check std::cin variable type

i am learning about functions and function arguments. i am attempting to right a function called int repeat(int rep){...} below is the code for the function int repeat().

int repeat(int rep){
    std::cout << "Do you wish to factorial another number? Y or N: " <<
        std::endl;
    char c1;
    restart:
    std::cin >> c1; /* pretend user input is "7". the below if statement should execute the `throw`            statement. However it does not evaluate the if statement as true. and passes over it.*/
    try{
        if (std::cin.fail()){
            throw std::runtime_error(" Must enter y for yes or n for no.");
        }
        if (c1 == 'y' || c1 == 'Y'){
            return rep = 1;
        }
        else{
            //system("pause");
            return rep = 0;
        }
    }
    catch (std::runtime_error err2){
        std::cout << err2.what() << std::endl;
        std::cin.clear();
        std::cin.ignore();
        goto restart;
    }
}

the issue in my code appears with the first if statement: if (std::cin.fail()){ throw std::runtime_error(" Must enter y for yes or n for no."); }

i am attempting to use std::cin.fail() to test rather the input that is stored in the variable char c1 is in fact a char type. Example: if the user inputs a number into c1 then the if statement should execute the throw statement. however it does not happen. Can anyone assist in making the if statement execute correctly if a user inputs a non char type into the std::cin >> c1?

Note: i have used this method in a different part of the program to test if an input intostd::cin is of type int and the if statement works as intended.