From e746df3caa1e7c3ff21d487742ca86c5ac6d9a99 Mon Sep 17 00:00:00 2001
From: MPins <pinsdorf@proton.me>
Date: Mon, 8 Sep 2025 16:24:33 -0700
Subject: [PATCH] zpay32: add support for P2TR fallback addresses

---
 zpay32/decode.go | 23 ++++++++++++++++++++---
 zpay32/encode.go |  6 ++++--
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/zpay32/decode.go b/zpay32/decode.go
index f97c77838..577f6a6d1 100644
--- a/zpay32/decode.go
+++ b/zpay32/decode.go
@@ -15,10 +15,18 @@ import (
 	"github.com/btcsuite/btcd/btcutil/bech32"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/btcsuite/btcd/txscript"
 	"github.com/lightningnetwork/lnd/fn/v2"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
+const (
+	fallbackVersionWitness    = txscript.BaseSegwitWitnessVersion
+	fallbackVersionTaproot    = txscript.TaprootWitnessVersion
+	fallbackVersionPubkeyHash = 17
+	fallbackVersionScriptHash = 18
+)
+
 var (
 	// ErrInvalidUTF8Description is returned if the invoice description is
 	// not valid UTF-8.
@@ -529,7 +537,7 @@ func parseFallbackAddr(data []byte, net *chaincfg.Params) (btcutil.Address, erro
 
 	version := data[0]
 	switch version {
-	case 0:
+	case fallbackVersionWitness:
 		witness, err := bech32.ConvertBits(data[1:], 5, 8, false)
 		if err != nil {
 			return nil, err
@@ -548,7 +556,16 @@ func parseFallbackAddr(data []byte, net *chaincfg.Params) (btcutil.Address, erro
 		if err != nil {
 			return nil, err
 		}
-	case 17:
+	case fallbackVersionTaproot:
+		witness, err := bech32.ConvertBits(data[1:], 5, 8, false)
+		if err != nil {
+			return nil, err
+		}
+		addr, err = btcutil.NewAddressTaproot(witness, net)
+		if err != nil {
+			return nil, err
+		}
+	case fallbackVersionPubkeyHash:
 		pubKeyHash, err := bech32.ConvertBits(data[1:], 5, 8, false)
 		if err != nil {
 			return nil, err
@@ -558,7 +575,7 @@ func parseFallbackAddr(data []byte, net *chaincfg.Params) (btcutil.Address, erro
 		if err != nil {
 			return nil, err
 		}
-	case 18:
+	case fallbackVersionScriptHash:
 		scriptHash, err := bech32.ConvertBits(data[1:], 5, 8, false)
 		if err != nil {
 			return nil, err
diff --git a/zpay32/encode.go b/zpay32/encode.go
index 43ccd5ecb..50f294e51 100644
--- a/zpay32/encode.go
+++ b/zpay32/encode.go
@@ -202,13 +202,15 @@ func writeTaggedFields(bufferBase32 *bytes.Buffer, invoice *Invoice) error {
 		var version byte
 		switch addr := invoice.FallbackAddr.(type) {
 		case *btcutil.AddressPubKeyHash:
-			version = 17
+			version = fallbackVersionPubkeyHash
 		case *btcutil.AddressScriptHash:
-			version = 18
+			version = fallbackVersionScriptHash
 		case *btcutil.AddressWitnessPubKeyHash:
 			version = addr.WitnessVersion()
 		case *btcutil.AddressWitnessScriptHash:
 			version = addr.WitnessVersion()
+		case *btcutil.AddressTaproot:
+			version = addr.WitnessVersion()
 		default:
 			return fmt.Errorf("unknown fallback address type")
 		}