Promote Kotlin type-inferring APIs to the main Moshi package (round 2!) (#1202)

* Make moshi-root a kotlin project

* Move moshi kotlin extensions to moshi core

* Add appropriate experimental annotations

* Add nextAdapter helper

* Add explicit return type on addAdapter

* Expression body for adapter

* Use nextAdapter helper

* Opportunistically fix a couple Util warnings

* Add Types extensions

* Spotless

* Use extensions in more places for added coverage

* Apply java versions on any java plugin type

This way the kotlin projects get this too

* Fix circularAdapters test?

* Use java 8 in java for code gen too

* Fixup with CircularAdaptersTest

* Add coverage for remaining

* Remove nextAdapter

* Remove leftover function

* Use asserts

left checkNotNull for the contract

* boxIfPrimitive

* Fixup docs

* Copyright fixes

* Add parameterized addAdapter

* Switch to using native javaType API

* Spotless

* Back to 2019

* Spotless

* Use rawType extension

* Fix rebase issues

build.gradle.kts

@@ -120,6 +120,7 @@ subprojects {
     }
   }
 
+  // Apply with "java" instead of just "java-library" so kotlin projects get it too
   pluginManager.withPlugin("java") {
     configure<JavaPluginExtension> {
       sourceCompatibility = JavaVersion.VERSION_1_7

buildSrc/src/main/kotlin/Dependencies.kt

@@ -36,6 +36,7 @@ object Dependencies {
 
   object Kotlin {
     const val version = "1.4.10"
+    const val stdlib = "org.jetbrains.kotlin:kotlin-stdlib:$version"
     const val metadata = "org.jetbrains.kotlinx:kotlinx-metadata-jvm:0.1.0"
   }
 

kotlin/codegen/build.gradle.kts

@@ -25,6 +25,11 @@ plugins {
   id("com.github.johnrengelman.shadow") version "6.0.0"
 }
 
+configure<JavaPluginExtension> {
+  sourceCompatibility = JavaVersion.VERSION_1_8
+  targetCompatibility = JavaVersion.VERSION_1_8
+}
+
 tasks.withType<KotlinCompile>().configureEach {
   kotlinOptions {
     jvmTarget = "1.8"

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

@@ -21,10 +21,10 @@ import com.squareup.moshi.JsonDataException
 import com.squareup.moshi.JsonReader
 import com.squareup.moshi.JsonWriter
 import com.squareup.moshi.Moshi
-import com.squareup.moshi.Types
 import com.squareup.moshi.internal.Util
 import com.squareup.moshi.internal.Util.generatedAdapter
 import com.squareup.moshi.internal.Util.resolve
+import com.squareup.moshi.rawType
 import java.lang.reflect.Modifier
 import java.lang.reflect.Type
 import java.util.AbstractMap.SimpleEntry
@@ -184,7 +184,7 @@ class KotlinJsonAdapterFactory : JsonAdapter.Factory {
     JsonAdapter<*>? {
       if (annotations.isNotEmpty()) return null
 
-      val rawType = Types.getRawType(type)
+      val rawType = type.rawType
       if (rawType.isInterface) return null
       if (rawType.isEnum) return null
       if (!rawType.isAnnotationPresent(KOTLIN_METADATA)) return null

kotlin/tests/src/test/kotlin/com/squareup/moshi/kotlin/DefaultConstructorTest.kt

@@ -17,6 +17,7 @@ package com.squareup.moshi.kotlin
 
 import com.squareup.moshi.JsonClass
 import com.squareup.moshi.Moshi
+import com.squareup.moshi.adapter
 import org.junit.Test
 
