[Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals)

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-11-21 12:40:29 UTC ---
1) is not valid constexpr IMHO, as the standard says that __func__ works as if
   static const char __func__[] = "function-name";
   has been provided, but when it is provided, it is not valid constexpr.
2) I don't see why you think you could use something like this at all.
   You are defining a non-constexpr operator "", so str there definitely isn't
   pointer to a string literal, it is a standalone function that might be
   inlined if the inliner choses so.  So it is obvious the static_assert
   which must be evaluated before the optimizations, without depending on
   whether the function is inlined or not, must fail.
   You want something like:
constexpr int
cstrlen (const char *c)
{
  return *c ? cstrlen (c + 1) + 1 : 0;
}

constexpr unsigned int
bin_number (const char *c)
{
  return *c ? (*c == '1' ? (1U << cstrlen (c + 1)) : *c == '0' ? 0 : throw 0) +
bin_number (c + 1) : 0;
}

constexpr unsigned int
operator"" _bin (const char *c)
{
  return bin_number (c);
}

int
main ()
{
  constexpr unsigned int a = 10000_bin;
  (void) a;
  return 0;
}

instead, then you can verify it the same at compile time.