Add built-in support for Kotlin's Unit type

This is built in a way such that its absence from the classpath will not cause failures.

pom.xml

@@ -46,6 +46,7 @@
 
     <!-- Compilation -->
     <java.version>1.7</java.version>
+    <kotlin.version>1.2.60</kotlin.version>
 
     <!-- Dependencies -->
     <android.version>4.1.1.4</android.version>
@@ -108,6 +109,11 @@
         <artifactId>android</artifactId>
         <version>${android.version}</version>
       </dependency>
+      <dependency>
+        <groupId>org.jetbrains.kotlin</groupId>
+        <artifactId>kotlin-stdlib</artifactId>
+        <version>${kotlin.version}</version>
+      </dependency>
       <dependency>
         <groupId>org.codehaus.mojo</groupId>
         <artifactId>animal-sniffer-annotations</artifactId>

retrofit/pom.xml

@@ -24,6 +24,11 @@
       <artifactId>android</artifactId>
       <optional>true</optional>
     </dependency>
+    <dependency>
+      <groupId>org.jetbrains.kotlin</groupId>
+      <artifactId>kotlin-stdlib</artifactId>
+      <optional>true</optional>
+    </dependency>
 
     <dependency>
       <groupId>org.codehaus.mojo</groupId>

retrofit/src/main/java/retrofit2/BuiltInConverters.java

@@ -18,11 +18,15 @@
 import java.io.IOException;
 import java.lang.annotation.Annotation;
 import java.lang.reflect.Type;
+import kotlin.Unit;
 import okhttp3.RequestBody;
 import okhttp3.ResponseBody;
 import retrofit2.http.Streaming;
 
 final class BuiltInConverters extends Converter.Factory {
+  /** Not volatile because we don't mind multiple threads discovering this. */
+  private boolean checkForKotlinUnit = true;
+
   @Override
   public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations,
       Retrofit retrofit) {
@@ -34,6 +38,15 @@ final class BuiltInConverters extends Converter.Factory {
     if (type == Void.class) {
       return VoidResponseBodyConverter.INSTANCE;
     }
+    if (checkForKotlinUnit) {
+      try {
+        if (type == Unit.class) {
+          return UnitResponseBodyConverter.INSTANCE;
+        }
+      } catch (NoClassDefFoundError ignored) {
+        checkForKotlinUnit = false;
+      }
+    }
     return null;
   }
 
@@ -55,6 +68,15 @@ static final class VoidResponseBodyConverter implements Converter<ResponseBody,
     }
   }
 
+  static final class UnitResponseBodyConverter implements Converter<ResponseBody, Unit> {
+    static final UnitResponseBodyConverter INSTANCE = new UnitResponseBodyConverter();
+
+    @Override public Unit convert(ResponseBody value) {
+      value.close();
+      return Unit.INSTANCE;
+    }
+  }
+
   static final class RequestBodyConverter implements Converter<RequestBody, RequestBody> {
     static final RequestBodyConverter INSTANCE = new RequestBodyConverter();
 

retrofit/src/main/resources/META-INF/proguard/retrofit2.pro

@@ -11,3 +11,6 @@
 
 # Ignore JSR 305 annotations for embedding nullability information.
 -dontwarn javax.annotation.**
+
+# Guarded by a NoClassDefFoundError try/catch and only used when on the classpath.
+-dontwarn kotlin.Unit

retrofit/src/test/java/retrofit2/KotlinUnitTest.java

@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) 2018 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
+ *
+ *      http://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 retrofit2;
+
+import java.io.IOException;
+import kotlin.Unit;
+import okhttp3.mockwebserver.MockResponse;
+import okhttp3.mockwebserver.MockWebServer;
+import org.junit.Ignore;
+import org.junit.Rule;
+import org.junit.Test;
+import retrofit2.http.GET;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public final class KotlinUnitTest {
+  @Rule public final MockWebServer server = new MockWebServer();
+
+  interface Service {
+    @GET("/")
+    Call<Unit> empty();
+  }
+
+  @Test public void unitOnClasspath() throws IOException {
+    Retrofit retrofit = new Retrofit.Builder()
+        .baseUrl(server.url("/"))
+        .build();
+    Service example = retrofit.create(Service.class);
+
+    server.enqueue(new MockResponse().setBody("Hi"));
+
+    Response<Unit> response = example.empty().execute();
+    assertThat(response.isSuccessful()).isTrue();
+    assertThat(response.body()).isSameAs(Unit.INSTANCE);
+  }
+
+  @Ignore("This is implicitly tested by integration tests of the adapters and converters")
+  @Test public void unitMissingFromClasspath() {
+  }
+}