 class DefaultConstructorTest {
@@ -25,7 +26,7 @@ class DefaultConstructorTest {
     val expected = TestClass("requiredClass")
     val json =
       """{"required":"requiredClass"}"""
-    val instance = Moshi.Builder().build().adapter<TestClass>(TestClass::class.java)
+    val instance = Moshi.Builder().build().adapter<TestClass>()
       .fromJson(json)!!
     check(instance == expected) {
       "No match:\nActual  : $instance\nExpected: $expected"
@@ -36,7 +37,7 @@ class DefaultConstructorTest {
     val expected = TestClass("requiredClass", "customOptional", 4, "setDynamic", 5, 6)
     val json =
       """{"required":"requiredClass","optional":"customOptional","optional2":4,"dynamicSelfReferenceOptional":"setDynamic","dynamicOptional":5,"dynamicInlineOptional":6}"""
-    val instance = Moshi.Builder().build().adapter<TestClass>(TestClass::class.java)
+    val instance = Moshi.Builder().build().adapter<TestClass>()
       .fromJson(json)!!
     check(instance == expected) {
       "No match:\nActual  : $instance\nExpected: $expected"
@@ -47,7 +48,7 @@ class DefaultConstructorTest {
     val expected = TestClass("requiredClass", "customOptional")
     val json =
       """{"required":"requiredClass","optional":"customOptional"}"""
-    val instance = Moshi.Builder().build().adapter<TestClass>(TestClass::class.java)
+    val instance = Moshi.Builder().build().adapter<TestClass>()
       .fromJson(json)!!
     check(instance == expected) {
       "No match:\nActual  : $instance\nExpected: $expected"

kotlin/tests/src/test/kotlin/com/squareup/moshi/kotlin/DualKotlinTest.kt

@@ -24,9 +24,10 @@ import com.squareup.moshi.JsonClass
 import com.squareup.moshi.JsonDataException
 import com.squareup.moshi.Moshi
 import com.squareup.moshi.ToJson
-import com.squareup.moshi.Types
+import com.squareup.moshi.adapter
 import com.squareup.moshi.kotlin.reflect.KotlinJsonAdapterFactory
-import com.squareup.moshi.kotlin.reflect.adapter
+import com.squareup.moshi.rawType
+import com.squareup.moshi.supertypeOf
 import org.intellij.lang.annotations.Language
 import org.junit.Assert.fail
 import org.junit.Test
@@ -62,11 +63,11 @@ class DualKotlinTest(useReflection: Boolean) {
           object : Factory {
             override fun create(
               type: Type,
-              annotations: MutableSet<out Annotation>,
+              annotations: Set<Annotation>,
               moshi: Moshi
             ): JsonAdapter<*>? {
               // Prevent falling back to generated adapter lookup
-              val rawType = Types.getRawType(type)
+              val rawType = type.rawType
               val metadataClass = Class.forName("kotlin.Metadata") as Class<out Annotation>
               check(rawType.isEnum || !rawType.isAnnotationPresent(metadataClass)) {
                 "Unhandled Kotlin type in reflective test! $rawType"
@@ -446,7 +447,7 @@ class DualKotlinTest(useReflection: Boolean) {
   @Test fun typeAliasUnwrapping() {
     val adapter = moshi
       .newBuilder()
-      .add(Types.supertypeOf(Int::class.javaObjectType), moshi.adapter<Int>())
+      .add(supertypeOf<Int>(), moshi.adapter<Int>())
       .build()
       .adapter<TypeAliasUnwrapping>()
 

kotlin/tests/src/test/kotlin/com/squareup/moshi/kotlin/codegen/ComplexGenericsInheritanceTest.kt

@@ -20,7 +20,7 @@ package com.squareup.moshi.kotlin.codegen
 import com.google.common.truth.Truth.assertThat
 import com.squareup.moshi.JsonClass
 import com.squareup.moshi.Moshi
-import com.squareup.moshi.kotlin.reflect.adapter
+import com.squareup.moshi.adapter
 import org.intellij.lang.annotations.Language
 import org.junit.Test
 

kotlin/tests/src/test/kotlin/com/squareup/moshi/kotlin/codegen/GeneratedAdaptersTest.kt

@@ -26,9 +26,8 @@ import com.squareup.moshi.JsonReader
 import com.squareup.moshi.JsonWriter
 import com.squareup.moshi.Moshi
 import com.squareup.moshi.ToJson
-import com.squareup.moshi.Types
+import com.squareup.moshi.adapter
 import com.squareup.moshi.internal.NullSafeJsonAdapter
-import com.squareup.moshi.kotlin.reflect.adapter
 import org.intellij.lang.annotations.Language
 import org.junit.Assert.assertNull
 import org.junit.Assert.fail
@@ -299,13 +298,7 @@ class GeneratedAdaptersTest {
 
