[Bug c++/80593] [7/8 Regression] GCC 7, aligned_storage and “dereferencing type-punned pointer will break strict-aliasing rules”

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80593

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2017-05-02
   Target Milestone|---                         |7.2
            Summary|GCC 7, aligned_storage and  |[7/8 Regression] GCC 7,
                   |“dereferencing type-punned  |aligned_storage and
                   |pointer will break          |“dereferencing type-punned
                   |strict-aliasing rules”      |pointer will break
                   |                            |strict-aliasing rules”
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  This likely regressed with the introduction of
TYPE_TYPELESS_STORAGE which aligned_storage now is, and thus 'storage' gets
alias-set zero itself running into

          /* warn_strict_aliasing >= 3.   This includes the default (3).
             Only warn if the cast is dereferenced immediately.  */
          alias_set_type set1
            = get_alias_set (TREE_TYPE (TREE_OPERAND (expr, 0)));
          alias_set_type set2 = get_alias_set (TREE_TYPE (type));

          if (set1 != set2 && set2 != 0
              && (set1 == 0
                  || (!alias_set_subset_of (set2, set1)
                      && !alias_sets_conflict_p (set1, set2))))
            {
              warning (OPT_Wstrict_aliasing, "dereferencing type-punned "
                       "pointer will break strict-aliasing rules");
              return true;

with set1 == 0.