Proposed resolution:

Add the following type traits to p3 [ratio]

// ratio arithmetic
template <class R1, class R2> struct ratio_add;
template <class R1, class R2> struct ratio_subtract;
template <class R1, class R2> struct ratio_multiply;
template <class R1, class R2> struct ratio_divide;
template <class R1, class ... RList> struct ratio_sum;
template <class R1, class ... RList> struct ratio_product;

after [ratio.arithmetic] p1: add

template <class R1, class ... RList> struct ratio_sum; // declared, never defined

template <class R1> struct ratio_sum<R1> : R1 {};

Requires: R1 is a specialization of class template ratio

template <class R1, class R2, class ... RList> 
 struct ratio_sum<R1, R2, RList...>
   : ratio_add< R1, ratio_sum<R2, RList...>> {
};

Requires: R1 and each element in parmater pack RList is a specialization of class template ratio

after [ratio.arithmetic] p3: add

template <class R1, class ... RList> struct ratio_product; // declared, never defined

template <class R1> struct ratio_product<R1> : R1 {};

Requires: R1 is a specialization of class template ratio

template <class R1, class R2, class ... RList> 
 struct ratio_sum<R1, R2, RList...>
   : ratio_add< R1, ratio_product<R2, RList...>> {
};

Requires: R1 and each element in parmater pack RList is a specialization of class template ratio