
+		// Non-message scalars replace the previous value.
+		knownFields.Set(num, v)
+	}
+	return n, nil
+}
+
+func (o UnmarshalOptions) unmarshalMap(b []byte, wtyp wire.Type, num wire.Number, mapv protoreflect.Map, field protoreflect.FieldDescriptor) (n int, err error) {
+	if wtyp != wire.BytesType {
+		return 0, errUnknown
+	}
+	b, n = wire.ConsumeBytes(b)
+	if n < 0 {
+		return 0, wire.ParseError(n)
+	}
+	var (
+		keyField = field.MessageType().Fields().ByNumber(1)
+		valField = field.MessageType().Fields().ByNumber(2)
+		key      protoreflect.Value
+		val      protoreflect.Value
+		haveKey  bool
+		haveVal  bool
+	)
+	switch valField.Kind() {
+	case protoreflect.GroupKind, protoreflect.MessageKind:
+		val = protoreflect.ValueOf(mapv.NewMessage().ProtoReflect())
+	}
+	// Map entries are represented as a two-element message with fields
+	// containing the key and value.
+	for len(b) > 0 {
+		num, wtyp, n := wire.ConsumeTag(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		b = b[n:]
+		err = errUnknown
+		switch num {
+		case 1:
+			key, n, err = o.unmarshalScalar(b, wtyp, num, keyField.Kind())
+			if err != nil {
+				break
+			}
+			haveKey = true
+		case 2:
+			var v protoreflect.Value
+			v, n, err = o.unmarshalScalar(b, wtyp, num, valField.Kind())
+			if err != nil {
+				break
+			}
+			switch valField.Kind() {
+			case protoreflect.GroupKind, protoreflect.MessageKind:
+				if err := o.unmarshalMessage(v.Bytes(), val.Message()); err != nil {
+					return 0, err
+				}
+			default:
+				val = v
+			}
+			haveVal = true
+		}
+		if err == errUnknown {
+			n = wire.ConsumeFieldValue(num, wtyp, b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+		} else if err != nil {
+			return 0, err
+		}
+		b = b[n:]
+	}
+	// Every map entry should have entries for key and value, but this is not strictly required.
+	if !haveKey {
+		key = keyField.Default()
+	}
+	if !haveVal {
+		switch valField.Kind() {
+		case protoreflect.GroupKind, protoreflect.MessageKind:
+			// Trigger required field checks by unmarshaling an empty message.
+			if err := o.unmarshalMessage(nil, val.Message()); err != nil {
+				return 0, err
+			}
+		default:
+			val = valField.Default()
+		}
+	}
+	mapv.Set(key.MapKey(), val)
+	return n, nil
+}
+
+// errUnknown is used internally to indicate fields which should be added
+// to the unknown field set of a message. It is never returned from an exported
+// function.
+var errUnknown = errors.New("unknown")
diff --git a/proto/decode_gen.go b/proto/decode_gen.go
new file mode 100644
index 0000000..130876f
--- /dev/null
+++ b/proto/decode_gen.go
@@ -0,0 +1,591 @@
+// Copyright 2018 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.
+
+// Code generated by generate-types. DO NOT EDIT.
+
+package proto
+
+import (
+	"math"
+
+	"github.com/golang/protobuf/v2/internal/encoding/wire"
+	"github.com/golang/protobuf/v2/reflect/protoreflect"
+)
+
+// unmarshalScalar decodes a value of the given kind.
+//
+// Message values are decoded into a []byte which aliases the input data.
+func (o UnmarshalOptions) unmarshalScalar(b []byte, wtyp wire.Type, num wire.Number, kind protoreflect.Kind) (val protoreflect.Value, n int, err error) {
+	switch kind {
+	case protoreflect.BoolKind:
+		if wtyp != wire.VarintType {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(wire.DecodeBool(v)), n, nil
+	case protoreflect.EnumKind:
+		if wtyp != wire.VarintType {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(protoreflect.EnumNumber(v)), n, nil
+	case protoreflect.Int32Kind:
+		if wtyp != wire.VarintType {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(int32(v)), n, nil
+	case protoreflect.Sint32Kind:
+		if wtyp != wire.VarintType {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(int32(wire.DecodeZigZag(v & math.MaxUint32))), n, nil
+	case protoreflect.Uint32Kind:
+		if wtyp != wire.VarintType {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(uint32(v)), n, nil
+	case protoreflect.Int64Kind:
+		if wtyp != wire.VarintType {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(int64(v)), n, nil
+	case protoreflect.Sint64Kind:
+		if wtyp != wire.VarintType {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(wire.DecodeZigZag(v)), n, nil
+	case protoreflect.Uint64Kind:
+		if wtyp != wire.VarintType {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(v), n, nil
+	case protoreflect.Sfixed32Kind:
+		if wtyp != wire.Fixed32Type {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeFixed32(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(int32(v)), n, nil
+	case protoreflect.Fixed32Kind:
+		if wtyp != wire.Fixed32Type {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeFixed32(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(uint32(v)), n, nil
+	case protoreflect.FloatKind:
+		if wtyp != wire.Fixed32Type {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeFixed32(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(math.Float32frombits(uint32(v))), n, nil
+	case protoreflect.Sfixed64Kind:
+		if wtyp != wire.Fixed64Type {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeFixed64(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(int64(v)), n, nil
+	case protoreflect.Fixed64Kind:
+		if wtyp != wire.Fixed64Type {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeFixed64(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(v), n, nil
+	case protoreflect.DoubleKind:
+		if wtyp != wire.Fixed64Type {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeFixed64(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(math.Float64frombits(v)), n, nil
+	case protoreflect.StringKind:
+		if wtyp != wire.BytesType {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeBytes(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(string(v)), n, nil
+	case protoreflect.BytesKind:
+		if wtyp != wire.BytesType {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeBytes(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(append(([]byte)(nil), v...)), n, nil
+	case protoreflect.MessageKind:
+		if wtyp != wire.BytesType {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeBytes(b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(v), n, nil
+	case protoreflect.GroupKind:
+		if wtyp != wire.StartGroupType {
+			return val, 0, errUnknown
+		}
+		v, n := wire.ConsumeGroup(num, b)
+		if n < 0 {
+			return val, 0, wire.ParseError(n)
+		}
+		return protoreflect.ValueOf(v), n, nil
+	default:
+		return val, 0, errUnknown
+	}
+}
+
+func (o UnmarshalOptions) unmarshalList(b []byte, wtyp wire.Type, num wire.Number, list protoreflect.List, kind protoreflect.Kind) (n int, err error) {
+	switch kind {
+	case protoreflect.BoolKind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeVarint(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(wire.DecodeBool(v)))
+			}
+			return n, nil
+		}
+		if wtyp != wire.VarintType {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(wire.DecodeBool(v)))
