For this code:

struct Vec2 {
  float x,y;
  Vec2():x(0),y(0) {
    printf( "Construct Vec2 %f %f",x,y );
  }
};
struct Rectangle {
  union {
    struct { float x,y; };
    Vec2 pos;
  };
  union {
    struct { float w,h; };
    Vec2 size;
  };

  RectF(){
    printf( "construct RectF %f %f %f %fn", x,y,w,h );
  }
}

int main() {
  
  Rectangle r;

}

I was very surprised to learn that the default constructors for Vec2 are not triggered for the Vec2s that are inside the unions inside the Rectangle

I was very surprised by that wonder why that is?