[expr.prim.id.general] p2 The first note in the rule is a bit misleading

[expr.prim.id.general] p2 says:

If an id-expression E denotes a member M of an anonymous union ([class.union.anon]) U:

• If U is a non-static data member, E refers to M as a member of the lookup context of the terminal name of E (after any transformation to a class member access expression ([class.mfct.non.static])).

[Example 1: o.x is interpreted as o.u.x, where u names the anonymous union member. — end example]

IIUC, the intent of the first bullet actually means the id-expression that would denote the member of U(conceptually designated by u.x) will be reinterpreted as o.x as if x is the member of o.

Consider this case:

struct A{
   union {
    int a;
  };
  void show(){
    int c =  this->a;  // #1
  }
};

The id-expression a denotes the member a of the anonymous union, and the terminal name of a is a, whose lookup context is just A, as per [basic.lookup.qual.general] p2. Hence, the intent of the rule may want to say a is interpreted as the member of A.

However, the note conveys a converse meaning with the intent, I think.

Start with [expr.prim.id.general] p2, the quoted first bullet, and Example 1, then compare them with the A::show example in this issue. Revise the note so it does not suggest the converse interpretation of the rule, and verify that the wording matches the intended treatment of the anonymous-union member.