+		return n, nil
+	case protoreflect.EnumKind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeVarint(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(protoreflect.EnumNumber(v)))
+			}
+			return n, nil
+		}
+		if wtyp != wire.VarintType {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(protoreflect.EnumNumber(v)))
+		return n, nil
+	case protoreflect.Int32Kind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeVarint(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(int32(v)))
+			}
+			return n, nil
+		}
+		if wtyp != wire.VarintType {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(int32(v)))
+		return n, nil
+	case protoreflect.Sint32Kind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeVarint(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(int32(wire.DecodeZigZag(v & math.MaxUint32))))
+			}
+			return n, nil
+		}
+		if wtyp != wire.VarintType {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(int32(wire.DecodeZigZag(v & math.MaxUint32))))
+		return n, nil
+	case protoreflect.Uint32Kind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeVarint(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(uint32(v)))
+			}
+			return n, nil
+		}
+		if wtyp != wire.VarintType {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(uint32(v)))
+		return n, nil
+	case protoreflect.Int64Kind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeVarint(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(int64(v)))
+			}
+			return n, nil
+		}
+		if wtyp != wire.VarintType {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(int64(v)))
+		return n, nil
+	case protoreflect.Sint64Kind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeVarint(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(wire.DecodeZigZag(v)))
+			}
+			return n, nil
+		}
+		if wtyp != wire.VarintType {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(wire.DecodeZigZag(v)))
+		return n, nil
+	case protoreflect.Uint64Kind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeVarint(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(v))
+			}
+			return n, nil
+		}
+		if wtyp != wire.VarintType {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeVarint(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(v))
+		return n, nil
+	case protoreflect.Sfixed32Kind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeFixed32(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(int32(v)))
+			}
+			return n, nil
+		}
+		if wtyp != wire.Fixed32Type {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeFixed32(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(int32(v)))
+		return n, nil
+	case protoreflect.Fixed32Kind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeFixed32(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(uint32(v)))
+			}
+			return n, nil
+		}
+		if wtyp != wire.Fixed32Type {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeFixed32(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(uint32(v)))
+		return n, nil
+	case protoreflect.FloatKind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeFixed32(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(math.Float32frombits(uint32(v))))
+			}
+			return n, nil
+		}
+		if wtyp != wire.Fixed32Type {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeFixed32(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(math.Float32frombits(uint32(v))))
+		return n, nil
+	case protoreflect.Sfixed64Kind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeFixed64(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(int64(v)))
+			}
+			return n, nil
+		}
+		if wtyp != wire.Fixed64Type {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeFixed64(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(int64(v)))
+		return n, nil
+	case protoreflect.Fixed64Kind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeFixed64(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(v))
+			}
+			return n, nil
+		}
+		if wtyp != wire.Fixed64Type {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeFixed64(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(v))
+		return n, nil
+	case protoreflect.DoubleKind:
+		if wtyp == wire.BytesType {
+			buf, n := wire.ConsumeBytes(b)
+			if n < 0 {
+				return 0, wire.ParseError(n)
+			}
+			for len(buf) > 0 {
+				v, n := wire.ConsumeFixed64(buf)
+				if n < 0 {
+					return 0, wire.ParseError(n)
+				}
+				buf = buf[n:]
+				list.Append(protoreflect.ValueOf(math.Float64frombits(v)))
+			}
+			return n, nil
+		}
+		if wtyp != wire.Fixed64Type {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeFixed64(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(math.Float64frombits(v)))
+		return n, nil
+	case protoreflect.StringKind:
+		if wtyp != wire.BytesType {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeBytes(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(string(v)))
+		return n, nil
+	case protoreflect.BytesKind:
+		if wtyp != wire.BytesType {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeBytes(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		list.Append(protoreflect.ValueOf(append(([]byte)(nil), v...)))
+		return n, nil
+	case protoreflect.MessageKind:
+		if wtyp != wire.BytesType {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeBytes(b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		m := list.NewMessage().ProtoReflect()
+		if err := o.unmarshalMessage(v, m); err != nil {
+			return 0, err
+		}
+		list.Append(protoreflect.ValueOf(m))
+		return n, nil
+	case protoreflect.GroupKind:
+		if wtyp != wire.StartGroupType {
+			return 0, errUnknown
+		}
+		v, n := wire.ConsumeGroup(num, b)
+		if n < 0 {
+			return 0, wire.ParseError(n)
+		}
+		m := list.NewMessage().ProtoReflect()
+		if err := o.unmarshalMessage(v, m); err != nil {
+			return 0, err
+		}
+		list.Append(protoreflect.ValueOf(m))
+		return n, nil
+	default:
+		return 0, errUnknown
+	}
+}
diff --git a/proto/decode_test.go b/proto/decode_test.go
new file mode 100644
index 0000000..9560ab8
--- /dev/null
+++ b/proto/decode_test.go
@@ -0,0 +1,749 @@
+// Copyright 2018 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 proto
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+
+	protoV1 "github.com/golang/protobuf/proto"
+	"github.com/golang/protobuf/v2/internal/encoding/pack"
+	_ "github.com/golang/protobuf/v2/internal/legacy"
+	"github.com/golang/protobuf/v2/internal/scalar"
+	testpb "github.com/golang/protobuf/v2/internal/testprotos/test"
+	pref "github.com/golang/protobuf/v2/reflect/protoreflect"
+)
+
+type testProto struct {
+	desc     string
+	decodeTo []Message
+	wire     []byte
+}
+
+func TestDecode(t *testing.T) {
+	for _, test := range testProtos {
+		for _, want := range test.decodeTo {
+			t.Run(fmt.Sprintf("%s (%T)", test.desc, want), func(t *testing.T) {
+				wire := append(([]byte)(nil), test.wire...)
