Check shouldEncodeElementDefault in InstantComponentSerializer

Author: dkhalanskyjb

core/commonMain/src/kotlinx/serialization/builtins/InstantComponentSerializer.kt

@@ -52,7 +52,7 @@ public object InstantComponentSerializer : KSerializer<Instant> {
     override fun serialize(encoder: Encoder, value: Instant) {
         encoder.encodeStructure(descriptor) {
             encodeLongElement(descriptor, 0, value.epochSeconds)
-            if (value.nanosecondsOfSecond != 0) {
+            if (value.nanosecondsOfSecond != 0 || shouldEncodeElementDefault(descriptor, 1)) {
                 encodeIntElement(descriptor, 1, value.nanosecondsOfSecond)
             }
         }

formats/json-tests/commonTest/src/kotlinx/serialization/InstantSerializationTest.kt

@@ -45,14 +45,21 @@ class InstantSerializationTest: JsonTestBase() {
             Pair(Instant.fromEpochSeconds(987654321, 123456789),
                 "{\"epochSeconds\":987654321,\"nanosecondsOfSecond\":123456789}"),
             Pair(Instant.fromEpochSeconds(987654321, 0),
-                "{\"epochSeconds\":987654321}"),
+                "{\"epochSeconds\":987654321,\"nanosecondsOfSecond\":0}"),
         )) {
             assertJsonFormAndRestored(serializer, instant, json)
         }
-        // check that having a `"nanosecondsOfSecond": 0` field doesn't break deserialization
+        // by default, `nanosecondsOfSecond` is optional
+        assertJsonFormAndRestored(serializer, Instant.fromEpochSeconds(987654321, 0),
+            "{\"epochSeconds\":987654321}", Json { })
+        // having a `"nanosecondsOfSecond": 0` field doesn't break deserialization
         assertEquals(Instant.fromEpochSeconds(987654321, 0),
             Json.decodeFromString(serializer,
                 "{\"epochSeconds\":987654321,\"nanosecondsOfSecond\":0}"))
+        // as does not having a `"nanosecondsOfSecond"` field if `encodeDefaults` is true
+        assertEquals(Instant.fromEpochSeconds(987654321, 0),
+            default.decodeFromString(serializer,
+                "{\"epochSeconds\":987654321}"))
         // "epochSeconds" should always be present
         assertFailsWith<SerializationException> { Json.decodeFromString(serializer, "{}") }
         assertFailsWith<SerializationException> { Json.decodeFromString(serializer, "{\"nanosecondsOfSecond\":3}") }