[Bug c++/67376] New: Comparison with pointer to past-the-end of array fails inside constant expression

[ldionne.2 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67376

            Bug ID: 67376
           Summary: Comparison with pointer to past-the-end of array fails
                    inside constant expression
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ldionne.2 at gmail dot com
  Target Milestone: ---

The following code fails to compile on GCC trunk:

    struct array {
        int elems_[1];
    };

    constexpr array a{{0}};
    static_assert(a.elems_ != a.elems_ + 1, "");

The error (formatted to fit the report) is

    [snip]: error: non-constant condition for static assertion
     static_assert(a.elems_ != a.elems_ + 1, "");
     ^
    [snip]: error: ‘(((const int*)(& a.array::elems_)) != 
                    (((const int*)(& a.array::elems_)) + 4u))’ 
            is not a constant expression
     static_assert(a.elems_ != a.elems_ + 1, "");
                            ^

It seems like GCC does not like the fact that we're comparing a past-the-end
pointer. But then again, that's weird because the following code works fine:

    constexpr int a[1] = {0};
    static_assert(a != a + 1, "");

Live example: http://melpon.org/wandbox/permlink/4QKMVN5vm3ePJcpY