+				got := reflect.New(reflect.TypeOf(want).Elem()).Interface().(Message)
+				if err := Unmarshal(wire, got); err != nil {
+					t.Errorf("Unmarshal error: %v\nMessage:\n%v", err, protoV1.MarshalTextString(want.(protoV1.Message)))
+					return
+				}
+
+				// Aliasing check: Modifying the original wire bytes shouldn't
+				// affect the unmarshaled message.
+				for i := range wire {
+					wire[i] = 0
+				}
+
+				if !protoV1.Equal(got.(protoV1.Message), want.(protoV1.Message)) {
+					t.Errorf("Unmarshal returned unexpected result; got:\n%v\nwant:\n%v", protoV1.MarshalTextString(got.(protoV1.Message)), protoV1.MarshalTextString(want.(protoV1.Message)))
+				}
+			})
+		}
+	}
+}
+
+var testProtos = []testProto{
+	{
+		desc: "basic scalar types",
+		decodeTo: []Message{&testpb.TestAllTypes{
+			OptionalInt32:      scalar.Int32(1001),
+			OptionalInt64:      scalar.Int64(1002),
+			OptionalUint32:     scalar.Uint32(1003),
+			OptionalUint64:     scalar.Uint64(1004),
+			OptionalSint32:     scalar.Int32(1005),
+			OptionalSint64:     scalar.Int64(1006),
+			OptionalFixed32:    scalar.Uint32(1007),
+			OptionalFixed64:    scalar.Uint64(1008),
+			OptionalSfixed32:   scalar.Int32(1009),
+			OptionalSfixed64:   scalar.Int64(1010),
+			OptionalFloat:      scalar.Float32(1011.5),
+			OptionalDouble:     scalar.Float64(1012.5),
+			OptionalBool:       scalar.Bool(true),
+			OptionalString:     scalar.String("string"),
+			OptionalBytes:      []byte("bytes"),
+			OptionalNestedEnum: testpb.TestAllTypes_BAR.Enum(),
+		}, build(
+			&testpb.TestAllExtensions{},
+			extend(testpb.E_OptionalInt32Extension, scalar.Int32(1001)),
+			extend(testpb.E_OptionalInt64Extension, scalar.Int64(1002)),
+			extend(testpb.E_OptionalUint32Extension, scalar.Uint32(1003)),
+			extend(testpb.E_OptionalUint64Extension, scalar.Uint64(1004)),
+			extend(testpb.E_OptionalSint32Extension, scalar.Int32(1005)),
+			extend(testpb.E_OptionalSint64Extension, scalar.Int64(1006)),
+			extend(testpb.E_OptionalFixed32Extension, scalar.Uint32(1007)),
+			extend(testpb.E_OptionalFixed64Extension, scalar.Uint64(1008)),
+			extend(testpb.E_OptionalSfixed32Extension, scalar.Int32(1009)),
+			extend(testpb.E_OptionalSfixed64Extension, scalar.Int64(1010)),
+			extend(testpb.E_OptionalFloatExtension, scalar.Float32(1011.5)),
+			extend(testpb.E_OptionalDoubleExtension, scalar.Float64(1012.5)),
+			extend(testpb.E_OptionalBoolExtension, scalar.Bool(true)),
+			extend(testpb.E_OptionalStringExtension, scalar.String("string")),
+			extend(testpb.E_OptionalBytesExtension, []byte("bytes")),
+			extend(testpb.E_OptionalNestedEnumExtension, testpb.TestAllTypes_BAR.Enum()),
+		)},
+		wire: pack.Message{
+			pack.Tag{1, pack.VarintType}, pack.Varint(1001),
+			pack.Tag{2, pack.VarintType}, pack.Varint(1002),
+			pack.Tag{3, pack.VarintType}, pack.Uvarint(1003),
+			pack.Tag{4, pack.VarintType}, pack.Uvarint(1004),
+			pack.Tag{5, pack.VarintType}, pack.Svarint(1005),
+			pack.Tag{6, pack.VarintType}, pack.Svarint(1006),
+			pack.Tag{7, pack.Fixed32Type}, pack.Uint32(1007),
+			pack.Tag{8, pack.Fixed64Type}, pack.Uint64(1008),
+			pack.Tag{9, pack.Fixed32Type}, pack.Int32(1009),
+			pack.Tag{10, pack.Fixed64Type}, pack.Int64(1010),
+			pack.Tag{11, pack.Fixed32Type}, pack.Float32(1011.5),
+			pack.Tag{12, pack.Fixed64Type}, pack.Float64(1012.5),
+			pack.Tag{13, pack.VarintType}, pack.Bool(true),
+			pack.Tag{14, pack.BytesType}, pack.String("string"),
+			pack.Tag{15, pack.BytesType}, pack.Bytes([]byte("bytes")),
+			pack.Tag{21, pack.VarintType}, pack.Varint(int(testpb.TestAllTypes_BAR)),
+		}.Marshal(),
+	},
+	{
+		desc: "groups",
+		decodeTo: []Message{&testpb.TestAllTypes{
+			Optionalgroup: &testpb.TestAllTypes_OptionalGroup{
+				A: scalar.Int32(1017),
+			},
+		}, build(
+			&testpb.TestAllExtensions{},
+			extend(testpb.E_OptionalgroupExtension, &testpb.OptionalGroupExtension{
+				A: scalar.Int32(1017),
+			}),
+		)},
+		wire: pack.Message{
+			pack.Tag{16, pack.StartGroupType},
+			pack.Tag{17, pack.VarintType}, pack.Varint(1017),
+			pack.Tag{16, pack.EndGroupType},
+		}.Marshal(),
+	},
+	{
+		desc: "groups (field overridden)",
+		decodeTo: []Message{&testpb.TestAllTypes{
+			Optionalgroup: &testpb.TestAllTypes_OptionalGroup{
+				A: scalar.Int32(2),
+			},
+		}, build(
+			&testpb.TestAllExtensions{},
+			extend(testpb.E_OptionalgroupExtension, &testpb.OptionalGroupExtension{
+				A: scalar.Int32(2),
+			}),
+		)},
+		wire: pack.Message{
+			pack.Tag{16, pack.StartGroupType},
+			pack.Tag{17, pack.VarintType}, pack.Varint(1),
+			pack.Tag{16, pack.EndGroupType},
