为什么这样写的话，编译器会提示vecsize is not a type

#include <iostream>
#include <vector>
struct A
{
    static const int vecSize = 20;
    static std::vector<double> vec(vecSize);
};

但如果改成这样就没问题

#include <iostream>
#include <vector>
struct A
{
    static const int vecSize = 20;
    static std::vector<double> vec;
};
std::vector<double> A::vec(vecSize);