Brace Initialize struct with virtual functions

Brace initialization

        struct A
        {
            int a;
            int b;
            void foo(){}
        };

        A a{1, 2};

It works fine. But if change foo to a virtual function, It will not compile with error,

Error C2440 'initializing': cannot convert from 'initializer list' to

I find this,

An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).

But what's the reason behind it?