[Bug c++/65685] Reducing alignment with alignas should be rejected

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65685

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org
      Known to fail|                            |4.5.3, 4.8.3, 4.9.3, 5.3.0,
                   |                            |6.0

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Still fails with the latest trunk and always has.

Below is a slightly modified test case to compare the behavior between C++11
(using alignas), C11 (using _Alignas), and prior standards (using GCC's
__attribute__ aligned).  As expected, GCC in C11 mode rejects the _Alignas
specifier iB with the error below:
u.cpp:10:32: error: â_Alignasâ specifiers cannot reduce alignment of âaâ

Both gcc and g++ silently accept but ignore __attribute__ aligned in B (it
would be nice if they issued a warning to point that out).

Finally, as noted, g++ in C++11 mode silently accepts the code:

$ cat u.cpp && /home/msebor/build/gcc-trunk-svn/gcc/xgcc
-B/home/msebor/build/gcc-trunk-svn/gcc -S -Wall -Wextra -Wpedantic -std=c++11
-xc++ u.cpp
#if __cplusplus >= 201103L
#  define Align(N)   alignas (N)
#elif __STDC_VERSION__ >= 201112L
#  define Align(N)   _Alignas (N)
#else
#  define Align(N) __attribute__ ((aligned (N)))
#endif

typedef struct A { Align (8) char c; } A;
typedef struct B { Align (1) A a; } B;

#define A(e) typedef char Assert [1 - 2 * !(e)]

A (__alignof__ (A) == 8);
A (__alignof__ (B) == 8);