llvm::SmallVectorImpl<Type>* region_arg_types) {
  // No operands or empty operand list.
  bool parsed_l_paren = succeeded(parser->parseOptionalLParen());
  if (!parsed_l_paren || succeeded(parser->parseOptionalRParen()))
    return success();

  // Parse comma separated operands of the following format:
  //   replicated_input
  //     [%a, ...] as %block_arg0: type
  //   packed_input
  //     %b as %block_arg1: type