test: add PSBT proprietary merge regression coverage

diff --git a/src/test/psbt_tests.cpp b/src/test/psbt_tests.cpp
index 74690d32..c8e1206e 100644
--- a/src/test/psbt_tests.cpp
+++ b/src/test/psbt_tests.cpp
@@ -9,6 +9,16 @@
 
 BOOST_FIXTURE_TEST_SUITE(psbt_tests, BasicTestingSetup)
 
+static PSBTProprietary MakeProprietary(uint64_t subtype, uint8_t key_data, uint8_t value)
+{
+    return PSBTProprietary{
+        .subtype = subtype,
+        .identifier = {'p', 's', 'b', 't'},
+        .key = {key_data},
+        .value = {value},
+    };
+}
+
 void CheckTimeLock(const std::string& base64_psbt, std::optional<uint32_t> timelock)
 {
     util::Result<PartiallySignedTransaction> psbt = DecodeBase64PSBT(base64_psbt);
@@ -167,4 +177,43 @@ BOOST_AUTO_TEST_CASE(psbt2_addoutput)
     BOOST_CHECK_EQUAL(psbt.outputs.size(), 2);
 }
 
+BOOST_AUTO_TEST_CASE(merge_proprietary_fields)
+{
+    CMutableTransaction tx;
+    tx.vin.emplace_back(COutPoint{});
+    tx.vout.emplace_back(0, CScript{});
+
+    PartiallySignedTransaction left(tx);
+    PartiallySignedTransaction right(tx);
+
+    const auto left_prop = MakeProprietary(/*subtype=*/1, /*key_data=*/0x01, /*value=*/0xaa);
+    const auto right_prop = MakeProprietary(/*subtype=*/2, /*key_data=*/0x02, /*value=*/0xbb);
+
+    left.m_proprietary.insert(left_prop);
+    left.inputs[0].m_proprietary.insert(left_prop);
+    left.outputs[0].m_proprietary.insert(left_prop);
+
+    right.m_proprietary.insert(right_prop);
+    right.inputs[0].m_proprietary.insert(right_prop);
+    right.outputs[0].m_proprietary.insert(right_prop);
+
+    BOOST_REQUIRE(left.Merge(right));
+
+    BOOST_REQUIRE_EQUAL(left.m_proprietary.size(), 2U);
+    BOOST_REQUIRE_EQUAL(left.inputs[0].m_proprietary.size(), 2U);
+    BOOST_REQUIRE_EQUAL(left.outputs[0].m_proprietary.size(), 2U);
+
+    const auto global_it = left.m_proprietary.find(right_prop);
+    BOOST_REQUIRE(global_it != left.m_proprietary.end());
+    BOOST_CHECK(global_it->value == right_prop.value);
+
+    const auto input_it = left.inputs[0].m_proprietary.find(right_prop);
+    BOOST_REQUIRE(input_it != left.inputs[0].m_proprietary.end());
+    BOOST_CHECK(input_it->value == right_prop.value);
+
+    const auto output_it = left.outputs[0].m_proprietary.find(right_prop);
+    BOOST_REQUIRE(output_it != left.outputs[0].m_proprietary.end());
+    BOOST_CHECK(output_it->value == right_prop.value);
+}
+
 BOOST_AUTO_TEST_SUITE_END()
diff --git a/test/functional/rpc_psbt.py b/test/functional/rpc_psbt.py
index 4703ee3b..9aad09a0 100755
--- a/test/functional/rpc_psbt.py
+++ b/test/functional/rpc_psbt.py
@@ -20,10 +20,12 @@ from test_framework.messages import (
     CTxOut,
     MAX_BIP125_RBF_SEQUENCE,
     WITNESS_SCALE_FACTOR,
+    ser_compact_size,
 )
 from test_framework.psbt import (
     PSBT,
     PSBTMap,
+    PSBT_GLOBAL_PROPRIETARY,
     PSBT_GLOBAL_UNSIGNED_TX,
     PSBT_GLOBAL_VERSION,
     PSBT_IN_RIPEMD160,
@@ -35,8 +37,10 @@ from test_framework.psbt import (
     PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS,
     PSBT_IN_MUSIG2_PUB_NONCE,
     PSBT_IN_NON_WITNESS_UTXO,
+    PSBT_IN_PROPRIETARY,
     PSBT_IN_WITNESS_UTXO,
     PSBT_OUT_MUSIG2_PARTICIPANT_PUBKEYS,
+    PSBT_OUT_PROPRIETARY,
     PSBT_OUT_TAP_TREE,
     PSBT_OUT_SCRIPT,
 )
@@ -294,6 +298,65 @@ class PSBTTest(BitcoinTestFramework):
         assert "participant_pubkeys" in out_participant_pks
         assert_equal(out_participant_pks["participant_pubkeys"], [out_pubkey1.hex(), out_pubkey2.hex()])
 
