[Bug c++/49399] [C++0x] substitution failure error

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49399

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-06-14 12:11:05 UTC ---
The program should not compile.

In C++03 it should fail to compile because it accesses a private member. SFINAE
does not take access control into account in C++03, so that is an error.  G++
fails to reject the program because access checking in templates is buggy.

There was a last minute change in the C++0x FDIS to make SFINAE consider access
control. The previous draft
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf) says in
section 14.8.2 [temp.deduct] p8 "Access checking is not done as part of the
substitution process. Consequently, when deduction succeeds, an access error
could still result when the function is instantiated."
That was changed in the FDIS, so accessing the private member should cause
deduction to fail, so there is no broken_fun and the call to it in main should
fail.  G++ 4.7 doesn't implement that change yet, so rejects the program for
the wrong reason.

If you had a second, less specific, overload of broken_fun then the program
would still be invalid in C++03 but should be well-formed in C++0x. G++ doesn't
accept it because access checking is not done as part of the substitution
process yet.

e.g. this is valid C++0x

struct broken
{
  private:
  typedef int value_type;
};

template <typename T>
int
broken_fun(int, typename T::value_type* = 0);

template <typename T>
char
broken_fun(...);

int main(int argc, char* argv[])
{

  return sizeof(broken_fun<broken>(5));
}