[Bug c++/104944] incorrect alignas(void) accepted (with warning if templated)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104944

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-03-16
             Status|UNCONFIRMED                 |NEW
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=85979
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Björn Fahller from comment #0)
> The erroneous code:
> 
> struct alignas(void) S{};
> 
> Is accepted without any diagnostic with g++
> (Compiler-Explorer-Build-gcc-98cd717fca9f21625b9c79c9231c2e909d1d93a3-
> binutils-2.36.1) 12.0.1 20220306 (experimental)
> 
> https://godbolt.org/z/q7das4G1W

Confirmed.

> With 'void' coming from a template, a confusing warning is issued:
> 
> template <typename T>
> struct alignas(T) S {};
> 
> S<void> s;
> 
> The message is:
> 
> In instantiation of 'struct S<void>':
> <source>:4:9:   required from here
> <source>:2:16: warning: invalid application of '__alignof__' to a void type
> [-Wpointer-arith]
>     2 | struct alignas(T) S {};
>       |                ^


This is accepted with a warning (rather than an error) because of the
https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html extension. 

> This is not entirely incorrect, but I believe most programmers would
> recognize that they have no `__alignof__` in their code, nor any pointer
> arithmetics.

That's PR 85979.