Re: Alignas broken when used with constexpr array data member for structure

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On 9 January 2018 at 14:26, Mason wrote:
> On 09/01/2018 14:39, Alec Teal wrote:
>
>> In GCC 4.8.4 I have something like the following:
>
> FWIW, 4.8 is no longer supported (2013)
>
>> constexpr int x = 5;
>> constexpr int y = 4;
>> struct alignas(y) my_data_block {
>>     char data[x];
>> };
>>
>>
>> And it causes some weird errors to the tune of "size of array 'data' is
>> not an integral constant-expression" in the presence of the alignas
>
> A good support request should specify
> - exact compiler version (GCC 4.8.4)
> - a small testcase (given above)
> - exact compilation flags (missing)
> - exact error message
>
>
> Compiling the above source on godbolt using:
> gcc 4.8.4 -Wall -Wextra
>
> 1 : <source>:1:1: warning: identifier 'constexpr' is a keyword in C++11 [-Wc++0x-compat]
>  constexpr int x = 5;
>  ^
> 1 : <source>:1:1: error: 'constexpr' does not name a type
> 1 : <source>:1:1: note: C++11 'constexpr' only available with -std=c++11 or -std=gnu++11
> 2 : <source>:2:1: error: 'constexpr' does not name a type
>  constexpr int y = 4;
>  ^
> 2 : <source>:2:1: note: C++11 'constexpr' only available with -std=c++11 or -std=gnu++11
> 3 : <source>:3:19: error: expected initializer before 'my_data_block'
>  struct alignas(y) my_data_block {
>                    ^
> Compiler returned: 1
>
>
>
> Compiling the above source on godbolt using:
> gcc 4.8.4 -Wall -Wextra -std=c++11
>
> No errors.
>
>
> So it seems I can't reproduce the issue?
>
> Looking at Bugzilla as jwakely suggested turns up
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58601
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70124

Don't give a help vampire a direct answer, see
http://slash7.com/2006/12/22/vampires/

You just enable their bad behaviour.