   @Test
   fun nullableTypeParams() {
-    val adapter = moshi.adapter<NullableTypeParams<Int>>(
-      Types.newParameterizedTypeWithOwner(
-        GeneratedAdaptersTest::class.java,
-        NullableTypeParams::class.java,
-        Int::class.javaObjectType
-      )
-    )
+    val adapter = moshi.adapter<NullableTypeParams<Int>>()
     val nullSerializing = adapter.serializeNulls()
 
     val nullableTypeParams = NullableTypeParams(
@@ -597,7 +590,7 @@ class GeneratedAdaptersTest {
 
   @Test fun multipleTransientConstructorParameters() {
     val moshi = Moshi.Builder().build()
-    val jsonAdapter = moshi.adapter(MultipleTransientConstructorParameters::class.java)
+    val jsonAdapter = moshi.adapter<MultipleTransientConstructorParameters>()
 
     val encoded = MultipleTransientConstructorParameters(3, 5, 7)
     assertThat(jsonAdapter.toJson(encoded)).isEqualTo("""{"b":5}""")
@@ -1346,6 +1339,7 @@ class GeneratedAdaptersTest {
 
   @Test fun typesSizeCheckMessages_noArgs() {
     try {
+      // Note: This is impossible to do if you use the reified adapter extension!
       moshi.adapter(MultipleGenerics::class.java)
       fail("Should have failed to construct the adapter due to missing generics")
     } catch (e: RuntimeException) {

kotlin/tests/src/test/kotlin/com/squareup/moshi/kotlin/codegen/MultipleMasksTest.kt

@@ -17,6 +17,7 @@ package com.squareup.moshi.kotlin.codegen
 
 import com.squareup.moshi.JsonClass
 import com.squareup.moshi.Moshi
+import com.squareup.moshi.adapter
 import org.intellij.lang.annotations.Language
 import org.junit.Assert.assertEquals
 import org.junit.Test
@@ -41,7 +42,7 @@ class MultipleMasksTest {
     val json =
       """{"arg50":500,"arg3":34,"arg11":11,"arg65":67}"""
 
-    val instance = Moshi.Builder().build().adapter(MultipleMasks::class.java)
+    val instance = Moshi.Builder().build().adapter<MultipleMasks>()
       .fromJson(json)!!
 
     assertEquals(instance.arg2, 2)

