Do not try to coerce input values for properties (#2530)

Do not try to coerce input values for properties that do not have default values.

Trying so leads to confusing errors about missing values despite a json key actually
present in the input.

Fixes #2529

9 files changed, 41 insertions, 8 deletions

diff --git a/docs/basic-serialization.md b/docs/basic-serialization.md
index 3853376e..96e70981 100644
--- a/docs/basic-serialization.md
+++ b/docs/basic-serialization.md
@@ -534,7 +534,7 @@ the `null` value to it.
 
 ```text
 Exception in thread "main" kotlinx.serialization.json.internal.JsonDecodingException: Unexpected JSON token at offset 52: Expected string literal but 'null' literal was found at path: $.language
-Use 'coerceInputValues = true' in 'Json {}' builder to coerce nulls to default values.
+Use 'coerceInputValues = true' in 'Json {}' builder to coerce nulls if property has a default value.
 ```
 
 <!--- TEST LINES_START -->
diff --git a/formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonCoerceInputValuesTest.kt b/formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonCoerceInputValuesTest.kt
index ecb946cb..3d7c3322 100644
--- a/formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonCoerceInputValuesTest.kt
+++ b/formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonCoerceInputValuesTest.kt
@@ -29,6 +29,16 @@ class JsonCoerceInputValuesTest : JsonTestBase() {
         val enum: SampleEnum?
     )
 
+    @Serializable
+    class Uncoercable(
+        val s: String
+    )
+
+    @Serializable
+    class UncoercableEnum(
+        val e: SampleEnum
+    )
+
     val json = Json {
         coerceInputValues = true
         isLenient = true
@@ -112,4 +122,24 @@ class JsonCoerceInputValuesTest : JsonTestBase() {
         decoded = decodeFromString<NullableEnumHolder>("""{"enum": OptionA}""")
         assertEquals(SampleEnum.OptionA, decoded.enum)
     }
+
+    @Test
+    fun propertiesWithoutDefaultValuesDoNotChangeErrorMsg() {
+        val json2 = Json(json) { coerceInputValues = false }
+        parametrizedTest { mode ->
+            val e1 = assertFailsWith<SerializationException>() { json.decodeFromString<Uncoercable>("""{"s":null}""", mode) }
+            val e2 = assertFailsWith<SerializationException>() { json2.decodeFromString<Uncoercable>("""{"s":null}""", mode) }
+            assertEquals(e2.message, e1.message)
+        }
+    }
+
+    @Test
+    fun propertiesWithoutDefaultValuesDoNotChangeErrorMsgEnum() {
+        val json2 = Json(json) { coerceInputValues = false }
+        parametrizedTest { mode ->
+            val e1 = assertFailsWith<SerializationException> { json.decodeFromString<UncoercableEnum>("""{"e":"UNEXPECTED"}""", mode) }
+            val e2 = assertFailsWith<SerializationException> { json2.decodeFromString<UncoercableEnum>("""{"e":"UNEXPECTED"}""", mode) }
+            assertEquals(e2.message, e1.message)
+        }
+    }
 }
diff --git a/formats/json/commonMain/src/kotlinx/serialization/json/Json.kt b/formats/json/commonMain/src/kotlinx/serialization/json/Json.kt
index 26c376ef..a510e8a3 100644
--- a/formats/json/commonMain/src/kotlinx/serialization/json/Json.kt
+++ b/formats/json/commonMain/src/kotlinx/serialization/json/Json.kt
@@ -287,7 +287,7 @@ public class JsonBuilder internal constructor(json: Json) {
     public var prettyPrintIndent: String = json.configuration.prettyPrintIndent
 
     /**
-     * Enables coercing incorrect JSON values to the default property value in the following cases:
+     * Enables coercing incorrect JSON values to the default property value (if exists) in the following cases:
      *   1. JSON value is `null` but the property type is non-nullable.
      *   2. Property type is an enum type, but JSON value contains unknown enum member.
      *
diff --git a/formats/json/commonMain/src/kotlinx/serialization/json/internal/JsonNamesMap.kt b/formats/json/commonMain/src/kotlinx/serialization/json/internal/JsonNamesMap.kt
index 8acd8fc4..9128f3a2 100644
--- a/formats/json/commonMain/src/kotlinx/serialization/json/internal/JsonNamesMap.kt
+++ b/formats/json/commonMain/src/kotlinx/serialization/json/internal/JsonNamesMap.kt
@@ -110,11 +110,14 @@ internal fun SerialDescriptor.getJsonNameIndexOrThrow(json: Json, name: String,
 