+    def test_combinepsbt_preserves_proprietary_fields(self):
+        self.log.info("Test that combining PSBTs preserves proprietary fields")
+
+        def proprietary_key(type_byte, identifier, subtype, key_data=b""):
+            return bytes([type_byte]) + ser_compact_size(len(identifier)) + identifier + ser_compact_size(subtype) + key_data
+
+        def proprietary_entry(key, value, identifier, subtype):
+            return {"identifier": identifier.hex(), "subtype": subtype, "key": key.hex(), "value": value.hex()}
+
+        tx = CTransaction()
+        tx.vin = [CTxIn(outpoint=COutPoint(hash=int('aa' * 32, 16), n=0), scriptSig=b"")]
+        tx.vout = [CTxOut(nValue=0, scriptPubKey=b"")]
+
+        global_key_a = proprietary_key(type_byte=PSBT_GLOBAL_PROPRIETARY, identifier=b"gc", subtype=1, key_data=b"\x01")
+        global_key_b = proprietary_key(type_byte=PSBT_GLOBAL_PROPRIETARY, identifier=b"gc", subtype=2, key_data=b"\x02")
+        input_key_a = proprietary_key(type_byte=PSBT_IN_PROPRIETARY, identifier=b"in", subtype=3, key_data=b"\x03")
+        input_key_b = proprietary_key(type_byte=PSBT_IN_PROPRIETARY, identifier=b"in", subtype=4, key_data=b"\x04")
+        output_key_a = proprietary_key(type_byte=PSBT_OUT_PROPRIETARY, identifier=b"out", subtype=5, key_data=b"\x05")
+        output_key_b = proprietary_key(type_byte=PSBT_OUT_PROPRIETARY, identifier=b"out", subtype=6, key_data=b"\x06")
+
+        psbt1 = PSBT(
+            g=PSBTMap({
+                PSBT_GLOBAL_UNSIGNED_TX: tx.serialize(),
+                global_key_a: b"\xaa",
+            }),
+            i=[PSBTMap({
+                input_key_a: b"\xbb",
+            })],
+            o=[PSBTMap({
+                output_key_a: b"\xcc",
+            })],
+        ).to_base64()
+        psbt2 = PSBT(
+            g=PSBTMap({
+                PSBT_GLOBAL_UNSIGNED_TX: tx.serialize(),
+                global_key_b: b"\xdd",
+            }),
+            i=[PSBTMap({
+                input_key_b: b"\xee",
+            })],
+            o=[PSBTMap({
+                output_key_b: b"\xff",
+            })],
+        ).to_base64()
+
+        decoded = self.nodes[0].decodepsbt(self.nodes[0].combinepsbt([psbt1, psbt2]))
+        assert_equal(decoded["proprietary"], [
+            proprietary_entry(key=global_key_a, value=b"\xaa", identifier=b"gc", subtype=1),
+            proprietary_entry(key=global_key_b, value=b"\xdd", identifier=b"gc", subtype=2),
+        ])
+        assert_equal(decoded["inputs"][0]["proprietary"], [
+            proprietary_entry(key=input_key_a, value=b"\xbb", identifier=b"in", subtype=3),
+            proprietary_entry(key=input_key_b, value=b"\xee", identifier=b"in", subtype=4),
+        ])
+        assert_equal(decoded["outputs"][0]["proprietary"], [
+            proprietary_entry(key=output_key_a, value=b"\xcc", identifier=b"out", subtype=5),
+            proprietary_entry(key=output_key_b, value=b"\xff", identifier=b"out", subtype=6),
+        ])
+
     def test_sighash_mismatch(self):
         self.log.info("Test sighash type mismatches")
         self.nodes[0].createwallet("sighash_mismatch")
@@ -1281,6 +1344,8 @@ class PSBTTest(BitcoinTestFramework):
 
         self.test_decodepsbt_musig2_input_output_types()
 
+        self.test_combinepsbt_preserves_proprietary_fields()
+
         self.log.info("Test that combining PSBTs with different transactions fails")
         tx = CTransaction()
         tx.vin = [CTxIn(outpoint=COutPoint(hash=int('aa' * 32, 16), n=0), scriptSig=b"")]