Как бы хотелось вместо (1) иметь возможность делать (2) или (3).

gistfile1.cpp

/* 1 */

template<template <size_t> class T, size_t... Indices> struct summa;
    
template<template <size_t> class T, size_t Index, size_t... Indices>
struct summa<T, Index, Indices...>
{
    static const decltype(T<Index>::value) value;
};

template<template <size_t> class T, size_t Index, size_t... Indices>
const decltype(T<Index>::value) summa<T, Index, Indices...>::value = T<Index>::value + summa<T, Indices...>::value;

template<template <size_t> class T, size_t Index>
struct summa<T, Index>
{
    static const decltype(T<Index>::value) value;
};

template<template <size_t> class T, size_t Index>
const decltype(T<Index>::value) summa<T, Index>::value = T<Index>::value;

/* 2 */

template<Signature>
struct summa
{
    static switch (template Signature)
    {
    case template <size_t> class T, size_t Index, size_t... Indices:
        static constexpr auto value = T<Index>::value + summa<T, Indices...>::value;
        break;
    case template <size_t> class T, size_t Index:
        static constexpr auto value = T<Index>::value;
        break;
    default:;
    }
};

/* 3 */

template<template <size_t> class T, size_t Index, size_t... Indices>
struct summa
{
    static if (sizeof...(Indices) > 0)
        static constexpr auto value = T<Index>::value + summa<T, Indices...>::value;
    else
        static constexpr auto value = T<Index>::value;
};