c++ - Can virtual functions be constexpr?

Question

Can virtual functions like X::f() in the following code

struct X 
{
    constexpr virtual int f() const 
    {
        return 0;
    }
};

be constexpr?

Answer 1

This answer is no longer correct as of C++20.

No. From [dcl.constexpr]/3 (7.1.5, "The constexpr specifier"):

The definition of a constexpr function shall satisfy the following requirements:

— it shall not be virtual