[Bug c++/70816] 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

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2016-06-08
     Ever confirmed|0                           |1
      Known to fail|6.0                         |6.1.0, 7.0

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
The same limitation affects other string builtins such as __builtin_strlen or
__builtin_strchr.

$ cat cst.C && /home/msebor/build/gcc-6-branch/gcc/xgcc
-B/home/msebor/build/gcc-6-branch/gcc -S -Wall -Wextra -Wpedantic cst.C
-fdump-tree-optimized=/dev/stdout -o/dev/stdout
constexpr int chr (int c)
{
  const char a[] = "123";
  return __builtin_strchr (a, c) - a;
}

constexpr int i = chr ('3');

constexpr int cmp (const char *a)
{
  const char b[] = "123";
  return __builtin_strcmp (a, b);
}

constexpr int j = cmp ("345");

constexpr unsigned len ()
{
  const char s[] = "123";
  return __builtin_strlen (s);
}

constexpr unsigned n = len ();
        .file   "cst.C"
cst.C:7:23:   in constexpr expansion of ‘chr(51)’
cst.C:4:27: error: ‘__builtin_strchr(((const char*)(& a)), 51)’ is not a
constant expression
   return __builtin_strchr (a, c) - a;
          ~~~~~~~~~~~~~~~~~^~~~~~
cst.C:15:23:   in constexpr expansion of ‘cmp(((const char*)"345"))’
cst.C:12:27: error: ‘__builtin_strcmp(((const char*)"345"), ((const char*)(&
b)))’ is not a constant expression
   return __builtin_strcmp (a, b);
          ~~~~~~~~~~~~~~~~~^~~~~~
cst.C:23:28:   in constexpr expansion of ‘len()’
cst.C:20:27: error: ‘__builtin_strlen(((const char*)(& s)))’ is not a constant
expression
   return __builtin_strlen (s);
          ~~~~~~~~~~~~~~~~~^~~