The basics

1.6 Constants

About constexpr

The standards require that a constexpr function must have at least one set of arguments for which it yields a constant expression.

std::cout operations are not constexpr and never will be (they require runtime hardware interaction). I/O operations are inherently runtime side effects - they interact with the console, which doesn't exist during compilation.

There, a constexpr function that always performs I/O cannot exist (Unconditional I/O). If the I/O is conditional, then the function can be used for constant expressions as long as the constant-evaluated path avoids the I/O.