Initialize Any Union Member (designated initializer)

Question

When declaring a union, eg

union u
{
  int x;
  float y;
};

you can construct the union as follows:

u integer { .x = 10 };
u fraction { .y = 10.f };

Suppose I have a pointer to member,

auto union_member_pointer = &u::y;

What syntax allows me to abstractly assign to that member of the union? ie

u unknown { .*union_member_pointer = { } }; // doesn't work

Answer 1

Designated initialization is not permitted to be dynamic. It is applied to the names of members, not of member pointers or anything of the like.