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

@@ -1,106 +0,0 @@
-/*
- * Copyright (C) 2019 Square, Inc.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *    https://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-package com.squareup.moshi.kotlin.reflect
-
-import com.squareup.moshi.JsonAdapter
-import com.squareup.moshi.Moshi
-import com.squareup.moshi.Types
-import com.squareup.moshi.internal.NonNullJsonAdapter
-import com.squareup.moshi.internal.NullSafeJsonAdapter
-import java.lang.reflect.Type
-import kotlin.reflect.KClass
-import kotlin.reflect.KType
-import kotlin.reflect.KTypeParameter
-import kotlin.reflect.KTypeProjection
-import kotlin.reflect.KVariance
-import kotlin.reflect.typeOf
-
-/**
- * @return a [JsonAdapter] for [T], creating it if necessary. Note that while nullability of [T]
- *         itself is handled, nested types (such as in generics) are not resolved.
- */
-inline fun <reified T> Moshi.adapter(): JsonAdapter<T> {
-  return adapter(typeOf<T>())
-}
-
-inline fun <reified T> Moshi.Builder.addAdapter(adapter: JsonAdapter<T>) = add(typeOf<T>().toType(), adapter)
-
-/**
- * @return a [JsonAdapter] for [ktype], creating it if necessary. Note that while nullability of
- *         [ktype] itself is handled, nested types (such as in generics) are not resolved.
- */
-fun <T> Moshi.adapter(ktype: KType): JsonAdapter<T> {
-  val adapter = adapter<T>(ktype.toType())
-  return if (adapter is NullSafeJsonAdapter || adapter is NonNullJsonAdapter) {
-    // TODO CR - Assume that these know what they're doing? Or should we defensively avoid wrapping for matching nullability?
-    adapter
-  } else if (ktype.isMarkedNullable) {
-    adapter.nullSafe()
-  } else {
-    adapter.nonNull()
-  }
-}
-
-@PublishedApi
-internal fun KType.toType(allowPrimitives: Boolean = true): Type {
-  classifier?.let {
-    when (it) {
-      is KTypeParameter -> throw IllegalArgumentException("Type parameters are not supported")
-      is KClass<*> -> {
-        val javaType = if (allowPrimitives) {
-          it.java
-        } else {
-          it.javaObjectType
-        }
-        if (javaType.isArray) {
-          return Types.arrayOf(javaType.componentType)
-        }
-
-        return if (arguments.isEmpty()) {
-          javaType
-        } else {
-          val typeArguments = arguments.toTypedArray { it.toType() }
-          val enclosingClass = javaType.enclosingClass
-          return if (enclosingClass != null) {
-            Types.newParameterizedTypeWithOwner(enclosingClass, javaType, *typeArguments)
-          } else {
-            Types.newParameterizedType(javaType, *typeArguments)
-          }
-        }
-      }
-      else -> throw IllegalArgumentException("Unsupported classifier: $this")
-    }
-  }
-
-  // Can happen for intersection types
-  throw IllegalArgumentException("Unrepresentable type: $this")
-}
-
-internal fun KTypeProjection.toType(): Type {
-  val javaType = type?.toType(allowPrimitives = false) ?: return Any::class.java
-  return when (variance) {
-    null -> Any::class.java
-    KVariance.INVARIANT -> javaType
-    KVariance.IN -> Types.subtypeOf(javaType)
-    KVariance.OUT -> Types.supertypeOf(javaType)
-  }
-}
-
-private inline fun <T, reified R> List<T>.toTypedArray(mapper: (T) -> R): Array<R> {
-  return Array(size) {
-    mapper.invoke(get(it))
-  }
-}

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

@@ -26,7 +26,7 @@ import com.squareup.moshi.JsonReader
 import com.squareup.moshi.JsonWriter
 import com.squareup.moshi.Moshi
 import com.squareup.moshi.ToJson
-import com.squareup.moshi.Types
+import com.squareup.moshi.adapter
 import org.assertj.core.api.Assertions
 import org.junit.Assert.fail
 import org.junit.Test
@@ -448,7 +448,7 @@ class KotlinJsonAdapterTest {
       fail()
     } catch (e: IllegalArgumentException) {
       assertThat(e).hasMessageThat().isEqualTo(
-        "Platform class kotlin.Triple in kotlin.Triple<java.lang.Object, java.lang.Object, java.lang.Object> requires explicit JsonAdapter to be registered"
+        "Platform class kotlin.Triple in kotlin.Triple<?, ?, ?> requires explicit JsonAdapter to be registered"
       )
     }
   }
@@ -857,13 +857,7 @@ class KotlinJsonAdapterTest {
 
   @Test fun genericTypes() {
     val moshi = Moshi.Builder().add(KotlinJsonAdapterFactory()).build()
-    val stringBoxAdapter = moshi.adapter<Box<String>>(
-      Types.newParameterizedTypeWithOwner(
-        KotlinJsonAdapterTest::class.java,
-        Box::class.java,
-        String::class.java
-      )
-    )
+    val stringBoxAdapter = moshi.adapter<Box<String>>()
     assertThat(stringBoxAdapter.fromJson("""{"data":"hello"}""")).isEqualTo(Box("hello"))
     assertThat(stringBoxAdapter.toJson(Box("hello"))).isEqualTo("""{"data":"hello"}""")
   }
@@ -872,18 +866,7 @@ class KotlinJsonAdapterTest {
 
