For each explicitly initialized element:

• If the element is an anonymous union member and the initializer list is a brace-enclosed designated-initializer-list, the element is initialized by the designated-initializer-list { D }, where D is the designated-initializer-clause naming a member of the anonymous union member. There shall be only one such designated-initializer-clause.

How could designated-initializer-list be the form { D }? According to the grammar of designated-initializer-list, it should be the comma-separated list of .identifier brace-or-equal-initializer. The above rule seems to phrase the following example

struct C{
   union {
     int a;
   };
};
C c = {{.a = 0}};

In this example, the corresponding initializer of the anonymous union member is the initializer-clasuse, which is a brace-enclosed designated-initializer-list. However, the formal example under [dcl.init.aggr]/4.1 is that

struct C {
  union {
    int a;
    const char* p;
  };
  int x;
} c = { .a = 1, .x = 3 };

That is, the original intent of the above rule is used to phrase the case where the explicitly initialized element is an anonymous union member and the corresponding initializer is a designated-initializer-clause naming the member of it. Should we reword the rule to that

If the element is an anonymous union member and the initializer D is a designated-initializer-clause naming a member of the anonymous union member, the element is initialized by the initializer { D }. There shall be only one such designated-initializer-clause for the anonymous union member.