diff --git a/internal/cmd/generate-protos/main.go b/internal/cmd/generate-protos/main.go
index 3ab6c30..cfc8c8a 100644
--- a/internal/cmd/generate-protos/main.go
+++ b/internal/cmd/generate-protos/main.go
@@ -105,10 +105,11 @@
 		path        string
 		grpcPlugin  bool
 		annotateFor map[string]bool
+		exclude     map[string]bool
 	}{
 		{path: "cmd/protoc-gen-go/testdata", annotateFor: map[string]bool{"annotations/annotations.proto": true}},
 		{path: "cmd/protoc-gen-go-grpc/testdata", grpcPlugin: true},
-		{path: "internal/testprotos"},
+		{path: "internal/testprotos", exclude: map[string]bool{"irregular/irregular.proto": true}},
 		{path: "encoding/testprotos"},
 		{path: "reflect/protoregistry/testprotos"},
 	}
@@ -128,6 +129,10 @@
 			check(err)
 			subDirs[filepath.Dir(relPath)] = true
 
+			if d.exclude[filepath.ToSlash(relPath)] {
+				return nil
+			}
+
 			// Emit a .meta file for certain files.
 			var opts string
 			if d.annotateFor[filepath.ToSlash(relPath)] {
diff --git a/internal/impl/encode_field.go b/internal/impl/encode_field.go
index 53a8eda..a6580ce 100644
--- a/internal/impl/encode_field.go
+++ b/internal/impl/encode_field.go
@@ -82,11 +82,11 @@
 	} else {
 		return pointerCoderFuncs{
 			size: func(p pointer, tagsize int, opts marshalOptions) int {
-				m := legacyWrapMessage(p.AsValueOf(ft).Elem())
+				m := asMessage(p.AsValueOf(ft).Elem())
 				return sizeMessage(m, tagsize, opts)
 			},
 			marshal: func(b []byte, p pointer, wiretag uint64, opts marshalOptions) ([]byte, error) {
-				m := legacyWrapMessage(p.AsValueOf(ft).Elem())
+				m := asMessage(p.AsValueOf(ft).Elem())
 				return appendMessage(b, m, wiretag, opts)
 			},
 		}
@@ -141,11 +141,11 @@
 	} else {
 		return pointerCoderFuncs{
 			size: func(p pointer, tagsize int, opts marshalOptions) int {
-				m := legacyWrapMessage(p.AsValueOf(ft).Elem())
+				m := asMessage(p.AsValueOf(ft).Elem())
 				return sizeGroup(m, tagsize, opts)
 			},
 			marshal: func(b []byte, p pointer, wiretag uint64, opts marshalOptions) ([]byte, error) {
-				m := legacyWrapMessage(p.AsValueOf(ft).Elem())
+				m := asMessage(p.AsValueOf(ft).Elem())
 				return appendGroup(b, m, wiretag, opts)
 			},
 		}
@@ -492,3 +492,10 @@
 	size:    sizeStringIfaceValidateUTF8,
 	marshal: appendStringIfaceValidateUTF8,
 }
+
+func asMessage(v reflect.Value) pref.ProtoMessage {
+	if m, ok := v.Interface().(pref.ProtoMessage); ok {
+		return m
+	}
+	return legacyWrapMessage(v)
+}
diff --git a/internal/testprotos/irregular/irregular.go b/internal/testprotos/irregular/irregular.go
new file mode 100644
index 0000000..cc9c549
--- /dev/null
+++ b/internal/testprotos/irregular/irregular.go
@@ -0,0 +1,141 @@
+// Copyright 2019 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 irregular
+
+import (
+	"google.golang.org/protobuf/encoding/prototext"
+	"google.golang.org/protobuf/reflect/protodesc"
+	pref "google.golang.org/protobuf/reflect/protoreflect"
+	"google.golang.org/protobuf/types/descriptorpb"
+)
+
+type IrregularMessage struct {
+	set   bool
+	value string
+}
+
+func (m *IrregularMessage) ProtoReflect() pref.Message { return (*message)(m) }
+
+type message IrregularMessage
+
+func (m *message) Descriptor() pref.MessageDescriptor { return descriptor.Messages().Get(0) }
+func (m *message) Type() pref.MessageType             { return nil }
+func (m *message) KnownFields() pref.KnownFields      { return (*known)(m) }
+func (m *message) UnknownFields() pref.UnknownFields  { return (*unknown)(m) }
+func (m *message) New() pref.Message                  { return &message{} }
+func (m *message) Interface() pref.ProtoMessage       { return (*IrregularMessage)(m) }
+
+type known IrregularMessage
