Applying type conversion to a list: is "const" special?

Question

The following applies the conversion of type F to a list C types E... :

 template <template <typename...> class F, typename P> struct apply_t; template < template <typename...> class F, template <typename...> class C, typename... E > struct apply_t <F, C <E...> > { typedef C <F <E>...> type; }; template <template <typename...> class F, typename P> using apply = typename apply_t<F, P>::type; 

Now by entering this input:

 template <typename T> using map1 = T*; template <typename T> using map2 = const T; template <typename T> using map3 = const T*; 

I get the following output in gcc 4.8.1 using, for example, std::tuple :

 apply <map1, tuple <int, char> > // -> tuple <int*, char*> (OK) apply <map2, tuple <int, char> > // -> tuple <int, char> // (tuple <const int, const char> expected) apply <map3, tuple <int, char> > // -> tuple <const int*, const char*> (OK) 

I see output types using a utility template function that calls static_assert() .

I tried all kinds of combinations. Everything works, except for the map const T , in particular, when you specify an apply type as the input map. But it still works fine if I insert it into the definition of apply , i.e. typedef C <const E...> type .

On the other hand, everything works as expected in clang. I might be missing something, but it really looks like a gcc error.

Any ideas?

Answer 1

Here's a complete but smaller test file, you can add this to your GCC error report (if you need to write one):

 template< typename T > using Map = const T; template< template<typename> class M, typename T > struct Apply; template< template<typename> class M, template<typename...> class C, typename... Ts > struct Apply< M, C<Ts...> > { typedef C<M<Ts>...> type; }; template< typename T > struct A {}; template<typename> void dummy(); int main() { dummy< Apply<Map,A<int>>::type >(); } 

will not contact due

 undefined reference to `void dummy<A<int> >()' 

instead of the expected

 undefined reference to `void dummy<A<const int> >()' 

(I know this does not solve the problem, but GCC people will most likely prefer a smaller test file without any dependency on std::tuple or any other header).