[Bug c++/57919] New: [C++11] Alias templates cause partial specialization to erroneously fail

[cheesear at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57919

            Bug ID: 57919
           Summary: [C++11] Alias templates cause partial specialization
                    to erroneously fail
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cheesear at gmail dot com

Created attachment 30516
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30516&action=edit
MWE that causes the aforementioned error.

The use of alias templates can cause the compiler to think that a valid
template class instantiation is ambiguous. The file attached contains an
example that, when compiled with g++ (version 4.8.1_1 from Macports), results
in the following error:

$ g++ -std=c++11 er.cpp
er.cpp: In function 'int main()':
er.cpp:39:41: error: ambiguous class template instantiation for 'struct foo<1,
1, sequence<> >'
  using t = typename foo<1, 1, extents<>>::type;
                                         ^
er.cpp:26:8: error: candidates are: struct foo<A, B, sequence<integer<Ts>...> >
 struct foo<A, B, extents<Ts...>>
        ^
er.cpp:32:8: error:                 struct foo<B, B, sequence<integer<Ts>...> >
 struct foo<B, B, extents<Ts...>>
        ^
er.cpp:39:43: error: 'type' in 'struct foo<1, 1, sequence<> >' does not name a
type
  using t = typename foo<1, 1, extents<>>::type;
                                           ^