+			pack.Tag{16, pack.StartGroupType},
+			pack.Tag{17, pack.VarintType}, pack.Varint(2),
+			pack.Tag{16, pack.EndGroupType},
+		}.Marshal(),
+	},
+	{
+		desc: "messages",
+		decodeTo: []Message{&testpb.TestAllTypes{
+			OptionalNestedMessage: &testpb.TestAllTypes_NestedMessage{
+				A: scalar.Int32(42),
+				Corecursive: &testpb.TestAllTypes{
+					OptionalInt32: scalar.Int32(43),
+				},
+			},
+		}, build(
+			&testpb.TestAllExtensions{},
+			extend(testpb.E_OptionalNestedMessageExtension, &testpb.TestAllTypes_NestedMessage{
+				A: scalar.Int32(42),
+				Corecursive: &testpb.TestAllTypes{
+					OptionalInt32: scalar.Int32(43),
+				},
+			}),
+		)},
+		wire: pack.Message{
+			pack.Tag{18, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(42),
+				pack.Tag{2, pack.BytesType}, pack.LengthPrefix(pack.Message{
+					pack.Tag{1, pack.VarintType}, pack.Varint(43),
+				}),
+			}),
+		}.Marshal(),
+	},
+	{
+		desc: "messages (split across multiple tags)",
+		decodeTo: []Message{&testpb.TestAllTypes{
+			OptionalNestedMessage: &testpb.TestAllTypes_NestedMessage{
+				A: scalar.Int32(42),
+				Corecursive: &testpb.TestAllTypes{
+					OptionalInt32: scalar.Int32(43),
+				},
+			},
+		}, build(
+			&testpb.TestAllExtensions{},
+			extend(testpb.E_OptionalNestedMessageExtension, &testpb.TestAllTypes_NestedMessage{
+				A: scalar.Int32(42),
+				Corecursive: &testpb.TestAllTypes{
+					OptionalInt32: scalar.Int32(43),
+				},
+			}),
+		)},
+		wire: pack.Message{
+			pack.Tag{18, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(42),
+			}),
+			pack.Tag{18, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{2, pack.BytesType}, pack.LengthPrefix(pack.Message{
+					pack.Tag{1, pack.VarintType}, pack.Varint(43),
+				}),
+			}),
+		}.Marshal(),
+	},
+	{
+		desc: "messages (field overridden)",
+		decodeTo: []Message{&testpb.TestAllTypes{
+			OptionalNestedMessage: &testpb.TestAllTypes_NestedMessage{
+				A: scalar.Int32(2),
+			},
+		}, build(
+			&testpb.TestAllExtensions{},
+			extend(testpb.E_OptionalNestedMessageExtension, &testpb.TestAllTypes_NestedMessage{
+				A: scalar.Int32(2),
+			}),
+		)},
+		wire: pack.Message{
+			pack.Tag{18, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(1),
+			}),
+			pack.Tag{18, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(2),
+			}),
+		}.Marshal(),
+	},
+	{
+		desc: "basic repeated types",
+		decodeTo: []Message{&testpb.TestAllTypes{
+			RepeatedInt32:    []int32{1001, 2001},
+			RepeatedInt64:    []int64{1002, 2002},
+			RepeatedUint32:   []uint32{1003, 2003},
+			RepeatedUint64:   []uint64{1004, 2004},
+			RepeatedSint32:   []int32{1005, 2005},
+			RepeatedSint64:   []int64{1006, 2006},
+			RepeatedFixed32:  []uint32{1007, 2007},
+			RepeatedFixed64:  []uint64{1008, 2008},
+			RepeatedSfixed32: []int32{1009, 2009},
+			RepeatedSfixed64: []int64{1010, 2010},
+			RepeatedFloat:    []float32{1011.5, 2011.5},
+			RepeatedDouble:   []float64{1012.5, 2012.5},
+			RepeatedBool:     []bool{true, false},
+			RepeatedString:   []string{"foo", "bar"},
+			RepeatedBytes:    [][]byte{[]byte("FOO"), []byte("BAR")},
+			RepeatedNestedEnum: []testpb.TestAllTypes_NestedEnum{
+				testpb.TestAllTypes_FOO,
+				testpb.TestAllTypes_BAR,
+			},
+		}, build(
+			&testpb.TestAllExtensions{},
+			extend(testpb.E_RepeatedInt32Extension, []int32{1001, 2001}),
+			extend(testpb.E_RepeatedInt64Extension, []int64{1002, 2002}),
+			extend(testpb.E_RepeatedUint32Extension, []uint32{1003, 2003}),
+			extend(testpb.E_RepeatedUint64Extension, []uint64{1004, 2004}),
+			extend(testpb.E_RepeatedSint32Extension, []int32{1005, 2005}),
+			extend(testpb.E_RepeatedSint64Extension, []int64{1006, 2006}),
+			extend(testpb.E_RepeatedFixed32Extension, []uint32{1007, 2007}),
+			extend(testpb.E_RepeatedFixed64Extension, []uint64{1008, 2008}),
+			extend(testpb.E_RepeatedSfixed32Extension, []int32{1009, 2009}),
+			extend(testpb.E_RepeatedSfixed64Extension, []int64{1010, 2010}),
+			extend(testpb.E_RepeatedFloatExtension, []float32{1011.5, 2011.5}),
+			extend(testpb.E_RepeatedDoubleExtension, []float64{1012.5, 2012.5}),
+			extend(testpb.E_RepeatedBoolExtension, []bool{true, false}),
+			extend(testpb.E_RepeatedStringExtension, []string{"foo", "bar"}),
+			extend(testpb.E_RepeatedBytesExtension, [][]byte{[]byte("FOO"), []byte("BAR")}),
+			extend(testpb.E_RepeatedNestedEnumExtension, []testpb.TestAllTypes_NestedEnum{
+				testpb.TestAllTypes_FOO,
+				testpb.TestAllTypes_BAR,
+			}),
+		)},
+		wire: pack.Message{
+			pack.Tag{31, pack.VarintType}, pack.Varint(1001),
