Clarify error for non-null Kotlin properties. (#376)

Instead of throwing an InvocationTargetException.

kotlin/src/main/java/com/squareup/moshi/KotlinJsonAdapter.kt

@@ -81,6 +81,9 @@ internal class KotlinJsonAdapter<T>(
               "Required value ${constructor.parameters[i].name} missing at ${reader.path}")
         }
         values[i] = null // Replace absent with null.
+      } else if (values[i] == null && !constructor.parameters[i].type.isMarkedNullable) {
+        throw JsonDataException("Non-null value ${constructor.parameters[i].name} " +
+            "was null at ${reader.path}")
       }
     }
 
@@ -89,7 +92,13 @@ internal class KotlinJsonAdapter<T>(
 
     // Set remaining properties.
     for (i in constructorSize until bindings.size) {
-      bindings[i]!!.set(result, values[i])
+      val binding = bindings[i]!!
+      val value = values[i]
+      if (value == null && !binding.property.returnType.isMarkedNullable) {
+        throw JsonDataException("Non-null value ${binding.property.name} " +
+            "was null at ${reader.path}")
+      }
+      binding.set(result, value)
     }
 
     return result

kotlin/src/test/java/com/squareup/moshi/KotlinJsonAdapterTest.kt

@@ -133,6 +133,32 @@ class KotlinJsonAdapterTest {
 
   class RequiredValueAbsent(var a: Int = 3, var b: Int)
 
+  @Test fun nonNullConstructorParameterCalledWithNullFailsWithJsonDataException() {
+    class Data(val a: String)
+    val moshi = Moshi.Builder().add(KotlinJsonAdapterFactory()).build()
+    val jsonAdapter = moshi.adapter(Data::class.java)
+
+    try {
+      jsonAdapter.fromJson("{\"a\":null}")
+      fail()
+    } catch (expected: JsonDataException) {
+      assertThat(expected).hasMessage("Non-null value a was null at \$")
+    }
+  }
+
+  @Test fun nonNullPropertySetToNullFailsWithJsonDataException() {
+    class Data { var a: String = "" }
+    val moshi = Moshi.Builder().add(KotlinJsonAdapterFactory()).build()
+    val jsonAdapter = moshi.adapter(Data::class.java)
+
+    try {
+      jsonAdapter.fromJson("{\"a\":null}")
+      fail()
+    } catch (expected: JsonDataException) {
+      assertThat(expected).hasMessage("Non-null value a was null at \$")
+    }
+  }
+
   @Test fun duplicatedValue() {
     val moshi = Moshi.Builder().add(KotlinJsonAdapterFactory()).build()
     val jsonAdapter = moshi.adapter(DuplicateValue::class.java)