[Bug c++/111379] New: comparison between unequal pointers to void should be illegal during constant evaluation

[Bug c++/111379] New: comparison between unequal pointers to void should be illegal during constant evaluation

[pkeir at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111379

            Bug ID: 111379
           Summary: comparison between unequal pointers to void should be
                    illegal during constant evaluation
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pkeir at outlook dot com
  Target Milestone: ---

[DR 2526](https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#2526)
clarifies that in C++23 comparison between unequal pointers to void has
unspecified result. As unspecified behaviour cannot be executed in a constant
expression, the code below should not compile.

In fact without DR 2526, the comparison would have undefined behaviour, which
also cannot be executed in a constant expression.

constexpr bool compare_void_pointers()
{
  int a[2];
  void* vp1 = &a[0], * vp2 = &a[1];
  return vp1 < vp2;
}

static_assert(compare_void_pointers());

[Bug c++/111379] comparison between unequal pointers to void should be illegal during constant evaluation

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111379

Jiang An <de34 at live dot cn> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |de34 at live dot cn

--- Comment #1 from Jiang An <de34 at live dot cn> ---
There's (or will be) a new DR CWG2749 which tentatively ready now.
https://cplusplus.github.io/CWG/issues/2749.html

It seems that the old resolution in CWG2526 was wrong, and the comparison
should be constexpr-friendly.

BTW I don't think there was anything specifying that "the comparison would have
*undefined* behaviour" before CWG2526.

[Bug c++/111379] comparison between unequal pointers to void should be illegal during constant evaluation

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111379

Xi Ruoyao <xry111 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |xry111 at gcc dot gnu.org

--- Comment #2 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
(In reply to Jiang An from comment #1)
> There's (or will be) a new DR CWG2749 which tentatively ready now.
> https://cplusplus.github.io/CWG/issues/2749.html
> 
> It seems that the old resolution in CWG2526 was wrong, and the comparison
> should be constexpr-friendly.
> 
> BTW I don't think there was anything specifying that "the comparison would
> have *undefined* behaviour" before CWG2526.

It is (or was) unspecified, not undefined.  And the standard explicitly
disallows "a relational operator where the result is unspecified" in
[expr.const].

[Bug c++/111379] comparison between unequal pointers to void should be illegal during constant evaluation

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111379

--- Comment #3 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
If CWG 2749 is accepted we should just close this as WONTFIX.