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

1 files changed, 30 insertions, 0 deletions

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)
+        }
+    }
 }