[Bug c++/70170] New: [6 regression] bogus not a constant expression error comparing pointer to array to null

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70170

            Bug ID: 70170
           Summary: [6 regression] bogus not a constant expression error
                    comparing pointer to array to null
           Product: gcc
           Version: 6.0
            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 example compiles successfully with 4.9.3 and 5 but fails with
6.0.  Note that the older compilers issue "warning: the address of âsâ will
ever be NULL [-Waddress]" for the "p0 == 0" expression.  GCC 6 does not.  That
seems like another regression.  Without bisecting it I suspect the delayed
folding changes to be responsible for both of these new problems.

$ cat t.c && /build/gcc-trunk/gcc/xgcc -B/build/gcc-trunk/gcc -S -Wall -Wextra
-Wpedantic -xc++ t.c
struct S { int a, b[1]; } s;

constexpr S *p0 = &s;
constexpr int *q0 = p0->b;

constexpr bool b0 = p0 == 0;
constexpr bool b1 = q0 == 0;

t.c:7:24: error: â((((int*)(& s)) + 4u) == 0u)â is not a constant expression
 constexpr bool b1 = q0 == 0;
                     ~~~^~~~