What should ellipsis in attribute-list in C++ be used for?

Question

In C++ reference I found information about allowed syntax of attributes in C++, it is:

[[attribute-list]]
[[ using attribute-namespace : attribute-list ]]

"where attribute-list is a comma-separated sequence of zero or more attributes (possibly ending with an ellipsis ... indicating a pack expansion)"

I've tried to use its, but I see no difference between:

[[deprecated]] void f() 
{
}

and

[[deprecated...]] void f() 
{
}

In both cases output is the same.

Answer 1

This was added to the specification for consistency and also because the future of attributes is still being discussed. Considering that we currently have pack expansion in variadic templates (see Variadic template pack expansion) like this:

// pack expansion in function arguments
template <typename... Args>
void f(Args... args) {}

// pack expansion in inheritance
template <typename... Inherited>
struct MyClass : Inherited... {};

Along the same lines, it also makes sense to think about pack expansion for attributes. A few example scenarios could be:

template <typename... Ts>
class [[Ts...]] MyClass {};

or

template <typename... Ts>
class [[Ts()...]] MyClass {};

But, again, this is only in the specification and currently there is no attribute that can be used like that.