[Bug c++/70563] SFINAE fails when trying invalid template instantiation

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70563

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-06-16
                 CC|                            |msebor at gcc dot gnu.org
            Summary|SFINEA fails when trying    |SFINAE fails when trying
                   |invalid template            |invalid template
                   |instantiation               |instantiation
     Ever confirmed|0                           |1
      Known to fail|                            |4.9.3, 5.3.0, 6.1.0, 7.0

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
I'm not 100% sure the code is valid but I can't find anything wrong with it and
I don't think the error message is correct.  The type it complains about being
incomplete seems complete to me.  I'm going to confirm this with the slightly
reduced test case that should also make clear which type is being instantiated
by which expression.  I note that Clang 3.8 does compile it as mentioned,
though EDG eccp does not (though the error it issues suggests it's confused by
the complicated templates).

All recent versions of GCC fail to compile the test case, although 4.9.3 with a
different error: âbâ is not a member of âC<D>::CC<B<int> >â.

$ cat u.C && /home/msebor/build/gcc-trunk-svn/gcc/xgcc -B
/home/msebor/build/gcc-trunk-svn/gcc -Wall -Wextra -Wpedantic u.C
template <class...> using A = void;

template <class> struct B { };

template <template <class...> class T> struct C
{
  template <class, class = A<>>
  struct CC { enum { b = 0 }; };

  template <template <class...> class U, class... Ts>
  struct CC<U<Ts...>, A<T<Ts..., float> > > { enum { a = 1 }; };
};

template <class> struct D { };
template <class, class> struct E { };

enum {
  a = C<E>::CC<B<int>>::a,
  b = C<D>::CC<B<int>>::b
};
u.C:19:7: error: incomplete type âC<D>::CC<B<int> >â used in nested name
specifier
   b = C<D>::CC<B<int>>::b
       ^~~~