tlv: modify RecordT type to ensure type param takes precedence

In this commit, we modify the RecordT type to allow callers to re-use
the Record definition of a different type, but use the new type param to
override the integer type used on the wire.

This will let use do things like encode a signature using the same
RecordProducer instance, but with a diff type in another context.

The upcoming use for this is allowing our `lnwire.Sig` type to be
encoded in the same message using distinct TLV integer types (new co-op
close protocol).

tlv/record_type.go

@@ -66,7 +66,18 @@ func (t *RecordT[T, V]) Record() Record {
 		)
 	}
 
-	return tlvRecord.Record()
+	// To enforce proper usage of the RecordT type, we'll make a wrapper
+	// record that uses the proper internal type value.
+	ogRecord := tlvRecord.Record()
+
+	return Record{
+		value:      ogRecord.value,
+		typ:        t.recordType.typeVal(),
+		staticSize: ogRecord.staticSize,
+		sizeFunc:   ogRecord.sizeFunc,
+		encoder:    ogRecord.encoder,
+		decoder:    ogRecord.decoder,
+	}
 }
 
 // OptionalRecordT is a high-order type that represents an optional TLV record.

tlv/record_type_test.go

@@ -63,6 +63,10 @@ type coolWireMsg struct {
 	CsvDelay RecordT[TlvType1, wireCsv]
 }
 
+type coolWireMsgDiffContext struct {
+	CsvDelay RecordT[TlvType3, wireCsv]
+}
+
 // TestRecordTFromRecord tests that we can create a RecordT type from an
 // existing record type and encode/decode as normal.
 func TestRecordTFromRecord(t *testing.T) {
@@ -91,3 +95,24 @@ func TestRecordTFromRecord(t *testing.T) {
 
 	require.Equal(t, wireMsg, wireMsg2)
 }
+
+// TestRecordTFromRecordTypeOverride tests that we can create a RecordT type
+// from an existing record type and encode/decode as normal. In this variant,
+// we make sure that we can use the type system to override the type of an
+// original record.
+func TestRecordTFromRecordTypeOverride(t *testing.T) {
+	t.Parallel()
+
+	// First, we'll make a new wire message. Instead of using the TLV type
+	// of 1 (hard coded in the Record() method defined above), we'll
+	// instead use TLvType3, as we want to use the same encode/decode, but
+	// in a context with a different integer type.
+	val := wireCsv(5)
+	wireMsg := coolWireMsgDiffContext{
+		CsvDelay: NewRecordT[TlvType3](val),
+	}
+
+	// If we extract the record, we should see that the type is now 3.
+	tlvRecord := wireMsg.CsvDelay.Record()
+	require.Equal(t, tlvRecord.Type(), Type(3))
+}