Is the order of support_copy and add_facade crucial?

[xuruilong100]

I'm trying sample basic_facade_builder, and I find that

template <class... Overloads>
struct CopyableCallable : pro::facade_builder::                              //
                          support_copy<pro::constraint_level::nontrivial>::  //
                          add_facade<MovableCallable<Overloads...>>::        //
                          build {};

is OK.

But, compiler tells me

template <class... Overloads>
struct CopyableCallable : pro::facade_builder::                              //
                          add_facade<MovableCallable<Overloads...>>::        //
                          support_copy<pro::constraint_level::nontrivial>::  //
                          build {};

is illegal.

[mingxwa]

@xuruilong100 Because the type add_facade<MovableCallable<Overloads...>> depends on template arguments Overloads, C++ requires keywork template to specify an alias template. You need to add template before support_copy to make it compile, or simply swapping add_facade and support_copy should also work (as you already tried).