+
+func (m *known) Len() int {
+	if m.set {
+		return 1
+	}
+	return 0
+}
+
+func (m *known) Has(num pref.FieldNumber) bool {
+	switch num {
+	case fieldS:
+		return m.set
+	}
+	return false
+}
+
+func (m *known) Get(num pref.FieldNumber) pref.Value {
+	switch num {
+	case fieldS:
+		return pref.ValueOf(m.value)
+	}
+	return pref.Value{}
+}
+
+func (m *known) Set(num pref.FieldNumber, v pref.Value) {
+	switch num {
+	case fieldS:
+		m.value = v.String()
+	default:
+		panic("unknown field")
+	}
+}
+
+func (m *known) Clear(num pref.FieldNumber) {
+	switch num {
+	case fieldS:
+		m.value = ""
+		m.set = false
+	default:
+		panic("unknown field")
+	}
+}
+
+func (m *known) WhichOneof(name pref.Name) pref.FieldNumber {
+	return 0
+}
+
+func (m *known) Range(f func(pref.FieldNumber, pref.Value) bool) {
+	if m.set {
+		f(fieldS, pref.ValueOf(m.value))
+	}
+}
+
+func (m *known) NewMessage(num pref.FieldNumber) pref.Message {
+	panic("not a message field")
+}
+
+func (m *known) ExtensionTypes() pref.ExtensionFieldTypes {
+	return (*exttypes)(m)
+}
+
+type unknown IrregularMessage
+
+func (m *unknown) Len() int                                          { return 0 }
+func (m *unknown) Get(pref.FieldNumber) pref.RawFields               { return nil }
+func (m *unknown) Set(pref.FieldNumber, pref.RawFields)              {}
+func (m *unknown) Range(func(pref.FieldNumber, pref.RawFields) bool) {}
+func (m *unknown) IsSupported() bool                                 { return false }
+
+type exttypes IrregularMessage
+
+func (m *exttypes) Len() int                                     { return 0 }
+func (m *exttypes) Register(pref.ExtensionType)                  { panic("not extendable") }
+func (m *exttypes) Remove(pref.ExtensionType)                    {}
+func (m *exttypes) ByNumber(pref.FieldNumber) pref.ExtensionType { return nil }
+func (m *exttypes) ByName(pref.FullName) pref.ExtensionType      { return nil }
+func (m *exttypes) Range(func(pref.ExtensionType) bool)          {}
+
+const fieldS = pref.FieldNumber(1)
+
+var descriptor = func() pref.FileDescriptor {
+	p := &descriptorpb.FileDescriptorProto{}
+	if err := prototext.Unmarshal([]byte(descriptorText), p); err != nil {
+		panic(err)
+	}
+	file, err := protodesc.NewFile(p, nil)
+	if err != nil {
+		panic(err)
+	}
+	return file
+}()
+
+func file_irregular_irregular_proto_init() { _ = descriptor }
+
+const descriptorText = `
+  name: "internal/testprotos/irregular/irregular.proto"
+  package: "goproto.proto.thirdparty"
+  message_type {
+    name: "IrregularMessage"
+    field {
+      name: "s"
+      number: 1
+      label: LABEL_OPTIONAL
+      type: TYPE_STRING
+      json_name: "s"
+    }
+  }
+  options {
+    go_package: "google.golang.org/protobuf/internal/testprotos/irregular"
+  }
+`
diff --git a/internal/testprotos/irregular/irregular.proto b/internal/testprotos/irregular/irregular.proto
new file mode 100644
index 0000000..9d8cf52
--- /dev/null
+++ b/internal/testprotos/irregular/irregular.proto
@@ -0,0 +1,16 @@
+// Copyright 2019 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.
+
+syntax = "proto2";
+
+package goproto.proto.irregular;
+
+option go_package = "google.golang.org/protobuf/internal/testprotos/irregular";
+
+// IrregularMessage is a message with an implementation that does not match the
+// usual structure of a generated message.
+message IrregularMessage {
+  optional string s = 1;
+}
+
diff --git a/internal/testprotos/irregular/test.pb.go b/internal/testprotos/irregular/test.pb.go
new file mode 100644
index 0000000..fa6aafd
--- /dev/null
+++ b/internal/testprotos/irregular/test.pb.go
@@ -0,0 +1,208 @@
+// Code generated by protoc-gen-go. DO NOT EDIT.
