Support more kinds of properties in KotlinJsonAdapter

This makes it possible to have synthetic properties that have no
state.

Also test properties that are synthetic and have no backing field.

Closes: https://github.com/square/moshi/issues/299

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

@@ -115,12 +115,6 @@ internal class KotlinJsonAdapter<T>(
       val adapter: JsonAdapter<P>,
       val property: KProperty1<K, P>,
       val parameter: KParameter?) {
-    init {
-      if (property !is KMutableProperty1 && parameter == null) {
-        throw IllegalArgumentException("No constructor or var property for ${property}")
-      }
-    }
-
     fun get(value: K) = property.get(value)
 
     fun set(result: K, value: P) {
@@ -171,11 +165,13 @@ object KotlinJsonAdapterFactory : JsonAdapter.Factory {
     for (property in rawType.kotlin.memberProperties) {
       if (Modifier.isTransient(property.javaField?.modifiers ?: 0)) continue
 
+      val parameter = parametersByName[property.name]
+      if (property !is KMutableProperty1 && parameter == null) continue
+
       property.isAccessible = true
       var allAnnotations = property.annotations
       var jsonAnnotation = property.findAnnotation<Json>()
 
-      val parameter = parametersByName[property.name]
       if (parameter != null) {
         allAnnotations += parameter.annotations
         if (jsonAnnotation == null) {

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

@@ -445,15 +445,17 @@ class KotlinJsonAdapterTest {
     }
   }
 
-  @Test fun unsettableProperty() {
+  @Test fun unsettablePropertyIgnored() {
     val moshi = Moshi.Builder().add(KotlinJsonAdapterFactory).build()
-    try {
+    val jsonAdapter = moshi.adapter(UnsettableProperty::class.java)
-      moshi.adapter(UnsettableProperty::class.java)
+
-      fail()
+    val encoded = UnsettableProperty()
-    } catch(expected: IllegalArgumentException) {
+    encoded.b = 5
-      assertThat(expected).hasMessage("No constructor or var property for " +
+    assertThat(jsonAdapter.toJson(encoded)).isEqualTo("{\"b\":5}")
-          "val ${UnsettableProperty::class.qualifiedName}.a: kotlin.Int")
+
-    }
+    val decoded = jsonAdapter.fromJson("{\"a\":4,\"b\":6}")!!
+    assertThat(decoded.a).isEqualTo(-1)
+    assertThat(decoded.b).isEqualTo(6)
   }
 
   class UnsettableProperty {
@@ -461,6 +463,52 @@ class KotlinJsonAdapterTest {
     var b: Int = -1
   }
 
+  @Test fun getterOnlyNoBackingField() {
+    val moshi = Moshi.Builder().add(KotlinJsonAdapterFactory).build()
+    val jsonAdapter = moshi.adapter(GetterOnly::class.java)
+
+    val encoded = GetterOnly(3, 5)
+    assertThat(jsonAdapter.toJson(encoded)).isEqualTo("{\"a\":3,\"b\":5}")
+
+    val decoded = jsonAdapter.fromJson("{\"a\":4,\"b\":6}")!!
+    assertThat(decoded.a).isEqualTo(4)
+    assertThat(decoded.b).isEqualTo(6)
+    assertThat(decoded.total).isEqualTo(10)
+  }
+
+  class GetterOnly(var a: Int, var b: Int) {
+    val total : Int
+      get() = a + b
+  }
+
+  @Test fun getterAndSetterNoBackingField() {
+    val moshi = Moshi.Builder().add(KotlinJsonAdapterFactory).build()
+    val jsonAdapter = moshi.adapter(GetterAndSetter::class.java)
+
+    val encoded = GetterAndSetter(3, 5)
+    assertThat(jsonAdapter.toJson(encoded)).isEqualTo("{\"a\":3,\"b\":5,\"total\":8}")
+
+    // Whether b is 6 or 7 is an implementation detail. Currently we call constructors then setters.
+    val decoded1 = jsonAdapter.fromJson("{\"a\":4,\"b\":6,\"total\":11}")!!
+    assertThat(decoded1.a).isEqualTo(4)
+    assertThat(decoded1.b).isEqualTo(7)
+    assertThat(decoded1.total).isEqualTo(11)
+
+    // Whether b is 6 or 7 is an implementation detail. Currently we call constructors then setters.
+    val decoded2 = jsonAdapter.fromJson("{\"a\":4,\"total\":11,\"b\":6}")!!
+    assertThat(decoded2.a).isEqualTo(4)
+    assertThat(decoded2.b).isEqualTo(7)
+    assertThat(decoded2.total).isEqualTo(11)
+  }
+
+  class GetterAndSetter(var a: Int, var b: Int) {
+    var total : Int
+      get() = a + b
+      set(value) {
+        b = value - a
+      }
+  }
+
   @Test fun nonPropertyConstructorParameter() {
     val moshi = Moshi.Builder().add(KotlinJsonAdapterFactory).build()
     try {