How to fix expression must have a constant value in C++

I am trying to write a program that hashes a string to use it in a switch and when I call the hash function I get the error : expression must have a constant value

The code in question is:

unsigned int hashString(const std::string string_input, int hash_seed)
{
  {
    return !string_input[0] 
    ? 33129 : (hashString(string_input, 1) 
    * 54) ^ string_input[0];
  }
}




bool evaluateString(std::string str)
{
  string first_word;
  stringstream inputStream { str };

    commandStream >> first_word;

    switch (hashString(first_word, 0))
    {
    case hashString(ipo::Io::STRING_HELLO, 0):
      /* code */
      break;

    default:
      break;
    }


}

The error occurs here: case hashString(ipo::Io::STRING_HELLO, 0):

It marks the ipo as a problem

How could I fix it

Thank you for your help