+// source: irregular/test.proto
+
+package irregular
+
+import (
+	protoreflect "google.golang.org/protobuf/reflect/protoreflect"
+	protoregistry "google.golang.org/protobuf/reflect/protoregistry"
+	protoiface "google.golang.org/protobuf/runtime/protoiface"
+	protoimpl "google.golang.org/protobuf/runtime/protoimpl"
+	sync "sync"
+)
+
+const (
+	// Verify that runtime/protoimpl is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(protoimpl.MaxVersion - 0)
+	// Verify that this generated code is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(0 - protoimpl.MinVersion)
+)
+
+type Message struct {
+	OptionalMessage *IrregularMessage            `protobuf:"bytes,1,opt,name=optional_message,json=optionalMessage" json:"optional_message,omitempty"`
+	RepeatedMessage []*IrregularMessage          `protobuf:"bytes,2,rep,name=repeated_message,json=repeatedMessage" json:"repeated_message,omitempty"`
+	RequiredMessage *IrregularMessage            `protobuf:"bytes,3,req,name=required_message,json=requiredMessage" json:"required_message,omitempty"`
+	MapMessage      map[string]*IrregularMessage `protobuf:"bytes,4,rep,name=map_message,json=mapMessage" json:"map_message,omitempty" protobuf_key:"bytes,1,opt,name=key" protobuf_val:"bytes,2,opt,name=value"`
+	// Types that are valid to be assigned to Union:
+	//	*Message_OneofMessage
+	Union                isMessage_Union         `protobuf_oneof:"union"`
+	XXX_NoUnkeyedLiteral struct{}                `json:"-"`
+	XXX_unrecognized     protoimpl.UnknownFields `json:"-"`
+	XXX_sizecache        protoimpl.SizeCache     `json:"-"`
+}
+
+func (x *Message) Reset() {
+	*x = Message{}
+}
+
+func (x *Message) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*Message) ProtoMessage() {}
+
+func (x *Message) ProtoReflect() protoreflect.Message {
+	return file_irregular_test_proto_msgTypes[0].MessageOf(x)
+}
+
+func (m *Message) XXX_Methods() *protoiface.Methods {
+	return file_irregular_test_proto_msgTypes[0].Methods()
+}
+
+// Deprecated: Use Message.ProtoReflect.Type instead.
+func (*Message) Descriptor() ([]byte, []int) {
+	return file_irregular_test_proto_rawDescGZIP(), []int{0}
+}
+
+func (x *Message) GetOptionalMessage() *IrregularMessage {
+	if x != nil {
+		return x.OptionalMessage
+	}
+	return nil
+}
+
+func (x *Message) GetRepeatedMessage() []*IrregularMessage {
+	if x != nil {
+		return x.RepeatedMessage
+	}
+	return nil
+}
+
+func (x *Message) GetRequiredMessage() *IrregularMessage {
+	if x != nil {
+		return x.RequiredMessage
+	}
+	return nil
+}
+
+func (x *Message) GetMapMessage() map[string]*IrregularMessage {
+	if x != nil {
+		return x.MapMessage
+	}
+	return nil
+}
+
+func (m *Message) GetUnion() isMessage_Union {
+	if m != nil {
+		return m.Union
+	}
+	return nil
+}
+
+func (x *Message) GetOneofMessage() *IrregularMessage {
+	if x, ok := x.GetUnion().(*Message_OneofMessage); ok {
+		return x.OneofMessage
+	}
+	return nil
+}
+
+// XXX_OneofWrappers is for the internal use of the proto package.
+func (*Message) XXX_OneofWrappers() []interface{} {
+	return []interface{}{
+		(*Message_OneofMessage)(nil),
+	}
+}
+
+type isMessage_Union interface {
+	isMessage_Union()
+}
+
+type Message_OneofMessage struct {
+	OneofMessage *IrregularMessage `protobuf:"bytes,5,opt,name=oneof_message,json=oneofMessage,oneof"`
+}
+
+func (*Message_OneofMessage) isMessage_Union() {}
+
+var File_irregular_test_proto protoreflect.FileDescriptor
+
+var file_irregular_test_proto_rawDesc = []byte{
+	0x0a, 0x14, 0x69, 0x72, 0x72, 0x65, 0x67, 0x75, 0x6c, 0x61, 0x72, 0x2f, 0x74, 0x65, 0x73, 0x74,
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+	0x65, 0x79, 0x12, 0x3f, 0x0a, 0x05, 0x76, 0x61, 0x6c, 0x75, 0x65, 0x18, 0x02, 0x20, 0x01, 0x28,
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+	0x67, 0x75, 0x6c, 0x61, 0x72, 0x4d, 0x65, 0x73, 0x73, 0x61, 0x67, 0x65, 0x52, 0x05, 0x76, 0x61,
+	0x6c, 0x75, 0x65, 0x3a, 0x02, 0x38, 0x01, 0x42, 0x07, 0x0a, 0x05, 0x75, 0x6e, 0x69, 0x6f, 0x6e,
+	0x42, 0x3a, 0x5a, 0x38, 0x67, 0x6f, 0x6f, 0x67, 0x6c, 0x65, 0x2e, 0x67, 0x6f, 0x6c, 0x61, 0x6e,
