error C2988: unrecognizable template declaration/definition

I looked at a bunch of documents but I cannot understand what the problem is, and how to resolve it. Here goes:

template<class T1, class T2>
class A { ... };

template<class T2>
class B
{
    ...

    template<class T1, class... Ts>
    A<T1, T2> f(Ts&&... args)
    { ... }

    ...
};

Everything is fine except:

A<T1, T2> f(Ts&&... args)

which causes

error C2988: unrecognizable template declaration/definition

whould could be the problem? How can I overcome this?