[Bug c++/69571] New: [C++11] invalid alignas on a typedef accepted, reduces alignment

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69571

            Bug ID: 69571
           Summary: [C++11] invalid alignas on a typedef accepted, reduces
                    alignment
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

GCC disregards some cases of invalid application of the alignment specifier
(issuing a warning pointing them out) but misses others.

In the program below, the last three instances the alignas specifier are
invalid for two reasons: first, the specifier is not permitted on typedefs, and
second, it's not allowed to relax the alignment requirement of a type.

$ cat z.c && /build/gcc-trunk/gcc/xgcc -B /build/gcc-trunk/gcc -O2 -S -Wall
-Wextra -Wpedantic -std=c++14 -xc++ z.c
typedef struct A { alignas (32) int c; } A;

#define Assert(e) static_assert (e, #e)

typedef alignas (1) A A1_1;
Assert (alignof (A1_1) == alignof (A));

typedef A alignas (1) A1_2;
Assert (alignof (A1_2) == alignof (A));

typedef A A1_3 alignas (1);
Assert (alignof (A1_3) == alignof (A));
z.c:5:9: warning: attribute ignored [-Wattributes]
 typedef alignas (1) A A1_1;
         ^~~~~~~
z.c:5:9: note: an attribute that appertains to a type-specifier is ignored
z.c:8:11: warning: attribute ignored [-Wattributes]
 typedef A alignas (1) A1_2;
           ^~~~~~~
z.c:8:11: note: an attribute that appertains to a type-specifier is ignored
z.c:3:19: error: static assertion failed: alignof (A1_3) == alignof (A)
 #define Assert(e) static_assert (e, #e)
                   ^
z.c:12:1: note: in expansion of macro ‘Assert’
 Assert (alignof (A1_3) == alignof (A));
 ^~~~~~


Clang rejects all three instances with a hard error.