Warning: Statemen has no effect; in C ++ program switch type

std::cout expects to receive all the information to be displayed on the screen using operator << . ends when you add ;

In your case, you are adding a string, but you close the flow before adding also the variable tot_pagar :

cout<< "El total a pagar es "; tot_pagar;

The correct thing would be:

cout << "El total a pagar es " << tot_pagar << endl;

It is also advisable to add endl at the end, to specify the end of the line.

Greetings

First you have some details how to use ";" instead of "< <" since you are not allowing to add the value of tot_pagar .

You can also use cin.get(); to get only the first character (either A, B, or C).

#include <iostream>
using namespace std;

    int main()
    {
       int precio, tot_pagar;
       char opcion;

        cout<< "Ingrese el precio ";
        cin>> precio;
        cout<< endl;
        cout<< "Ingrese la opción de descuento ";
        cin>> opcion;
        cin.get(); //*** Haz uso de cin.get()
        switch (opcion)
        {
                case 'A':  tot_pagar = precio-precio*0.05;
                    cout<< "El total a pagar es " << tot_pagar;
                        break;
                case 'B':  tot_pagar = precio-precio*0.10;
                    cout<< "El total a pagar es " << tot_pagar;
                        break;
                case 'C':  tot_pagar = precio-precio*0.15;
                    cout<< "El total a pagar es " << tot_pagar;
                        break;
                default: cout<< "Su artículo no tiene descuento";
        }
        return 0;
    }