   @Test fun nestedGenericTypes() {
     val moshi = Moshi.Builder().add(KotlinJsonAdapterFactory()).build()
-    val type = Types.newParameterizedTypeWithOwner(
-      KotlinJsonAdapterTest::class.java,
-      NestedGenerics::class.java,
-      String::class.java,
-      Int::class.javaObjectType,
-      Types.newParameterizedTypeWithOwner(
-        KotlinJsonAdapterTest::class.java,
-        Box::class.java,
-        String::class.java
-      )
-    )
-    val adapter = moshi.adapter<NestedGenerics<String, Int, Box<String>>>(type).indent("  ")
+    val adapter = moshi.adapter<NestedGenerics<String, Int, Box<String>>>().indent("  ")
     val json =
       """
       |{

moshi/build.gradle.kts

@@ -14,15 +14,30 @@
  * limitations under the License.
  */
 
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
 plugins {
-  `java-library`
+  kotlin("jvm")
   id("com.vanniktech.maven.publish")
 }
 
+tasks.withType<KotlinCompile>()
+  .matching { it.name.contains("test", true) }
+  .configureEach {
+    kotlinOptions {
+      @Suppress("SuspiciousCollectionReassignment") // It's not suspicious
+      freeCompilerArgs += listOf(
+        "-Xopt-in=kotlin.ExperimentalStdlibApi"
+      )
+    }
+  }
+
 dependencies {
   compileOnly(Dependencies.jsr305)
+  compileOnly(Dependencies.Kotlin.stdlib)
   api(Dependencies.okio)
 
+  testImplementation(Dependencies.Kotlin.stdlib)
   testCompileOnly(Dependencies.jsr305)
   testImplementation(Dependencies.Testing.junit)
   testImplementation(Dependencies.Testing.truth)

moshi/gradle.properties

@@ -18,3 +18,7 @@ POM_NAME=Moshi
 POM_ARTIFACT_ID=moshi
 POM_PACKAGING=jar
 AUTOMATIC_MODULE_NAME=com.squareup.moshi
+
+# Kotlin adds the stdlib dep by default in 1.4.0+, but we want to effectively make it compileOnly
+# for our case to avoid imposing it on consumers.
+kotlin.stdlib.default.dependency=false

moshi/src/main/java/com/squareup/moshi/-MoshiKotlinExtensions.kt

@@ -0,0 +1,49 @@
+/*
+ * Copyright (C) 2019 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.moshi
+
+import com.squareup.moshi.internal.NonNullJsonAdapter
+import com.squareup.moshi.internal.NullSafeJsonAdapter
+import kotlin.reflect.KType
+import kotlin.reflect.javaType
+import kotlin.reflect.typeOf
+
+/**
+ * @return a [JsonAdapter] for [T], creating it if necessary. Note that while nullability of [T]
+ *         itself is handled, nested types (such as in generics) are not resolved.
+ */
+@ExperimentalStdlibApi
+inline fun <reified T> Moshi.adapter(): JsonAdapter<T> = adapter(typeOf<T>())
+
+@ExperimentalStdlibApi
+inline fun <reified T> Moshi.Builder.addAdapter(adapter: JsonAdapter<T>): Moshi.Builder = add(typeOf<T>().javaType, adapter)
+
+/**
+ * @return a [JsonAdapter] for [ktype], creating it if necessary. Note that while nullability of
+ *         [ktype] itself is handled, nested types (such as in generics) are not resolved.
+ */
+@ExperimentalStdlibApi
+fun <T> Moshi.adapter(ktype: KType): JsonAdapter<T> {
+  val adapter = adapter<T>(ktype.javaType)
+  return if (adapter is NullSafeJsonAdapter || adapter is NonNullJsonAdapter) {
+    // TODO CR - Assume that these know what they're doing? Or should we defensively avoid wrapping for matching nullability?
+    adapter
+  } else if (ktype.isMarkedNullable) {
+    adapter.nullSafe()
+  } else {
+    adapter.nonNull()
+  }
+}

