[Bug c++/69585] New: [C++ 11] parser errors mixing alignas, C++ 11 and GNU attributes on class declaration

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69585

            Bug ID: 69585
           Summary: [C++ 11] parser errors mixing alignas, C++ 11 and GNU
                    attributes on class declaration
           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: ---

The C++ parser seems to have trouble combining the C++11 alignas specifier and
the GNU __attribute__ specifier in the same class declaration.  It also doesn't
like pairing C++11 attribute specifiers with GNU __attribute__.  Pairing
alignas and the C++ 11 [[attribute]] syntax seems to work.

Clang does somewhat better (it handles GNU __attribute__ and C++11
[[attribute]] but it too chokes on alignas and GNU attributes and on C++
[[attribute]] and GNU __attribute__.

The following example shows the problematic interactions between C++ 11
alignas, C++ 11 [[gnu::may_alias]], and GNU __attribute__((mayalias)).

Since other similar syntactic combinations are accepted (such as the common
inline and __attribute__((always_inline))), I expected alignas and
__attribute__((whatever)) to be as well.  I couldn't find anything about this
restriction is in manual.

$ cat v.c && /build/gcc-trunk/gcc/xgcc -B /build/gcc-trunk/gcc -O2 -S -Wall
-Wextra -Wpedantic -Wuninitialized -std=c++11 -xc++ v.c
struct __attribute__ ((aligned (2))) __attribute__ ((may_alias))
S1 { };

struct __attribute__ ((aligned (2))) [[gnu::may_alias]]
S2 { };

struct alignas (2) __attribute__ ((may_alias))
S3 { };

struct alignas (2) [[gnu::may_alias]]
S4 { };


struct __attribute__ ((may_alias)) __attribute__ ((aligned (2)))
S1_2 { };

struct [[gnu::may_alias]] __attribute__ ((aligned (2)))
S2_2 { };

struct __attribute__ ((may_alias)) alignas (2)
S3_2 { };

struct [[gnu::may_alias]] alignas (2)
S4_2 { };

v.c:4:38: error: expected identifier before ‘[’ token
 struct __attribute__ ((aligned (2))) [[gnu::may_alias]]
                                      ^
v.c:4:38: warning: attribute ignored [-Wattributes]
v.c:4:38: note: an attribute that appertains to a type-specifier is ignored
v.c:7:20: error: expected identifier before ‘__attribute__’
 struct alignas (2) __attribute__ ((may_alias))
                    ^~~~~~~~~~~~~
v.c:17:27: error: expected identifier before ‘__attribute__’
 struct [[gnu::may_alias]] __attribute__ ((aligned (2)))
                           ^~~~~~~~~~~~~
v.c:20:36: error: expected identifier before ‘alignas’
 struct __attribute__ ((may_alias)) alignas (2)
                                    ^~~~~~~
v.c:20:36: warning: attribute ignored [-Wattributes]
v.c:20:36: note: an attribute that appertains to a type-specifier is ignored