Adding support for generic builder interface.

integration-tests/src/main/java/com/example/GenericComponentBuilderInterface.java

@@ -0,0 +1,18 @@
+package com.example;
+
+import dagger.BindsInstance;
+import dagger.Component;
+
+@Component
+public interface GenericComponentBuilderInterface {
+  String value();
+
+  @Component.Builder
+  interface Builder extends GenericBuilder<GenericComponentBuilderInterface> {
+    Builder bindString(@BindsInstance String instance);
+  }
+
+  public interface GenericBuilder<Component> {
+    Component build();
+  }
+}

integration-tests/src/test/java/com/example/IntegrationTest.java

@@ -1450,6 +1450,14 @@ public void subcomponentFactoryProvision() {
     assertThat(nested.two()).isEqualTo(2L);
   }
 
+  @Test
+  public void genericComponentBuilderInterface() {
+    GenericComponentBuilderInterface component =
+        backend.builder(GenericComponentBuilderInterface.Builder.class).bindString("one").build();
+
+    assertThat(component.value()).isEqualTo("one");
+  }
+
   @Test
   public void componentBindingInstance() {
     ComponentBindingInstance instance = backend.create(ComponentBindingInstance.class);

reflect/src/main/java/dagger/reflect/ComponentBuilderInvocationHandler.java

@@ -28,7 +28,9 @@
 import java.lang.reflect.InvocationHandler;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
+import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
+import java.lang.reflect.TypeVariable;
 
 final class ComponentBuilderInvocationHandler implements InvocationHandler {
   static <B> B forComponentBuilder(Class<B> builderClass) {
@@ -87,7 +89,7 @@ public Object invoke(Object proxy, Method method, Object[] args) throws Throwabl
       return method.invoke(proxy, args);
     }
 
-    Class<?> returnType = method.getReturnType();
+    Type returnType = resolveReturnType(method);
     Type[] parameterTypes = method.getGenericParameterTypes();
     Annotation[][] parameterAnnotations = method.getParameterAnnotations();
 
@@ -166,4 +168,26 @@ public Object invoke(Object proxy, Method method, Object[] args) throws Throwabl
 
     throw new IllegalStateException(method.toString()); // TODO report unsupported method shape
   }
+
+  private Type resolveReturnType(Method method) {
+    Type genericReturnType = method.getGenericReturnType();
+
+    if (genericReturnType != null) {
+      Class<?> declaringClass = method.getDeclaringClass();
+      TypeVariable<? extends Class<?>>[] typeParameters = declaringClass.getTypeParameters();
+      for (int i = 0; i < typeParameters.length; i++) {
+        if (typeParameters[i].equals(genericReturnType)) {
+          Type[] genericInterfaces = builderClass.getGenericInterfaces();
+          for (Type genericInterface : genericInterfaces) {
+            ParameterizedType implementationType = (ParameterizedType) genericInterface;
+            if (implementationType.getRawType().equals(declaringClass)) {
+              return implementationType.getActualTypeArguments()[i];
+            }
+          }
+        }
+      }
+    }
+
+    return method.getReturnType();
+  }
 }