+			pack.Tag{31, pack.VarintType}, pack.Varint(2001),
+			pack.Tag{32, pack.VarintType}, pack.Varint(1002),
+			pack.Tag{32, pack.VarintType}, pack.Varint(2002),
+			pack.Tag{33, pack.VarintType}, pack.Uvarint(1003),
+			pack.Tag{33, pack.VarintType}, pack.Uvarint(2003),
+			pack.Tag{34, pack.VarintType}, pack.Uvarint(1004),
+			pack.Tag{34, pack.VarintType}, pack.Uvarint(2004),
+			pack.Tag{35, pack.VarintType}, pack.Svarint(1005),
+			pack.Tag{35, pack.VarintType}, pack.Svarint(2005),
+			pack.Tag{36, pack.VarintType}, pack.Svarint(1006),
+			pack.Tag{36, pack.VarintType}, pack.Svarint(2006),
+			pack.Tag{37, pack.Fixed32Type}, pack.Uint32(1007),
+			pack.Tag{37, pack.Fixed32Type}, pack.Uint32(2007),
+			pack.Tag{38, pack.Fixed64Type}, pack.Uint64(1008),
+			pack.Tag{38, pack.Fixed64Type}, pack.Uint64(2008),
+			pack.Tag{39, pack.Fixed32Type}, pack.Int32(1009),
+			pack.Tag{39, pack.Fixed32Type}, pack.Int32(2009),
+			pack.Tag{40, pack.Fixed64Type}, pack.Int64(1010),
+			pack.Tag{40, pack.Fixed64Type}, pack.Int64(2010),
+			pack.Tag{41, pack.Fixed32Type}, pack.Float32(1011.5),
+			pack.Tag{41, pack.Fixed32Type}, pack.Float32(2011.5),
+			pack.Tag{42, pack.Fixed64Type}, pack.Float64(1012.5),
+			pack.Tag{42, pack.Fixed64Type}, pack.Float64(2012.5),
+			pack.Tag{43, pack.VarintType}, pack.Bool(true),
+			pack.Tag{43, pack.VarintType}, pack.Bool(false),
+			pack.Tag{44, pack.BytesType}, pack.String("foo"),
+			pack.Tag{44, pack.BytesType}, pack.String("bar"),
+			pack.Tag{45, pack.BytesType}, pack.Bytes([]byte("FOO")),
+			pack.Tag{45, pack.BytesType}, pack.Bytes([]byte("BAR")),
+			pack.Tag{51, pack.VarintType}, pack.Varint(int(testpb.TestAllTypes_FOO)),
+			pack.Tag{51, pack.VarintType}, pack.Varint(int(testpb.TestAllTypes_BAR)),
+		}.Marshal(),
+	},
+	{
+		desc: "basic repeated types (packed encoding)",
+		decodeTo: []Message{&testpb.TestAllTypes{
+			RepeatedInt32:    []int32{1001, 2001},
+			RepeatedInt64:    []int64{1002, 2002},
+			RepeatedUint32:   []uint32{1003, 2003},
+			RepeatedUint64:   []uint64{1004, 2004},
+			RepeatedSint32:   []int32{1005, 2005},
+			RepeatedSint64:   []int64{1006, 2006},
+			RepeatedFixed32:  []uint32{1007, 2007},
+			RepeatedFixed64:  []uint64{1008, 2008},
+			RepeatedSfixed32: []int32{1009, 2009},
+			RepeatedSfixed64: []int64{1010, 2010},
+			RepeatedFloat:    []float32{1011.5, 2011.5},
+			RepeatedDouble:   []float64{1012.5, 2012.5},
+			RepeatedBool:     []bool{true, false},
+			RepeatedNestedEnum: []testpb.TestAllTypes_NestedEnum{
+				testpb.TestAllTypes_FOO,
+				testpb.TestAllTypes_BAR,
+			},
+		}, build(
+			&testpb.TestAllExtensions{},
+			extend(testpb.E_RepeatedInt32Extension, []int32{1001, 2001}),
+			extend(testpb.E_RepeatedInt64Extension, []int64{1002, 2002}),
+			extend(testpb.E_RepeatedUint32Extension, []uint32{1003, 2003}),
+			extend(testpb.E_RepeatedUint64Extension, []uint64{1004, 2004}),
+			extend(testpb.E_RepeatedSint32Extension, []int32{1005, 2005}),
+			extend(testpb.E_RepeatedSint64Extension, []int64{1006, 2006}),
+			extend(testpb.E_RepeatedFixed32Extension, []uint32{1007, 2007}),
+			extend(testpb.E_RepeatedFixed64Extension, []uint64{1008, 2008}),
+			extend(testpb.E_RepeatedSfixed32Extension, []int32{1009, 2009}),
+			extend(testpb.E_RepeatedSfixed64Extension, []int64{1010, 2010}),
+			extend(testpb.E_RepeatedFloatExtension, []float32{1011.5, 2011.5}),
+			extend(testpb.E_RepeatedDoubleExtension, []float64{1012.5, 2012.5}),
+			extend(testpb.E_RepeatedBoolExtension, []bool{true, false}),
+			extend(testpb.E_RepeatedNestedEnumExtension, []testpb.TestAllTypes_NestedEnum{
+				testpb.TestAllTypes_FOO,
+				testpb.TestAllTypes_BAR,
+			}),
+		)},
+		wire: pack.Message{
+			pack.Tag{31, pack.BytesType}, pack.LengthPrefix{
+				pack.Varint(1001), pack.Varint(2001),
+			},
+			pack.Tag{32, pack.BytesType}, pack.LengthPrefix{
+				pack.Varint(1002), pack.Varint(2002),
+			},
+			pack.Tag{33, pack.BytesType}, pack.LengthPrefix{
+				pack.Uvarint(1003), pack.Uvarint(2003),
+			},
+			pack.Tag{34, pack.BytesType}, pack.LengthPrefix{
+				pack.Uvarint(1004), pack.Uvarint(2004),
+			},
+			pack.Tag{35, pack.BytesType}, pack.LengthPrefix{
+				pack.Svarint(1005), pack.Svarint(2005),
+			},
+			pack.Tag{36, pack.BytesType}, pack.LengthPrefix{
+				pack.Svarint(1006), pack.Svarint(2006),
+			},
+			pack.Tag{37, pack.BytesType}, pack.LengthPrefix{
+				pack.Uint32(1007), pack.Uint32(2007),
+			},
+			pack.Tag{38, pack.BytesType}, pack.LengthPrefix{
+				pack.Uint64(1008), pack.Uint64(2008),
+			},
+			pack.Tag{39, pack.BytesType}, pack.LengthPrefix{
+				pack.Int32(1009), pack.Int32(2009),