+	0x67, 0x2e, 0x6f, 0x72, 0x67, 0x2f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x62, 0x75, 0x66, 0x2f, 0x69,
+	0x6e, 0x74, 0x65, 0x72, 0x6e, 0x61, 0x6c, 0x2f, 0x74, 0x65, 0x73, 0x74, 0x70, 0x72, 0x6f, 0x74,
+	0x6f, 0x73, 0x2f, 0x69, 0x72, 0x72, 0x65, 0x67, 0x75, 0x6c, 0x61, 0x72,
+}
+
+var (
+	file_irregular_test_proto_rawDescOnce sync.Once
+	file_irregular_test_proto_rawDescData = file_irregular_test_proto_rawDesc
+)
+
+func file_irregular_test_proto_rawDescGZIP() []byte {
+	file_irregular_test_proto_rawDescOnce.Do(func() {
+		file_irregular_test_proto_rawDescData = protoimpl.X.CompressGZIP(file_irregular_test_proto_rawDescData)
+	})
+	return file_irregular_test_proto_rawDescData
+}
+
+var file_irregular_test_proto_msgTypes = make([]protoimpl.MessageInfo, 2)
+var file_irregular_test_proto_goTypes = []interface{}{
+	(*Message)(nil),          // 0: goproto.proto.irregular.Message
+	nil,                      // 1: goproto.proto.irregular.Message.MapMessageEntry
+	(*IrregularMessage)(nil), // 2: goproto.proto.irregular.IrregularMessage
+}
+var file_irregular_test_proto_depIdxs = []int32{
+	2, // goproto.proto.irregular.Message.optional_message:type_name -> goproto.proto.irregular.IrregularMessage
+	2, // goproto.proto.irregular.Message.repeated_message:type_name -> goproto.proto.irregular.IrregularMessage
+	2, // goproto.proto.irregular.Message.required_message:type_name -> goproto.proto.irregular.IrregularMessage
+	1, // goproto.proto.irregular.Message.map_message:type_name -> goproto.proto.irregular.Message.MapMessageEntry
+	2, // goproto.proto.irregular.Message.oneof_message:type_name -> goproto.proto.irregular.IrregularMessage
+	2, // goproto.proto.irregular.Message.MapMessageEntry.value:type_name -> goproto.proto.irregular.IrregularMessage
+}
+
+func init() { file_irregular_test_proto_init() }
+func file_irregular_test_proto_init() {
+	if File_irregular_test_proto != nil {
+		return
+	}
+	file_irregular_irregular_proto_init()
+	File_irregular_test_proto = protoimpl.FileBuilder{
+		RawDescriptor:      file_irregular_test_proto_rawDesc,
+		GoTypes:            file_irregular_test_proto_goTypes,
+		DependencyIndexes:  file_irregular_test_proto_depIdxs,
+		MessageOutputTypes: file_irregular_test_proto_msgTypes,
+		FilesRegistry:      protoregistry.GlobalFiles,
+		TypesRegistry:      protoregistry.GlobalTypes,
+	}.Init()
+	file_irregular_test_proto_rawDesc = nil
+	file_irregular_test_proto_goTypes = nil
+	file_irregular_test_proto_depIdxs = nil
+}
diff --git a/internal/testprotos/irregular/test.proto b/internal/testprotos/irregular/test.proto
new file mode 100644
index 0000000..16ad47c
--- /dev/null
+++ b/internal/testprotos/irregular/test.proto
@@ -0,0 +1,25 @@
+// Copyright 2019 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.
+
+// This file contains a message which references a message that implements the
+// proto.Message interface but does not have the structure of a normal generated
+// message.
+
+syntax = "proto2";
+
+package goproto.proto.irregular;
+
+import "irregular/irregular.proto";
+
+option go_package = "google.golang.org/protobuf/internal/testprotos/irregular";
+
+message Message {
+  optional IrregularMessage optional_message = 1;
+  repeated IrregularMessage repeated_message = 2;
+  required IrregularMessage required_message = 3;
+  map<string,IrregularMessage> map_message = 4;
+  oneof union {
+    IrregularMessage oneof_message = 5;
+  }
+}
diff --git a/testing/prototest/prototest_test.go b/testing/prototest/prototest_test.go
index 82eb5b2..ec502c4 100644
--- a/testing/prototest/prototest_test.go
+++ b/testing/prototest/prototest_test.go
@@ -8,6 +8,7 @@
 	"fmt"
 	"testing"
 
+	irregularpb "google.golang.org/protobuf/internal/testprotos/irregular"
 	testpb "google.golang.org/protobuf/internal/testprotos/test"
 	test3pb "google.golang.org/protobuf/internal/testprotos/test3"
 	"google.golang.org/protobuf/proto"
@@ -20,6 +21,7 @@
 		(*test3pb.TestAllTypes)(nil),
 		(*testpb.TestRequired)(nil),
 		(*testpb.TestWeak)(nil),
+		(*irregularpb.Message)(nil),
 	} {
 		t.Run(fmt.Sprintf("%T", m), func(t *testing.T) {
 			prototest.TestMessage(t, m)