Address the review

Author: dkhalanskyjb

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

@@ -27,23 +27,28 @@ public object InstantComponentSerializer : KSerializer<Instant> {
     @OptIn(ExperimentalSerializationApi::class)
     override fun deserialize(decoder: Decoder): Instant =
         decoder.decodeStructure(descriptor) {
-            var epochSeconds: Long? = null
+            var epochSecondsNotSeen = true
+            var epochSeconds: Long = 0
             var nanosecondsOfSecond = 0
             while (true) {
                 when (val index = decodeElementIndex(descriptor)) {
-                    0 -> epochSeconds = decodeLongElement(descriptor, 0)
+                    0 -> {
+                        epochSecondsNotSeen = false
+                        epochSeconds = decodeLongElement(descriptor, 0)
+                    }
                     1 -> nanosecondsOfSecond = decodeIntElement(descriptor, 1)
                     CompositeDecoder.DECODE_DONE -> break
                     else -> throw SerializationException("Unexpected index: $index")
                 }
             }
-            if (epochSeconds == null) throw MissingFieldException(
+            if (epochSecondsNotSeen) throw MissingFieldException(
                 missingField = "epochSeconds",
                 serialName = descriptor.serialName
             )
             Instant.fromEpochSeconds(epochSeconds, nanosecondsOfSecond)
         }
 
+    @OptIn(ExperimentalSerializationApi::class)
     override fun serialize(encoder: Encoder, value: Instant) {
         encoder.encodeStructure(descriptor) {
             encodeLongElement(descriptor, 0, value.epochSeconds)

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

@@ -26,6 +26,14 @@ class InstantSerializationTest: JsonTestBase() {
         )) {
             assertJsonFormAndRestored(serializer, instant, json)
         }
+        for ((instant, json) in listOf(
+            Pair(Instant.fromEpochSeconds(987654321, 123456789),
+                "\"2001-04-19T07:55:21.123456789+03:30\""),
+            Pair(Instant.fromEpochSeconds(987654321, 123456789),
+                "\"2001-04-19T00:55:21.123456789-03:30\""),
+        )) {
+            assertRestoredFromJsonForm(serializer, json, instant)
+        }
     }
 
     private fun componentSerialization(serializer: KSerializer<Instant>) {

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

@@ -190,4 +190,15 @@ abstract class JsonTestBase {
             assertTrue("Failed with streaming = $jsonTestingMode\n\tsource value =$data\n\tdeserialized value=$deserialized") { check(data, deserialized) }
         }
     }
+
+    internal fun <T> assertRestoredFromJsonForm(
+        serializer: KSerializer<T>,
+        jsonForm: String,
+        expected: T,
+    ) {
+        parametrizedTest { jsonTestingMode ->
+            val deserialized: T = Json.decodeFromString(serializer, jsonForm, jsonTestingMode)
+            assertEquals(expected, deserialized, "Failed with streaming = $jsonTestingMode")
+        }
+    }
 }