 @OptIn(ExperimentalSerializationApi::class)
 internal inline fun Json.tryCoerceValue(
-    elementDescriptor: SerialDescriptor,
+    descriptor: SerialDescriptor,
+    index: Int,
     peekNull: (consume: Boolean) -> Boolean,
     peekString: () -> String?,
     onEnumCoercing: () -> Unit = {}
 ): Boolean {
+    if (!descriptor.isElementOptional(index)) return false
+    val elementDescriptor = descriptor.getElementDescriptor(index)
     if (!elementDescriptor.isNullable && peekNull(true)) return true
     if (elementDescriptor.kind == SerialKind.ENUM) {
         if (elementDescriptor.isNullable && peekNull(false)) {
diff --git a/formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonDecoder.kt b/formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonDecoder.kt
index 0018fce1..caa1f4a5 100644
--- a/formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonDecoder.kt
+++ b/formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonDecoder.kt
@@ -213,7 +213,7 @@ internal open class StreamingJsonDecoder(
      * Checks whether JSON has `null` value for non-null property or unknown enum value for enum property
      */
     private fun coerceInputValue(descriptor: SerialDescriptor, index: Int): Boolean = json.tryCoerceValue(
-        descriptor.getElementDescriptor(index),
+        descriptor, index,
         { lexer.tryConsumeNull(it) },
         { lexer.peekString(configuration.isLenient) },
         { lexer.consumeString() /* skip unknown enum string*/ }
diff --git a/formats/json/commonMain/src/kotlinx/serialization/json/internal/TreeJsonDecoder.kt b/formats/json/commonMain/src/kotlinx/serialization/json/internal/TreeJsonDecoder.kt
index aedfb95f..690b35e1 100644
--- a/formats/json/commonMain/src/kotlinx/serialization/json/internal/TreeJsonDecoder.kt
+++ b/formats/json/commonMain/src/kotlinx/serialization/json/internal/TreeJsonDecoder.kt
@@ -190,7 +190,7 @@ private open class JsonTreeDecoder(
      */
     private fun coerceInputValue(descriptor: SerialDescriptor, index: Int, tag: String): Boolean =
         json.tryCoerceValue(
-            descriptor.getElementDescriptor(index),
+            descriptor, index,
             { currentElement(tag) is JsonNull },
             { (currentElement(tag) as? JsonPrimitive)?.contentOrNull }
         )
diff --git a/formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/AbstractJsonLexer.kt b/formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/AbstractJsonLexer.kt
index c83bdef9..f90ee1a0 100644
--- a/formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/AbstractJsonLexer.kt
+++ b/formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/AbstractJsonLexer.kt
@@ -11,7 +11,7 @@ import kotlin.jvm.*
 import kotlin.math.*
 
 internal const val lenientHint = "Use 'isLenient = true' in 'Json {}' builder to accept non-compliant JSON."
-internal const val coerceInputValuesHint = "Use 'coerceInputValues = true' in 'Json {}' builder to coerce nulls to default values."
+internal const val coerceInputValuesHint = "Use 'coerceInputValues = true' in 'Json {}' builder to coerce nulls if property has a default value."
 internal const val specialFlowingValuesHint =
     "It is possible to deserialize them using 'JsonBuilder.allowSpecialFloatingPointValues = true'"
 internal const val ignoreUnknownKeysHint = "Use 'ignoreUnknownKeys = true' in 'Json {}' builder to ignore unknown keys."
diff --git a/formats/json/jsMain/src/kotlinx/serialization/json/internal/DynamicDecoders.kt b/formats/json/jsMain/src/kotlinx/serialization/json/internal/DynamicDecoders.kt
index 86c7a85c..1ff1e40d 100644
--- a/formats/json/jsMain/src/kotlinx/serialization/json/internal/DynamicDecoders.kt
+++ b/formats/json/jsMain/src/kotlinx/serialization/json/internal/DynamicDecoders.kt
@@ -73,7 +73,7 @@ private open class DynamicInput(
 
     private fun coerceInputValue(descriptor: SerialDescriptor, index: Int, tag: String): Boolean =
         json.tryCoerceValue(
-            descriptor.getElementDescriptor(index),
+            descriptor, index,
             { getByTag(tag) == null },
             { getByTag(tag) as? String }
         )
diff --git a/guide/test/BasicSerializationTest.kt b/guide/test/BasicSerializationTest.kt
index dc89feb6..11f9e9f2 100644
--- a/guide/test/BasicSerializationTest.kt
+++ b/guide/test/BasicSerializationTest.kt
@@ -110,7 +110,7 @@ class BasicSerializationTest {
     fun testExampleClasses12() {
         captureOutput("ExampleClasses12") { example.exampleClasses12.main() }.verifyOutputLinesStart(
             "Exception in thread \"main\" kotlinx.serialization.json.internal.JsonDecodingException: Unexpected JSON token at offset 52: Expected string literal but 'null' literal was found at path: $.language",
-            "Use 'coerceInputValues = true' in 'Json {}' builder to coerce nulls to default values."
+            "Use 'coerceInputValues = true' in 'Json {}' builder to coerce nulls if property has a default value."
         )
     }