[Bug c++/110312] New: -Wcast-align=strict warning despite alignas

[Bug c++/110312] New: -Wcast-align=strict warning despite alignas

[f.heckenbach@fh-soft.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110312

            Bug ID: 110312
           Summary: -Wcast-align=strict warning despite alignas
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: f.heckenbach@fh-soft.de
  Target Milestone: ---

% cat test.cpp
alignas (int) char a[sizeof (int)];
auto b = reinterpret_cast <int &> (a);
% g++ -c -Wcast-align=strict test.cpp  
test.cpp:2:10: warning: cast from 'char (*)[4]' to 'int*' increases required
alignment of target type [-Wcast-align]

I'd kind of understand the warning if I was casting &a to int* because the
alignas information might get lost when taking the address. So I tried casting
a reference, but apparently (telling from the message), gcc implements this via
a pointer cast anyway, so it doesn't help.

FWIW, I got this when I tried to replace aligned_storage_t (which is deprecated
in C++23 for reasons I don't really understand TBH) with a std::byte array as
suggested. Does this mean that I simply can't use -Wcast-align=strict then?

[Bug c++/110312] -Wcast-align=strict warning despite alignas

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110312

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The decl has the increased alignment but the type does not in this case.

So I think the warning is still correct.

[Bug c++/110312] -Wcast-align=strict warning despite alignas

[f.heckenbach@fh-soft.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110312

--- Comment #2 from Frank Heckenbach <f.heckenbach@fh-soft.de> ---
(In reply to Andrew Pinski from comment #1)
> The decl has the increased alignment but the type does not in this case.
> 
> So I think the warning is still correct.

So there's no way around it other then disabling the warning, correct?

[Bug c++/110312] -Wcast-align=strict warning despite alignas

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110312

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-06-20
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think the warning isn't useful at least, maybe it can be improved to not
diagnose in the case where we can see an appropriately aligned decl (with some
effort).