+			},
+			pack.Tag{40, pack.BytesType}, pack.LengthPrefix{
+				pack.Int64(1010), pack.Int64(2010),
+			},
+			pack.Tag{41, pack.BytesType}, pack.LengthPrefix{
+				pack.Float32(1011.5), pack.Float32(2011.5),
+			},
+			pack.Tag{42, pack.BytesType}, pack.LengthPrefix{
+				pack.Float64(1012.5), pack.Float64(2012.5),
+			},
+			pack.Tag{43, pack.BytesType}, pack.LengthPrefix{
+				pack.Bool(true), pack.Bool(false),
+			},
+			pack.Tag{51, pack.BytesType}, pack.LengthPrefix{
+				pack.Varint(int(testpb.TestAllTypes_FOO)),
+				pack.Varint(int(testpb.TestAllTypes_BAR)),
+			},
+		}.Marshal(),
+	},
+	{
+		desc: "repeated messages",
+		decodeTo: []Message{&testpb.TestAllTypes{
+			RepeatedNestedMessage: []*testpb.TestAllTypes_NestedMessage{
+				{A: scalar.Int32(1)},
+				{A: scalar.Int32(2)},
+			},
+		}, build(
+			&testpb.TestAllExtensions{},
+			extend(testpb.E_RepeatedNestedMessageExtension, []*testpb.TestAllTypes_NestedMessage{
+				{A: scalar.Int32(1)},
+				{A: scalar.Int32(2)},
+			}),
+		)},
+		wire: pack.Message{
+			pack.Tag{48, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(1),
+			}),
+			pack.Tag{48, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(2),
+			}),
+		}.Marshal(),
+	},
+	{
+		desc: "repeated groups",
+		decodeTo: []Message{&testpb.TestAllTypes{
+			Repeatedgroup: []*testpb.TestAllTypes_RepeatedGroup{
+				{A: scalar.Int32(1017)},
+				{A: scalar.Int32(2017)},
+			},
+		}, build(
+			&testpb.TestAllExtensions{},
+			extend(testpb.E_RepeatedgroupExtension, []*testpb.RepeatedGroupExtension{
+				{A: scalar.Int32(1017)},
+				{A: scalar.Int32(2017)},
+			}),
+		)},
+		wire: pack.Message{
+			pack.Tag{46, pack.StartGroupType},
+			pack.Tag{47, pack.VarintType}, pack.Varint(1017),
+			pack.Tag{46, pack.EndGroupType},
+			pack.Tag{46, pack.StartGroupType},
+			pack.Tag{47, pack.VarintType}, pack.Varint(2017),
+			pack.Tag{46, pack.EndGroupType},
+		}.Marshal(),
+	},
+	{
+		desc: "maps",
+		decodeTo: []Message{&testpb.TestAllTypes{
+			MapInt32Int32:       map[int32]int32{1056: 1156, 2056: 2156},
+			MapInt64Int64:       map[int64]int64{1057: 1157, 2057: 2157},
+			MapUint32Uint32:     map[uint32]uint32{1058: 1158, 2058: 2158},
+			MapUint64Uint64:     map[uint64]uint64{1059: 1159, 2059: 2159},
+			MapSint32Sint32:     map[int32]int32{1060: 1160, 2060: 2160},
+			MapSint64Sint64:     map[int64]int64{1061: 1161, 2061: 2161},
+			MapFixed32Fixed32:   map[uint32]uint32{1062: 1162, 2062: 2162},
+			MapFixed64Fixed64:   map[uint64]uint64{1063: 1163, 2063: 2163},
+			MapSfixed32Sfixed32: map[int32]int32{1064: 1164, 2064: 2164},
+			MapSfixed64Sfixed64: map[int64]int64{1065: 1165, 2065: 2165},
+			MapInt32Float:       map[int32]float32{1066: 1166.5, 2066: 2166.5},
+			MapInt32Double:      map[int32]float64{1067: 1167.5, 2067: 2167.5},
+			MapBoolBool:         map[bool]bool{true: false, false: true},
+			MapStringString:     map[string]string{"69.1.key": "69.1.val", "69.2.key": "69.2.val"},
+			MapStringBytes:      map[string][]byte{"70.1.key": []byte("70.1.val"), "70.2.key": []byte("70.2.val")},
+			MapStringNestedMessage: map[string]*testpb.TestAllTypes_NestedMessage{
+				"71.1.key": {A: scalar.Int32(1171)},
+				"71.2.key": {A: scalar.Int32(2171)},
+			},
+			MapStringNestedEnum: map[string]testpb.TestAllTypes_NestedEnum{
+				"73.1.key": testpb.TestAllTypes_FOO,
+				"73.2.key": testpb.TestAllTypes_BAR,
+			},
+		}},
+		wire: pack.Message{
+			pack.Tag{56, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(1056),
+				pack.Tag{2, pack.VarintType}, pack.Varint(1156),
+			}),
+			pack.Tag{56, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(2056),
+				pack.Tag{2, pack.VarintType}, pack.Varint(2156),
+			}),
+			pack.Tag{57, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(1057),
+				pack.Tag{2, pack.VarintType}, pack.Varint(1157),
+			}),
+			pack.Tag{57, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(2057),
+				pack.Tag{2, pack.VarintType}, pack.Varint(2157),
+			}),
+			pack.Tag{58, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(1058),
+				pack.Tag{2, pack.VarintType}, pack.Varint(1158),
+			}),
+			pack.Tag{58, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(2058),
+				pack.Tag{2, pack.VarintType}, pack.Varint(2158),
+			}),
+			pack.Tag{59, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(1059),
+				pack.Tag{2, pack.VarintType}, pack.Varint(1159),
+			}),
+			pack.Tag{59, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(2059),
+				pack.Tag{2, pack.VarintType}, pack.Varint(2159),
+			}),
+			pack.Tag{60, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Svarint(1060),
