[Possible duplicate] Small inconsistency in the logic of encoding of fields with defaults

[orchestr7]

(!) I am aware of the encoding of defaults and understand that they are not encoded by default: Encoding Defaults Documentation.

Please close this issue as a duplicate with a reference to the original one, if it exists.

Consider the following scenario where I have a default value for my field b:

@Serializable
data class A(
    val a: String?,
    val b: String = "b"
)

If I pass a different value explicitly:

val value = A("a", "b1")
Json.encodeToString(value)

The encoded output is as expected:

{ "a": "a", "b": "b1" }

However, if I pass the default value explicitly:

val value = A("a", "b")
Json.encodeToString(value)

The encoded output is not what you might expect:

{ "a": "a" }

My team encountered a production error because they did not check if the default compile-time value was equal to the non-default runtime value, and they expected this field to be serialized.

The motivation behind this behavior is clear: it is challenging to distinguish between the default value and an explicitly set value at the frontend level. However, it would be beneficial to mention this explicitly in the README.

[sandwwraith]

There's no way to distinguish between these two "b"s. In the same way, you can't distinguish between these calls in Kotlin:

fun foo(b: String = "b") {
// you can't tell if "b" is a default value or not
}


foo()
foo("b") // identical call

Actually, the very purpose of default values is to be indistinguishable from user-passed ones. Consider using nulls or wrappers if you need a special differentiation.