[Bug c++/113919] New: Nested anonymous unions should be prohibited in ISO C++

[Bug c++/113919] New: Nested anonymous unions should be prohibited in ISO C++

[tsqurt at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113919

            Bug ID: 113919
           Summary: Nested anonymous unions should be prohibited in ISO
                    C++
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tsqurt at outlook dot com
  Target Milestone: ---

Created attachment 57425
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57425&action=edit
example.cpp

Anonymous unions shall not be declared within an anonymous union. But gcc never
warns about this. 

Demo source code as a C++17 source file attached. Note: attached code is a
valid C 
source code but not a valid C++ source code. GCC is expected to REJECT or WARN,
but it compiles without any message.

$gcc example.cpp -std=c++17 --pedantic -Wall -Wextra

expect:
warning: ISO C++ prohibits anonymous unions within anonymous unions[-Wpedantic]

But gcc exited with code 0, leaving empty message.

Reference: N4849, Working Draft, Standard for Programming Language C++, Chapter
11, Section 5.1 Anonymous unions. (Different versions of modern C++
specification mention this issue in their correspondent chapters.)

[Bug c++/113919] Nested anonymous unions should be prohibited in ISO C++

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113919

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-02-14
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |accepts-invalid
     Ever confirmed|0                           |1
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
[class.union.anon]/1: Nested types, anonymous unions, and functions shall not
be declared within an anonymous union.

[Bug c++/113919] Nested anonymous unions should be prohibited in ISO C++

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113919

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I suppose this should fix it but there are testsuite FAILs.

--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -5522,6 +5522,12 @@ fixup_anonymous_aggr (tree t)
        }
        }
    }
+      /* [class.union.anon]/1: Nested types, anonymous unions, and functions
+    shall not be declared within an anonymous union.  */
+      else if (ANON_UNION_TYPE_P (TREE_TYPE (probe)))
+   pedwarn (DECL_SOURCE_LOCATION (probe), OPT_Wpedantic,
+        "anonymous unions cannot be declared within an anonymous "
+        "union");
       }

   /* Splice all functions out of CLASSTYPE_MEMBER_VEC.  */

[Bug c++/113919] Nested anonymous unions should be prohibited in ISO C++

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113919

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
clang++ has -Wnested-anon-types so I think we need that too.

[Bug c++/113919] Nested anonymous unions should be prohibited in ISO C++

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113919

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Ah, we have a dup.

*** This bug has been marked as a duplicate of bug 72751 ***