usage of constexpr variable as a case label

Is it correct to use constexpr variable as a case label.

#include <iostream>

int main() {
    constexpr int x = 5;

    int y = 4;

    switch (y) {
        case x - 1:
            std::cout << "case " << x << std::endl;

            break;

        case 20:
            std::cout << "case 20" << std::endl;

            break;

        default:
            std::cout << "case default" << std::endl;

            break;
    }
}

>Solution :

It is. cppreference says the case value must be

a constant expression of the same type as the type of condition after conversions and integral promotions

That includes constexpr expressions like a constexpr variable to which you subtract 1 like in your example.

In the standard, that’s [stmt.switch]:

Any statement within the switch statement can be labeled with one or more case labels as follows:
case constant-expression :
where the constant-expression shall be a converted constant expression of the adjusted type of the switch condition.
No two of the case constants in the same switch shall have the same value after conversion.

And a constant expression is anything in [expr.const].

For example, you can even use a constexpr function call:

constexpr int caseLabel() {
    return 4;
}
// ...
switch (y) {
    case caseLabel():