Fix error message when missing primary constructor.

moshi-kotlin-tests/src/test/kotlin/com/squareup/moshi/kotlin/reflect/KotlinJsonAdapterTest.kt

@@ -1074,6 +1074,24 @@ class KotlinJsonAdapterTest {
 
   sealed class SealedClass
 
+  @Test fun requirePrimaryConstructor() {
+    val moshi: Moshi = Moshi.Builder()
+      .add(KotlinJsonAdapterFactory())
+      .build()
+    try {
+      moshi.adapter(SubClass::class.java)
+    } catch (e: IllegalArgumentException) {
+      assertThat(e).hasMessageThat()
+        .contains("Cannot reflectively serialize class without the primary constructor")
+    }
+  }
+
+  open class SuperClass
+
+  class SubClass : SuperClass {
+    constructor() : super()
+  }
+
   private fun <T> mapWildcardsParameterizedTest(type: Class<T>, json: String, value: T) {
     // Ensure the map was created with the expected wildcards of a Kotlin map.
     val fieldType = type.getDeclaredField("map").genericType

moshi-kotlin/src/main/java/com/squareup/moshi/kotlin/reflect/KotlinJsonAdapterFactory.kt

@@ -213,6 +213,9 @@ public class KotlinJsonAdapterFactory : JsonAdapter.Factory {
       // Fall back to a reflective adapter when the generated adapter is not found.
     }
 
+    require(!rawType.isAnonymousClass) {
+      "Cannot serialize anonymous class ${rawType.name}"
+    }
     require(!rawType.isLocalClass) {
       "Cannot serialize local class or object expression ${rawType.name}"
     }
@@ -230,7 +233,11 @@ public class KotlinJsonAdapterFactory : JsonAdapter.Factory {
       "Cannot reflectively serialize sealed class ${rawType.name}. Please register an adapter."
     }
 
-    val constructor = rawTypeKotlin.primaryConstructor ?: return null
+    val constructor = rawTypeKotlin.primaryConstructor
+    requireNotNull(constructor) {
+      "Cannot reflectively serialize class without the primary constructor ${rawType.name}." +
+        " Please register an adapter."
+    }
     val parametersByName = constructor.parameters.associateBy { it.name }
     constructor.isAccessible = true