moshi/src/main/java/com/squareup/moshi/-MoshiKotlinTypesExtensions.kt

@@ -0,0 +1,71 @@
+/*
+ * Copyright (C) 2020 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.moshi
+
+import com.squareup.moshi.internal.Util
+import java.lang.reflect.GenericArrayType
+import java.lang.reflect.Type
+import java.lang.reflect.WildcardType
+import kotlin.reflect.KClass
+import kotlin.reflect.KType
+import kotlin.reflect.javaType
+import kotlin.reflect.typeOf
+
+/** Returns the raw [Class] type of this type. */
+val Type.rawType: Class<*> get() = Types.getRawType(this)
+
+/**
+ * Checks if [this] contains [T]. Returns the subset of [this] without [T], or null if
+ * [this] does not contain [T].
+ */
+inline fun <reified T : Annotation> Set<Annotation>.nextAnnotations(): Set<Annotation>? = Types.nextAnnotations(this, T::class.java)
+
+/**
+ * Returns a type that represents an unknown type that extends [T]. For example, if
+ * [T] is [CharSequence], this returns `out CharSequence`. If
+ * [T] is [Any], this returns `*`, which is shorthand for `out Any?`.
+ */
+@ExperimentalStdlibApi
+inline fun <reified T> subtypeOf(): WildcardType {
+  var type = typeOf<T>().javaType
+  if (type is Class<*>) {
+    type = Util.boxIfPrimitive(type)
+  }
+  return Types.subtypeOf(type)
+}
+
+/**
+ * Returns a type that represents an unknown supertype of [T] bound. For example, if [T] is
+ * [String], this returns `in String`.
+ */
+@ExperimentalStdlibApi
+inline fun <reified T> supertypeOf(): WildcardType {
+  var type = typeOf<T>().javaType
+  if (type is Class<*>) {
+    type = Util.boxIfPrimitive(type)
+  }
+  return Types.supertypeOf(type)
+}
+
+/** Returns a [GenericArrayType] with [this] as its [GenericArrayType.getGenericComponentType]. */
+@ExperimentalStdlibApi
+fun KType.asArrayType(): GenericArrayType = javaType.asArrayType()
+
+/** Returns a [GenericArrayType] with [this] as its [GenericArrayType.getGenericComponentType]. */
+fun KClass<*>.asArrayType(): GenericArrayType = java.asArrayType()
+
+/** Returns a [GenericArrayType] with [this] as its [GenericArrayType.getGenericComponentType]. */
+fun Type.asArrayType(): GenericArrayType = Types.arrayOf(this)

moshi/src/main/java/com/squareup/moshi/internal/Util.java

@@ -39,7 +39,9 @@ import java.lang.reflect.WildcardType;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.LinkedHashMap;
 import java.util.LinkedHashSet;
+import java.util.Map;
 import java.util.NoSuchElementException;
 import java.util.Set;
 import javax.annotation.Nullable;
@@ -50,6 +52,9 @@ public final class Util {
   @Nullable public static final Class<?> DEFAULT_CONSTRUCTOR_MARKER;
   @Nullable private static final Class<? extends Annotation> METADATA;
 
+  /** A map from primitive types to their corresponding wrapper types. */
+  private static final Map<Class<?>, Class<?>> PRIMITIVE_TO_WRAPPER_TYPE;
+
   static {
     Class<? extends Annotation> metadata = null;
     try {
@@ -67,6 +72,20 @@ public final class Util {
     } catch (ClassNotFoundException ignored) {
     }
     DEFAULT_CONSTRUCTOR_MARKER = defaultConstructorMarker;
+
+    Map<Class<?>, Class<?>> primToWrap = new LinkedHashMap<>(16);
+
+    primToWrap.put(boolean.class, Boolean.class);
+    primToWrap.put(byte.class, Byte.class);
+    primToWrap.put(char.class, Character.class);
+    primToWrap.put(double.class, Double.class);
+    primToWrap.put(float.class, Float.class);
+    primToWrap.put(int.class, Integer.class);
+    primToWrap.put(long.class, Long.class);
+    primToWrap.put(short.class, Short.class);
+    primToWrap.put(void.class, Void.class);
+
+    PRIMITIVE_TO_WRAPPER_TYPE = Collections.unmodifiableMap(primToWrap);
   }
 
