[Bug c++/53162] New: unexpanded parameter packs not diagnosed in alias template used in default argument

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53162

             Bug #: 53162
           Summary: unexpanded parameter packs not diagnosed in alias
                    template used in default argument
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid, diagnostic
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: redi@gcc.gnu.org


template<bool B>
struct bool_constant
{
  static const bool value = B;
};

template<bool> struct enable_if { };
template<> struct enable_if<true> { typedef void type; };

template<typename... A>
struct F
{
  template<typename... B>
    using SameSize = bool_constant<sizeof...(A) == sizeof...(B)>;

  template<typename... B,
           typename = SameSize<B>>
  F(int, B...) { }

  template<typename... B,
           typename = typename enable_if<SameSize<B>::value>::type>
  F(char, B...) { }
};

int main()
{
  F<int, int> f1(1, 2, 3);
  F<int, int> f2('1', 2, 3);
}

$ g++ -std=c++11 t.cc
t.cc: In instantiation of âstruct F<int, int>â:
t.cc:27:17:   required from here
t.cc:22:3: error: no type named âtypeâ in âstruct enable_if<false>â
   F(char, B...) { }
   ^

The declaration of f1 should be rejected because the F(int, B...) constructor's
default template argument refers to the parameter pack B without expanding it,
but there is no error.

The declaration of f1 is rejected but with an unhelpful diagnostic, if the
problem was that there's no 'type' in enable_if then it should result in a
substitution failure not an error. The actual problem is that B is not expanded
in SameSize<B>, so the error should say so.