Deduce the template parameter of an outer class in a nested class function call?

Question

Is there a way to deduce the outer template in a nested class?

template<class T>
struct A{
    struct B{};
};

template<class T> void f(typename A<T>::B b){} // hard to deduce T?

int main(){
  A<double>::B b;
  f(b); // no matching function for call to 'f(A<double>::B&)', meant to call f<double>(b);
}

Or I am forced to declare the nested class outside? Is this the only workaround?

template<class T> struct B_impl{};

template<class T>
struct A{
    using B = B_impl<T>;
};

template<class T> void f(B_impl<T> b){} // to bad there is no mention of A here

int main(){
  A<double>::B b;
  f(b);
}

@P.W I want the function to deduce the parameter. I wonder if this this outside the domain of template deduction or is that compilers just don't implement it. (I tried with gcc and clang) — alfC, Jan 16 '19 at 11:57
Possible duplicate of [What is a nondeduced context?](https://stackoverflow.com/questions/25245453/what-is-a-nondeduced-context) — Holt, Jan 16 '19 at 15:27
Somewhere in the program, there could be `template <> struct A { using B = A::B; };` What should `T` be now in `f(b)` call? — Igor Tandetnik, Jan 17 '19 at 04:17
@IgorTandetnik, it could say that it can't deduce the type in that case. :) Seriously, good point. — alfC, Jan 17 '19 at 04:42
`A` could be a complicated template using partial specializations, SFINAE, recursion and what not. In general, determining that `is_same::B, A::B>` for some pair of types `T`, `U` would require instantiating `A` for every possible type, of which there's an infinite number. Since that is clearly impossible, the standard says that the compiler shouldn't even try; hence the concept of non-deduced context. — Igor Tandetnik, Jan 17 '19 at 05:45
@IgorTandetnik, clear enough. I guess this is a good reason to avoid nested classes because in the long run they have their limitations. After all, all that can be implemented with a nested class can be implemented outside and `typedef`ed inside the class. The problem is how to name these classes. — alfC, Jan 17 '19 at 07:43

Deduce the template parameter of an outer class in a nested class function call?

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