   // Extracted as a method with a keep rule to prevent R8 from keeping Kotlin Metada
@@ -182,14 +201,14 @@ public final class Util {
   }
 
   public static Type resolve(Type context, Class<?> contextRawType, Type toResolve) {
-    return resolve(context, contextRawType, toResolve, new LinkedHashSet<TypeVariable>());
+    return resolve(context, contextRawType, toResolve, new LinkedHashSet<TypeVariable<?>>());
   }
 
   private static Type resolve(
       Type context,
       Class<?> contextRawType,
       Type toResolve,
-      Collection<TypeVariable> visitedTypeVariables) {
+      Collection<TypeVariable<?>> visitedTypeVariables) {
     // This implementation is made a little more complicated in an attempt to avoid object-creation.
     while (true) {
       if (toResolve instanceof TypeVariable) {
@@ -643,4 +662,12 @@ public final class Util {
     }
     return new JsonDataException(message);
   }
+
+  // Public due to inline access in MoshiKotlinTypesExtensions
+  public static <T> Class<T> boxIfPrimitive(Class<T> type) {
+    // cast is safe: long.class and Long.class are both of type Class<Long>
+    @SuppressWarnings("unchecked")
+    Class<T> wrapped = (Class<T>) PRIMITIVE_TO_WRAPPER_TYPE.get(type);
+    return (wrapped == null) ? type : wrapped;
+  }
 }

moshi/src/test/java/com/squareup/moshi/KotlinExtensionsTest.kt

@@ -0,0 +1,99 @@
+/*
+ * Copyright (C) 2020 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.moshi
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import kotlin.annotation.AnnotationRetention.RUNTIME
+import kotlin.reflect.typeOf
+
+@JsonQualifier
+@Retention(RUNTIME)
+annotation class TestAnnotation1
+
+@JsonQualifier
+@Retention(RUNTIME)
+annotation class TestAnnotation2
+
+@TestAnnotation1
+@TestAnnotation2
+class KotlinExtensionsTest {
+
+  @Test
+  fun nextAnnotationsShouldWork() {
+    val annotations = KotlinExtensionsTest::class.java.annotations
+      .filterTo(mutableSetOf()) {
+        it.annotationClass.java.isAnnotationPresent(JsonQualifier::class.java)
+      }
+    assertEquals(2, annotations.size)
+    val next = annotations.nextAnnotations<TestAnnotation2>()
+    checkNotNull(next)
+    assertEquals(1, next.size)
+    assertTrue(next.first() is TestAnnotation1)
+  }
+
+  @Test
+  fun arrayType() {
+    val stringArray = String::class.asArrayType()
+    check(stringArray.genericComponentType == String::class.java)
+
+    val stringListType = typeOf<List<String>>()
+    val stringListArray = stringListType.asArrayType()
+    val expected = Types.arrayOf(Types.newParameterizedType(List::class.java, String::class.java))
+    assertEquals(stringListArray, expected)
+  }
+
+  @Test
+  fun addAdapterInferred() {
+    // An adapter that always returns -1
+    val customIntdapter = object : JsonAdapter<Int>() {
+      override fun fromJson(reader: JsonReader): Int? {
+        reader.skipValue()
+        return -1
+      }
+
+      override fun toJson(writer: JsonWriter, value: Int?) {
+        throw NotImplementedError()
+      }
+    }
+    val moshi = Moshi.Builder()
+      .addAdapter(customIntdapter)
+      .build()
+
+    assertEquals(-1, moshi.adapter<Int>().fromJson("5"))
+  }
+
+  @Test
+  fun addAdapterInferred_parameterized() {
+    // An adapter that always returns listOf(-1)
+    val customIntListAdapter = object : JsonAdapter<List<Int>>() {
+      override fun fromJson(reader: JsonReader): List<Int>? {
+        reader.skipValue()
+        return listOf(-1)
+      }
+
+      override fun toJson(writer: JsonWriter, value: List<Int>?) {
+        throw NotImplementedError()
+      }
+    }
+    val moshi = Moshi.Builder()
+      .addAdapter(customIntListAdapter)
+      .build()
+
+    assertEquals(listOf(-1), moshi.adapter<List<Int>>().fromJson("[5]"))
+  }
+}