+				pack.Tag{2, pack.VarintType}, pack.Svarint(1160),
+			}),
+			pack.Tag{60, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Svarint(2060),
+				pack.Tag{2, pack.VarintType}, pack.Svarint(2160),
+			}),
+			pack.Tag{61, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Svarint(1061),
+				pack.Tag{2, pack.VarintType}, pack.Svarint(1161),
+			}),
+			pack.Tag{61, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Svarint(2061),
+				pack.Tag{2, pack.VarintType}, pack.Svarint(2161),
+			}),
+			pack.Tag{62, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.Fixed32Type}, pack.Int32(1062),
+				pack.Tag{2, pack.Fixed32Type}, pack.Int32(1162),
+			}),
+			pack.Tag{62, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.Fixed32Type}, pack.Int32(2062),
+				pack.Tag{2, pack.Fixed32Type}, pack.Int32(2162),
+			}),
+			pack.Tag{63, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.Fixed64Type}, pack.Int64(1063),
+				pack.Tag{2, pack.Fixed64Type}, pack.Int64(1163),
+			}),
+			pack.Tag{63, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.Fixed64Type}, pack.Int64(2063),
+				pack.Tag{2, pack.Fixed64Type}, pack.Int64(2163),
+			}),
+			pack.Tag{64, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.Fixed32Type}, pack.Int32(1064),
+				pack.Tag{2, pack.Fixed32Type}, pack.Int32(1164),
+			}),
+			pack.Tag{64, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.Fixed32Type}, pack.Int32(2064),
+				pack.Tag{2, pack.Fixed32Type}, pack.Int32(2164),
+			}),
+			pack.Tag{65, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.Fixed64Type}, pack.Int64(1065),
+				pack.Tag{2, pack.Fixed64Type}, pack.Int64(1165),
+			}),
+			pack.Tag{65, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.Fixed64Type}, pack.Int64(2065),
+				pack.Tag{2, pack.Fixed64Type}, pack.Int64(2165),
+			}),
+			pack.Tag{66, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(1066),
+				pack.Tag{2, pack.Fixed32Type}, pack.Float32(1166.5),
+			}),
+			pack.Tag{66, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(2066),
+				pack.Tag{2, pack.Fixed32Type}, pack.Float32(2166.5),
+			}),
+			pack.Tag{67, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(1067),
+				pack.Tag{2, pack.Fixed64Type}, pack.Float64(1167.5),
+			}),
+			pack.Tag{67, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(2067),
+				pack.Tag{2, pack.Fixed64Type}, pack.Float64(2167.5),
+			}),
+			pack.Tag{68, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Bool(true),
+				pack.Tag{2, pack.VarintType}, pack.Bool(false),
+			}),
+			pack.Tag{68, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Bool(false),
+				pack.Tag{2, pack.VarintType}, pack.Bool(true),
+			}),
+			pack.Tag{69, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.BytesType}, pack.String("69.1.key"),
+				pack.Tag{2, pack.BytesType}, pack.String("69.1.val"),
+			}),
+			pack.Tag{69, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.BytesType}, pack.String("69.2.key"),
+				pack.Tag{2, pack.BytesType}, pack.String("69.2.val"),
+			}),
+			pack.Tag{70, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.BytesType}, pack.String("70.1.key"),
+				pack.Tag{2, pack.BytesType}, pack.String("70.1.val"),
+			}),
+			pack.Tag{70, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.BytesType}, pack.String("70.2.key"),
+				pack.Tag{2, pack.BytesType}, pack.String("70.2.val"),
+			}),
+			pack.Tag{71, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.BytesType}, pack.String("71.1.key"),
+				pack.Tag{2, pack.BytesType}, pack.LengthPrefix(pack.Message{
+					pack.Tag{1, pack.VarintType}, pack.Varint(1171),
+				}),
+			}),
+			pack.Tag{71, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.BytesType}, pack.String("71.2.key"),
+				pack.Tag{2, pack.BytesType}, pack.LengthPrefix(pack.Message{
+					pack.Tag{1, pack.VarintType}, pack.Varint(2171),
+				}),
+			}),
+			pack.Tag{73, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.BytesType}, pack.String("73.1.key"),
+				pack.Tag{2, pack.VarintType}, pack.Varint(int(testpb.TestAllTypes_FOO)),
+			}),
+			pack.Tag{73, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.BytesType}, pack.String("73.2.key"),
+				pack.Tag{2, pack.VarintType}, pack.Varint(int(testpb.TestAllTypes_BAR)),
+			}),
+		}.Marshal(),
+	},
+	{
+		desc:     "oneof (uint32)",
+		decodeTo: []Message{&testpb.TestAllTypes{OneofField: &testpb.TestAllTypes_OneofUint32{1111}}},
+		wire:     pack.Message{pack.Tag{111, pack.VarintType}, pack.Varint(1111)}.Marshal(),
+	},
+	{
+		desc: "oneof (message)",
