code: private boolean checkImplementer(TypeElement providerImplementer, TypeElement providerType) {
  
      String verify = processingEnv.getOptions().get("verify");
      if (verify == null || !Boolean.valueOf(verify)) {
        return true;
      }
  
      // TODO: We're currently only enforcing the subtype relationship
      // constraint. It would be nice to enforce them all.
  
      Types types = processingEnv.getTypeUtils();
  
      return types.isSubtype(providerImplementer.asType(), providerType.asType());
    }
docs: Verifies {@link ServiceProvider} constraints on the concrete provider class.
  Note that these constraints are enforced at runtime via the ServiceLoader,
  we're just checking them at compile time to be extra nice to our users.