Replace @Optional with @Required

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/resolve/KSerializationUtil.kt

@@ -85,8 +85,8 @@ val Annotations.serialNameValue: String?
         return value
     }
 
-val Annotations.serialOptional: Boolean
-    get() = hasAnnotation(SerializationAnnotations.serialOptionalFqName)
+val Annotations.serialRequired: Boolean
+    get() = hasAnnotation(SerializationAnnotations.requiredAnnotationFqName)
 
 val Annotations.serialTransient: Boolean
     get() = hasAnnotation(SerializationAnnotations.serialTransientFqName)

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/resolve/NamingConventions.kt

@@ -17,7 +17,7 @@ object SerializationAnnotations {
     val serializableAnnotationFqName = FqName("kotlinx.serialization.Serializable")
     val serializerAnnotationFqName = FqName("kotlinx.serialization.Serializer")
     internal val serialNameAnnotationFqName = FqName("kotlinx.serialization.SerialName")
-    internal val serialOptionalFqName = FqName("kotlinx.serialization.Optional")
+    internal val requiredAnnotationFqName = FqName("kotlinx.serialization.Required")
     internal val serialTransientFqName = FqName("kotlinx.serialization.Transient")
     internal val serialInfoFqName = FqName("kotlinx.serialization.SerialInfo")
 

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/resolve/SerializableProperty.kt

@@ -37,7 +37,7 @@ class SerializableProperty(
     val genericIndex = type.genericIndex
     val module = descriptor.module
     val serializableWith = extractSerializableWith(descriptor.annotations)
-    val optional = descriptor.declaresDefaultValue
+    val optional = !descriptor.annotations.serialRequired && descriptor.declaresDefaultValue
     val transient = descriptor.annotations.serialTransient || !hasBackingField
     val annotationsWithArguments: List<Triple<ClassDescriptor, List<ValueArgument>, List<ValueParameterDescriptor>>> =
         descriptor.annotationsWithArguments()