[Bug c++/82877] New: negative array index accepted in a pointer difference expression in constexpr context

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82877

            Bug ID: 82877
           Summary: negative array index accepted in a pointer difference
                    expression in constexpr context
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The following invalid program is required to be diagnosed because it has
undefined behavior due to the negative array index.  However, it is silently
accepted by G++.   It is rejected with an error by Clang 5.0, Intel ICC 17, and
Visual C++.

constexpr int f ()
{
  struct S { int a[1]; };
  S s = { 0 };

  return &s.a[0] - &s.a[-1];   // undefined, should be rejected
}

constexpr int i = f ();