+		decodeTo: []Message{&testpb.TestAllTypes{OneofField: &testpb.TestAllTypes_OneofNestedMessage{
+			&testpb.TestAllTypes_NestedMessage{A: scalar.Int32(1112)},
+		}}},
+		wire: pack.Message{pack.Tag{112, pack.BytesType}, pack.LengthPrefix(pack.Message{
+			pack.Message{pack.Tag{1, pack.VarintType}, pack.Varint(1112)},
+		})}.Marshal(),
+	},
+	{
+		desc: "oneof (overridden message)",
+		decodeTo: []Message{&testpb.TestAllTypes{OneofField: &testpb.TestAllTypes_OneofNestedMessage{
+			&testpb.TestAllTypes_NestedMessage{
+				Corecursive: &testpb.TestAllTypes{
+					OptionalInt32: scalar.Int32(43),
+				},
+			},
+		}}},
+		wire: pack.Message{
+			pack.Tag{112, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Message{pack.Tag{1, pack.VarintType}, pack.Varint(1)},
+			}),
+			pack.Tag{112, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{2, pack.BytesType}, pack.LengthPrefix(pack.Message{
+					pack.Tag{1, pack.VarintType}, pack.Varint(43),
+				}),
+			}),
+		}.Marshal(),
+	},
+	{
+		desc:     "oneof (string)",
+		decodeTo: []Message{&testpb.TestAllTypes{OneofField: &testpb.TestAllTypes_OneofString{"1113"}}},
+		wire:     pack.Message{pack.Tag{113, pack.BytesType}, pack.String("1113")}.Marshal(),
+	},
+	{
+		desc:     "oneof (bytes)",
+		decodeTo: []Message{&testpb.TestAllTypes{OneofField: &testpb.TestAllTypes_OneofBytes{[]byte("1114")}}},
+		wire:     pack.Message{pack.Tag{114, pack.BytesType}, pack.String("1114")}.Marshal(),
+	},
+	{
+		desc:     "oneof (bool)",
+		decodeTo: []Message{&testpb.TestAllTypes{OneofField: &testpb.TestAllTypes_OneofBool{true}}},
+		wire:     pack.Message{pack.Tag{115, pack.VarintType}, pack.Bool(true)}.Marshal(),
+	},
+	{
+		desc:     "oneof (uint64)",
+		decodeTo: []Message{&testpb.TestAllTypes{OneofField: &testpb.TestAllTypes_OneofUint64{116}}},
+		wire:     pack.Message{pack.Tag{116, pack.VarintType}, pack.Varint(116)}.Marshal(),
+	},
+	{
+		desc:     "oneof (float)",
+		decodeTo: []Message{&testpb.TestAllTypes{OneofField: &testpb.TestAllTypes_OneofFloat{117.5}}},
+		wire:     pack.Message{pack.Tag{117, pack.Fixed32Type}, pack.Float32(117.5)}.Marshal(),
+	},
+	{
+		desc:     "oneof (double)",
+		decodeTo: []Message{&testpb.TestAllTypes{OneofField: &testpb.TestAllTypes_OneofDouble{118.5}}},
+		wire:     pack.Message{pack.Tag{118, pack.Fixed64Type}, pack.Float64(118.5)}.Marshal(),
+	},
+	{
+		desc:     "oneof (enum)",
+		decodeTo: []Message{&testpb.TestAllTypes{OneofField: &testpb.TestAllTypes_OneofEnum{testpb.TestAllTypes_BAR}}},
+		wire:     pack.Message{pack.Tag{119, pack.VarintType}, pack.Varint(int(testpb.TestAllTypes_BAR))}.Marshal(),
+	},
+	{
+		desc:     "oneof (overridden value)",
+		decodeTo: []Message{&testpb.TestAllTypes{OneofField: &testpb.TestAllTypes_OneofUint64{2}}},
+		wire: pack.Message{
+			pack.Tag{111, pack.VarintType}, pack.Varint(1),
+			pack.Tag{116, pack.VarintType}, pack.Varint(2),
+		}.Marshal(),
+	},
+	// TODO: More unknown field tests for ordering, repeated fields, etc.
+	//
+	// It is currently impossible to produce results that the v1 Equal
+	// considers equivalent to those of the v1 decoder. Figure out if
+	// that's a problem or not.
+	{
+		desc: "unknown fields",
+		decodeTo: []Message{build(
+			&testpb.TestAllTypes{},
+			unknown(100000, pack.Message{
+				pack.Tag{100000, pack.VarintType}, pack.Varint(1),
+			}.Marshal()),
+		)},
+		wire: pack.Message{
+			pack.Tag{100000, pack.VarintType}, pack.Varint(1),
+		}.Marshal(),
+	},
+	{
+		desc: "field type mismatch",
+		decodeTo: []Message{build(
+			&testpb.TestAllTypes{},
+			unknown(1, pack.Message{
+				pack.Tag{1, pack.BytesType}, pack.String("string"),
+			}.Marshal()),
+		)},
+		wire: pack.Message{
+			pack.Tag{1, pack.BytesType}, pack.String("string"),
+		}.Marshal(),
+	},
+	{
+		desc: "map field element mismatch",
+		decodeTo: []Message{
+			&testpb.TestAllTypes{
+				MapInt32Int32: map[int32]int32{1: 0},
+			},
+		},
+		wire: pack.Message{
+			pack.Tag{56, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.VarintType}, pack.Varint(1),
+				pack.Tag{2, pack.BytesType}, pack.String("string"),
+			}),
+		}.Marshal(),
+	},
+}
+
+func build(m Message, opts ...buildOpt) Message {
+	for _, opt := range opts {
+		opt(m)
+	}
+	return m
+}
+
+type buildOpt func(Message)
+
+func unknown(num pref.FieldNumber, raw pref.RawFields) buildOpt {
+	return func(m Message) {
+		m.ProtoReflect().UnknownFields().Set(num, raw)
+	}
+}
+
+func extend(desc *protoV1.ExtensionDesc, value interface{}) buildOpt {
+	return func(m Message) {
+		if err := protoV1.SetExtension(m.(protoV1.Message), desc, value); err != nil {
+			panic(err)
+		}
+	}
+}