[Bug c++/70816] New: bogus error __builtin_strcmp is not a constant expression in a constexpr function

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70816

            Bug ID: 70816
           Summary: bogus error __builtin_strcmp is not a constant
                    expression in a constexpr function
           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: ---

G++ accepts most calls to __builtin_strcmp with constant arguments in constexpr
contexts, except when the intrinsic is invoked on local arrays defined in a
constexpr function.  The test case below shows a number of accepted calls along
with the one that's incorrectly rejected.

$ cat u.c && /home/msebor/build/gcc-trunk-git/gcc/xgcc
-B/home/msebor/build/gcc-trunk-git/gcc -Wall -Wextra -Wpedantic -xc++ u.c
constexpr int f0 ()
{
  return __builtin_strcmp ("a", "b");
}

constexpr int f1 ()
{
  const char a [] = "a";
  const char b [] = "b";
  return __builtin_strcmp (a, b);
}

constexpr int f2 (const char *a, const char *b)
{
  int d = 0;
  do {
    d = *b - *a;
  } while (*a++ && *b++);
  return d;
}

constexpr int f3 ()
{
  const char s [] = "a";
  const char t [] = "b";

  const char *a = s;
  const char *b = t;

  int d = 0;
  do {
    d = *b - *a;
  } while (*a++ && *b++);
  return d;
}

constexpr char a [] = "a";
constexpr char b [] = "b";

constexpr int i0 = __builtin_strcmp ("a", "b");
constexpr int i1 = __builtin_strcmp (a, b);
constexpr int i2 = f1 ();
constexpr int i3 = f2 (a, b);
constexpr int i4 = f3 ();

u.c:42:23:   in constexpr expansion of ‘f1()’
u.c:10:27: error: ‘__builtin_strcmp(((const char*)(& a)), ((const char*)(&
b)))’ is not a constant expression
   return __builtin_strcmp (a, b);
          ~~~~~~~~~~~~~~~~~^~~~~~