// (args, kwargs), where args is an empty tuple, and kwargs is a dictionary of
  // string keys to TensorSpecs.
  if (!canonicalized_input_signature.has_tuple_value()) {
    return absl::FailedPreconditionError(absl::StrCat(
        "SignatureDefFunction's canonicalized_input_signature should be "
        "of form tuple(tuple(), dict()), but was instead: \n",

  // conditions 2 and 3 are trivially satisfied.
  // We need to ensure that:
  // flatten(input_signature).size() + captures.size() = fdef.signature().size()
  // A concrete function's serialized "canonicalized_input_signature" comes
  // from encoding its "structured_input_signature" field:
  // https://github.com/tensorflow/tensorflow/blob/1c064ab76064c58e54261b805027474885a1534d/tensorflow/python/saved_model/function_serialization.py#L70-L71