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

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

[gccbugbjorn at fahller dot se]

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

            Bug ID: 104944
           Summary: incorrect alignas(void) accepted (with warning if
                    templated)
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gccbugbjorn at fahller dot se
  Target Milestone: ---

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

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

[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.

[Bug c++/104944] [9/10/11/12 Regression] 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
----------------------------------------------------------------------------
            Summary|incorrect alignas(void)     |[9/10/11/12 Regression]
                   |accepted (with warning if   |incorrect alignas(void)
                   |templated)                  |accepted (with warning if
                   |                            |templated)
      Known to fail|                            |12.0, 5.1.0
                 CC|                            |mpolacek at gcc dot gnu.org
      Known to work|                            |4.9.4

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Before GCC 5.1 the non-template case was rejected:

104944.C:1:22: error: requested alignment is not an integer constant
 struct alignas(void) S{};
                      ^

That changed with r210262:


Author: Marek Polacek
Date:   Fri May 9 09:24:37 2014

    re PR c/50459 (alignof doesn't work on plain old constant, works with
expressions containing it)

[Bug c++/104944] [9/10/11/12 Regression] incorrect alignas(void) accepted (with warning if templated)

[mpolacek at gcc dot gnu.org]

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

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |9.5
             Status|NEW                         |ASSIGNED
           Priority|P3                          |P2
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Mine then.

[Bug c++/104944] [9/10/11/12 Regression] incorrect alignas(void) accepted (with warning if templated)

[cvs-commit at gcc dot gnu.org]

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

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:d0b938a7612fb7acf1f181da9577235c83ede59e

commit r12-7815-gd0b938a7612fb7acf1f181da9577235c83ede59e
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Mar 23 17:12:29 2022 -0400

    c++: alignas and alignof void [PR104944]

    I started looking into this PR because in GCC 4.9 we were able to
    detect the invalid

      struct alignas(void) S{};

    but I broke it in r210262.

    It's ill-formed code in C++:
    [dcl.align]/3: "An alignment-specifier of the form alignas(type-id) has
    the same effect as alignas(alignof(type-id))", and [expr.align]/1:
    "The operand shall be a type-id representing a complete object type,
    or an array thereof, or a reference to one of those types." and void
    is not a complete type.

    It's also invalid in C:
    6.7.5: _Alignas(type-name) is equivalent to _Alignas(_Alignof(type-name))
    6.5.3.4: "The _Alignof operator shall not be applied to a function type
    or an incomplete type."

    We have a GNU extension whereby we treat sizeof(void) as 1, but I assume
    it doesn't apply to alignof, at least in C++.  However, __alignof__(void)
    is still accepted with a -Wpedantic warning.

    We still say "invalid application of 'alignof'" rather than 'alignas' in
the
    void diagnostic, but I felt that fixing that may not be suitable as part of
    this patch.  The "incomplete type" diagnostic still always prints
    '__alignof__'.

            PR c++/104944

    gcc/cp/ChangeLog:

            * typeck.cc (cxx_sizeof_or_alignof_type): Diagnose alignof(void).
            (cxx_alignas_expr): Call cxx_sizeof_or_alignof_type with
            complain == true.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/alignas20.C: New test.

[Bug c++/104944] [9/10/11 Regression] incorrect alignas(void) accepted (with warning if templated)

[mpolacek at gcc dot gnu.org]

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

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[9/10/11/12 Regression]     |[9/10/11 Regression]
                   |incorrect alignas(void)     |incorrect alignas(void)
                   |accepted (with warning if   |accepted (with warning if
                   |templated)                  |templated)

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed on trunk so far.  I guess I might backport to 11 too.

[Bug c++/104944] [9/10/11 Regression] incorrect alignas(void) accepted (with warning if templated)

[cvs-commit at gcc dot gnu.org]

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

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Marek Polacek
<mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:fe641f6a44959ff8b9d9f7af11b8b806c5f1375a

commit r11-9714-gfe641f6a44959ff8b9d9f7af11b8b806c5f1375a
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Mar 23 17:12:29 2022 -0400

    c++: alignas and alignof void [PR104944]

    I started looking into this PR because in GCC 4.9 we were able to
    detect the invalid

      struct alignas(void) S{};

    but I broke it in r210262.

    It's ill-formed code in C++:
    [dcl.align]/3: "An alignment-specifier of the form alignas(type-id) has
    the same effect as alignas(alignof(type-id))", and [expr.align]/1:
    "The operand shall be a type-id representing a complete object type,
    or an array thereof, or a reference to one of those types." and void
    is not a complete type.

    It's also invalid in C:
    6.7.5: _Alignas(type-name) is equivalent to _Alignas(_Alignof(type-name))
    6.5.3.4: "The _Alignof operator shall not be applied to a function type
    or an incomplete type."

    We have a GNU extension whereby we treat sizeof(void) as 1, but I assume
    it doesn't apply to alignof, at least in C++.  However, __alignof__(void)
    is still accepted with a -Wpedantic warning.

    We still say "invalid application of 'alignof'" rather than 'alignas' in
the
    void diagnostic, but I felt that fixing that may not be suitable as part of
    this patch.  The "incomplete type" diagnostic still always prints
    '__alignof__'.

            PR c++/104944

    gcc/cp/ChangeLog:

            * typeck.c (cxx_sizeof_or_alignof_type): Diagnose alignof(void).
            (cxx_alignas_expr): Call cxx_sizeof_or_alignof_type with
            complain == true.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/alignas20.C: New test.

    (cherry picked from commit d0b938a7612fb7acf1f181da9577235c83ede59e)

[Bug c++/104944] [9/10 Regression] incorrect alignas(void) accepted (with warning if templated)

[mpolacek at gcc dot gnu.org]

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

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[9/10/11 Regression]        |[9/10 Regression] incorrect
                   |incorrect alignas(void)     |alignas(void) accepted
                   |accepted (with warning if   |(with warning if templated)
                   |templated)                  |

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.

[Bug c++/104944] [9/10 Regression] incorrect alignas(void) accepted (with warning if templated)

[mpolacek at gcc dot gnu.org]

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

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
.

[Bug c++/104944] [9/10 Regression] incorrect alignas(void) accepted (with warning if templated)

[pinskia at gcc dot gnu.org]

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

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|12.0                        |11.2.0
   Target Milestone|9.5                         |11.3
      Known to work|                            |11.3.0, 12.1.0