[Bug c/77650] struct with a nested flexible array followed by another member accepted

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77650

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Qing Zhao <qinzhao@gcc.gnu.org>:

https://gcc.gnu.org/g:070a6bf0bdc6761ad77ac97404c98f00a7007d54

commit r14-2197-g070a6bf0bdc6761ad77ac97404c98f00a7007d54
Author: Qing Zhao <qing.zhao@oracle.com>
Date:   Thu Jun 29 17:07:26 2023 +0000

    Update documentation to clarify a GCC extension [PR c/77650]

    on a structure with a C99 flexible array member being nested in
    another structure.

    "The GCC extension accepts a structure containing an ISO C99 "flexible
array
    member", or a union containing such a structure (possibly recursively)
    to be a member of a structure.

     There are two situations:

       * A structure containing a C99 flexible array member, or a union
         containing such a structure, is the last field of another structure,
         for example:

              struct flex  { int length; char data[]; };
              union union_flex { int others; struct flex f; };

              struct out_flex_struct { int m; struct flex flex_data; };
              struct out_flex_union { int n; union union_flex flex_data; };

         In the above, both 'out_flex_struct.flex_data.data[]' and
         'out_flex_union.flex_data.f.data[]' are considered as flexible
         arrays too.

       * A structure containing a C99 flexible array member, or a union
         containing such a structure, is not the last field of another
structure,
         for example:

              struct flex  { int length; char data[]; };

              struct mid_flex { int m; struct flex flex_data; int n; };

         In the above, accessing a member of the array
'mid_flex.flex_data.data[]'
         might have undefined behavior.  Compilers do not handle such a case
         consistently, Any code relying on this case should be modified to
ensure
         that flexible array members only end up at the ends of structures.

         Please use the warning option '-Wflex-array-member-not-at-end' to
         identify all such cases in the source code and modify them.  This
extension
         is now deprecated.
    "

            PR c/77650

    gcc/c-family/ChangeLog:

            * c.opt: New option -Wflex-array-member-not-at-end.

    gcc/c/ChangeLog:

            * c-decl.cc (finish_struct): Issue warnings for new option.

    gcc/ChangeLog:

            * doc/extend.texi: Document GCC extension on a structure containing
            a flexible array member to be a member of another structure.

    gcc/testsuite/ChangeLog:

            * gcc.dg/variable-sized-type-flex-array.c: New test.