[Bug c++/77539] New: gcc-5/6: comparison to nullptr failure in constexpr (fixed by r235506 on trunk)

[trippels at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77539

            Bug ID: 77539
           Summary: gcc-5/6: comparison to nullptr failure in constexpr
                    (fixed by r235506 on trunk)
           Product: gcc
           Version: 6.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: trippels at gcc dot gnu.org
                CC: nathan at gcc dot gnu.org
  Target Milestone: ---

In
http://stackoverflow.com/questions/39405241/constexpr-comparision-to-nullptr-bug-or-feature/
a user posted the following testcase:

trippels@gcc2-power8 ~ % cat const2.ii
constexpr int foobar()
{
  int array[100] = {};
  int *ar = array;
  if (ar == nullptr) // Error...
  {
    return 0;
  }
  return 1;
}
static_assert(foobar(),"");

trippels@gcc2-power8 ~ % ~/gcc_6/usr/local/bin/g++ -c const2.ii
const2.ii:12:1: error: non-constant condition for static assertion
 static_assert(foobar(),"");
 ^~~~~~~~~~~~~
const2.ii:12:21:   in constexpr expansion of ‘foobar()’
const2.ii:5:10: error: ‘(((int*)(& array)) == 0u)’ is not a constant expression
   if (ar == nullptr) // Error...
       ~~~^~~~~~~~~~

This was fixed on trunk by r235506.

Nathan, would could you please backport your fix to gcc-6 branch?