[C++ Patch] PR 44625

[Jason Merrill]

Actually, 9.5 says

A union of the form
   union { member-specification } ;
is called an anonymous union; it defines an unnamed object of unnamed 
type. The member-specification of an anonymous union shall only define 
non-static data members. [ Note: Nested types and functions cannot
be declared within an anonymous union. — end note ]

So we should be able to just reject nested anonymous aggregates and not 
worry about how to make them work.

Jason