Static initialization of std::map to unique_ptr

Question

I need a map to a polymorphic set of objects, which requires a map to pointers to avoid slicing. Since I would like the life-cycle of the value-objects to be tied to that of the map object, I would like to use unique_ptr.

This leads me to the following construction:

#include <memory>
#include <string>
#include <map>
#include <vector>

struct Shape {
    virtual double area() const = 0;
    virtual ~Shape() {};
};
struct Square : public Shape {
    double a;
    Square(double a) { this->a = a; };
    double area() const { return a*a; };
};

typedef std::map<std::string, std::unique_ptr<Shape>> ShapeMap;

// Creating an instance of ShapeMap with a factory is ok:
ShapeMap buildShapeMap() {
    ShapeMap m;
    m.emplace("square1", std::make_unique<Square>(2));
    return m;
}
ShapeMap m1 = buildShapeMap();

But, what I would really like to do is a static constructor:

/* Fails: std::pair<const _Kty,_Ty>::pair(const std::pair<const _Kty,_Ty> &)': attempting to reference a deleted function
ShapeMap m2{
    { "square1", std::make_unique<Square>(2) }
};
*/

I assume the move semantics are somehow different here -- but is it something I can get around, or should I just stick with the factory-approach?

As a followup question: Should I expect similar issues in the following?

struct World {
    std::string name;
    ShapeMap shapes;
    World(std::string const &name, ShapeMap &shapes) : name(name), shapes{ shapes } {
    };
};

World world1{ 
    ShapeMap{
        { "square2", std::make_unique<Square>(3) }
    } 
};