could not convert {...} from <brace-enclosed initializer list> to struct

The problem is here:

struct Test
{
    int x = 0; // <==
    Vector2 v;
};

Until recently, default member initializer prevent the class from being an aggregate, so you cannot use aggregate initialization on them. Gcc 4.9 still implements the old rules here, whereas gcc 5 uses the new ones.

You missed ; after your class definition and after int x = 0. Then you got many errors and apparently only considered the last one. But your compiler was confused because Vector2 was not defined (due to missing ;).

This compiles:

int main()
{
    class Vector2
    {
    public:
        Vector2(float x, float y)
        {
            this->x = x;
            this->y = y;
        }
        float x = 0.f;
        float y = 0.f;
    };

    struct Test
    {
        int x;
        Vector2 v;
    };

    Test tst = {0,Vector2(4,5)};
    return 0;
}