Why is the struct-like constructor removed when I define virtual function?

Question

Suppose I define the following class:

class A
{
public:
    int x;
};

Now I can initialize it like a struct:

A a { .x = 1; }

However, when I define any virtual function, the constructor goes away:

class A
{
public:
    int x;
    virtual int f() {}
};

A a { .x = 1; }

The message is:

# error: no matching function for call to ‘A::A(<brace-enclosed initializer list>)’

Why does a virtual function delete that constructor?