Coerce property value from correct type after retrieving it

#KT-25497 fixed
 Fixes https://github.com/Kotlin/kotlinx.serialization/issues/434

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/jvm/JVMCodegenUtil.kt

@@ -102,15 +102,15 @@ fun InstructionAdapter.genKOutputMethodCall(
     val sti = generator.getSerialTypeInfo(property, propertyType)
     val useSerializer = if (fromClassStartVar == null) stackValueSerializerInstanceFromSerializer(classCodegen, sti, generator)
     else stackValueSerializerInstanceFromClass(classCodegen, sti, fromClassStartVar, generator)
-    if (!sti.unit) ImplementationBodyCodegen.genPropertyOnStack(
+    val actualType = if (!sti.unit) ImplementationBodyCodegen.genPropertyOnStack(
         this,
         expressionCodegen.context,
         property.descriptor,
         propertyOwnerType,
         ownerVar,
         classCodegen.state
-    )
-    StackValue.coerce(propertyType, sti.type, this)
+    ) else null
+    actualType?.type?.let { type -> StackValue.coerce(type, sti.type, this) }
     invokeinterface(
         kOutputType.internalName,
         CallingConventions.encode + sti.elementMethodPrefix + (if (useSerializer) "Serializable" else "") + CallingConventions.elementPostfix,

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/jvm/SerializableCodegenImpl.kt

@@ -143,7 +143,8 @@ class SerializableCodegenImpl(
                 val writeLabel = Label()
                 val nonWriteLabel = Label()
                 // obj.prop != DEFAULT_VAL
-                ImplementationBodyCodegen.genPropertyOnStack(
+                val propAsmType = classCodegen.typeMapper.mapType(property.type)
+                val actualType: JvmKotlinType = ImplementationBodyCodegen.genPropertyOnStack(
                     this,
                     exprCodegen.context,
                     property.descriptor,
@@ -151,7 +152,7 @@ class SerializableCodegenImpl(
                     thisI,
                     classCodegen.state
                 )
-                val propAsmType = classCodegen.typeMapper.mapType(property.type)
+                StackValue.coerce(actualType.type, propAsmType, this)
                 val lhs = StackValue.onStack(propAsmType)
                 val (expr, _) = initializersMapper(property)
                 exprCodegen.gen(expr, propAsmType)