[Bug c++/72751] New: anonymous union within an anonymous union accepted

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72751

            Bug ID: 72751
           Summary: anonymous union within an anonymous union accepted
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Quoting from C++14, [class.union.anon], p1:

Each member-declaration in the member-specification of an anonymous union shall
either define a non-static data member or be a static_assert-declaration.
[Note: Nested types, anonymous unions, and functions cannot be declared within
an anonymous union. -- end note]

That makes the following program ill-formed.  In C mode (where the program is
also ill-formed) GCC issues a warning for the nested union and an error for the
use of a.  However in C++ mode, G++ fails to issue a diagnostic for it, even in
pedantic mode.  Clang issues:

  warning: anonymous types declared in an anonymous union are an extension
[-Wnested-anon-types]

and EDG eccp 4.11:

   error: types cannot be declared in anonymous unions

$ cat t.C && gcc -S -Wall -Wextra -Wpedantic t.C
void f ()
{
  union { union { int a, b; }; };
  a = 0;
}