From be64e92530e52a862671e6e443767032e736ba05 Mon Sep 17 00:00:00 2001
From: Ignacio Van Droogenbroeck <ignacio@vandroogenbroeck.net>
Date: Mon, 2 Mar 2026 15:02:49 -0600
Subject: [PATCH] fix(decode): choose TextUnmarshaler over BinaryUnmarshaler
 for str format (#10)

When a type implements both interfaces, the decoder now peeks at the
wire format and dispatches to TextUnmarshaler for str codes and
BinaryUnmarshaler for bin codes.
---
 decode_value.go | 22 ++++++++++++++++++++++
 types_test.go   | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/decode_value.go b/decode_value.go
index 445532c..47c81fd 100644
--- a/decode_value.go
+++ b/decode_value.go
@@ -5,6 +5,8 @@ import (
 	"errors"
 	"fmt"
 	"reflect"
+
+	"github.com/vmihailenco/msgpack/v5/msgpcode"
 )
 
 var (
@@ -71,6 +73,9 @@ func _getDecoder(typ reflect.Type) decoderFunc {
 		return nilAwareDecoder(typ, unmarshalValue)
 	}
 	if typ.Implements(binaryUnmarshalerType) {
+		if typ.Implements(textUnmarshalerType) {
+			return nilAwareDecoder(typ, unmarshalBinaryOrTextValue)
+		}
 		return nilAwareDecoder(typ, unmarshalBinaryValue)
 	}
 	if typ.Implements(textUnmarshalerType) {
@@ -87,6 +92,9 @@ func _getDecoder(typ reflect.Type) decoderFunc {
 			return addrDecoder(nilAwareDecoder(typ, unmarshalValue))
 		}
 		if ptr.Implements(binaryUnmarshalerType) {
+			if ptr.Implements(textUnmarshalerType) {
+				return addrDecoder(nilAwareDecoder(typ, unmarshalBinaryOrTextValue))
+			}
 			return addrDecoder(nilAwareDecoder(typ, unmarshalBinaryValue))
 		}
 		if ptr.Implements(textUnmarshalerType) {
@@ -238,6 +246,20 @@ func unmarshalValue(d *Decoder, v reflect.Value) error {
 	return unmarshaler.UnmarshalMsgpack(b)
 }
 
+// unmarshalBinaryOrTextValue peeks at the wire format to choose between
+// BinaryUnmarshaler (bin) and TextUnmarshaler (str). Used when a type
+// implements both interfaces.
+func unmarshalBinaryOrTextValue(d *Decoder, v reflect.Value) error {
+	c, err := d.PeekCode()
+	if err != nil {
+		return err
+	}
+	if msgpcode.IsString(c) {
+		return unmarshalTextValue(d, v)
+	}
+	return unmarshalBinaryValue(d, v)
+}
+
 func unmarshalBinaryValue(d *Decoder, v reflect.Value) error {
 	data, err := d.DecodeBytes()
 	if err != nil {
diff --git a/types_test.go b/types_test.go
index 46a051c..33d001a 100644
--- a/types_test.go
+++ b/types_test.go
@@ -1294,6 +1294,40 @@ func TestPoolReleasesOversizedBuffers(t *testing.T) {
 	require.Equal(t, "small", s)
 }
 
+// Issue #10: types implementing both TextUnmarshaler and BinaryUnmarshaler
+// should use TextUnmarshaler when wire format is str.
+type dualUnmarshaler struct {
+	text   string
+	binary []byte
+}
+
+func (d *dualUnmarshaler) MarshalText() ([]byte, error)           { return []byte(d.text), nil }
+func (d *dualUnmarshaler) UnmarshalText(b []byte) error           { d.text = string(b); return nil }
+func (d *dualUnmarshaler) MarshalBinary() ([]byte, error)         { return d.binary, nil }
+func (d *dualUnmarshaler) UnmarshalBinary(b []byte) error         { d.binary = b; return nil }
+
+func TestTextUnmarshalerWithStrFormat(t *testing.T) {
+	// Encode a plain string (str format) and decode into a type that
+	// implements both BinaryUnmarshaler and TextUnmarshaler.
+	b, err := msgpack.Marshal("hello")
+	require.NoError(t, err)
+
+	var du dualUnmarshaler
+	require.NoError(t, msgpack.Unmarshal(b, &du))
+	// Should have used TextUnmarshaler because the wire format is str.
+	require.Equal(t, "hello", du.text)
+	require.Nil(t, du.binary)
+
+	// Encode as binary (bin format) and decode — should use BinaryUnmarshaler.
+	b, err = msgpack.Marshal([]byte{0xDE, 0xAD})
+	require.NoError(t, err)
+
+	var du2 dualUnmarshaler
+	require.NoError(t, msgpack.Unmarshal(b, &du2))
+	require.Equal(t, []byte{0xDE, 0xAD}, du2.binary)
+	require.Equal(t, "", du2.text)
+}
+
 func mustParseTime(format, s string) time.Time {
 	tm, err := time.Parse(format, s)
 	if err != nil {