final <T> T isSubtype(T sub) {
    Type returnType = method.getGenericReturnType();
    Type paramType = getOnlyParameterType();
    TestSubtype spec = method.getAnnotation(TestSubtype.class);
    assertWithMessage("%s is subtype of %s", paramType, returnType)
        .that(TypeToken.of(paramType).isSubtypeOf(returnType))
        .isTrue();
    assertWithMessage("%s is supertype of %s", returnType, paramType)

      } catch (NoSuchMethodException e) {
        throw new AssertionError("Annotation is missing required method", e);
      }
      Class<?> returnType = method.getReturnType();
      assertTrue(
          rootLocaleFormat("%s.%s() must return an array.", annotationClass, propertyName),
          returnType.isArray());
      assertSame(
          rootLocaleFormat(
              "%s.%s() must return an array of %s.",

   * }
   */
  public final <R1 extends R> Invokable<T, R1> returning(Class<R1> returnType) {
    return returning(TypeToken.of(returnType));
  }

  /** Explicitly specifies the return type of this {@code Invokable}. */
  public final <R1 extends R> Invokable<T, R1> returning(TypeToken<R1> returnType) {
    if (!returnType.isSupertypeOf(getReturnType())) {
      throw new IllegalArgumentException(

    ): Any? {
      val callArgs = args ?: arrayOf<Any?>()
      val methodName = method.name
      val returnType = method.returnType
      if (methodName == "supports" && Boolean::class.javaPrimitiveType == returnType) {
        return true // ALPN is supported.
      } else if (methodName == "unsupported" && Void.TYPE == returnType) {
        this.unsupported = true // Peer doesn't support ALPN.
        return null

    protected final String methodName;

    /** Array of method parameter types */
    protected final Class<?>[] parameterTypes;

    /** The return type of the method */
    protected final Class<?> returnType;

    /** Information about parameterized argument types */
    protected final ParameterizedClassDesc[] parameterizedClassDescs;

    /** Information about the parameterized return type */

    @SuppressWarnings("unchecked") // interfaceType is T
    T result = (T) dummy;
    return result;
  }

  /** Returns the dummy return value for {@code returnType}. */
  abstract <R> @Nullable R dummyReturnValue(TypeToken<R> returnType);

  private final class DummyHandler extends AbstractInvocationHandler implements Serializable {
    private final TypeToken<?> interfaceType;

    public static Class<?> getElementTypeOfCollectionFromReturnType(final Method method) {
        assertArgumentNotNull("method", method);

        final Type returnType = method.getGenericReturnType();
        return GenericsUtil.getRawClass(GenericsUtil.getElementTypeOfCollection(returnType));
    }

    /**
     * Returns the key type of the parameterized map declared as the method's argument type.
     *
     * @param method

    }

    /**
     * Specifies that only the methods that are declared to return {@code returnType} or its subtype
     * are tested.
     *
     * @return this tester object
     */
    @CanIgnoreReturnValue
    public FactoryMethodReturnValueTester thatReturn(Class<?> returnType) {
      this.returnTypeToTest = returnType;
      return this;
    }

    /**

      } catch (NoSuchMethodException e) {
        throw new AssertionError("Annotation is missing required method", e);
      }
      Class<?> returnType = method.getReturnType();
      assertTrue(
          rootLocaleFormat("%s.%s() must return an array.", annotationClass, propertyName),
          returnType.isArray());
      assertSame(
          rootLocaleFormat(
              "%s.%s() must return an array of %s.",

    }

    /**
     * Specifies that only the methods that are declared to return {@code returnType} or its subtype
     * are tested.
     *
     * @return this tester object
     */
    @CanIgnoreReturnValue
    public FactoryMethodReturnValueTester thatReturn(Class<?> returnType) {
      this.returnTypeToTest = returnType;
      return this;
    }

    /**