I only see some possible stylistic/reusability improvements:

• You use decltype(std::declval<T>() + std::declval<T>()) five times in the function (I did not count the one in the return type). You could probably typedef that to improve readability. I don't know how you would call it, but I am pretty sure that you can find a meaningful name.

• By the way, are there many types for which decltype(std::declval<T>() + std::declval<T>) is different from T while still being amp compatible? If it is the case, that may break some other pieces of your code anyway (I didn't check though; that's a wild guess).

• Whenever possible, try to use std::begin and std::end instead of the member functions. It helps to write reusable code; you might someday want to extract parts of your function to write smaller and more generic functions that could work on old C arrays.

• By default, if you don't specify the rank of an array_view, it is one. Therefore, I actually don't have to specify it since you always use 1.

• Your lines are really long. As I said in the comments, you might want to split some of them to improve readability:

    futTailSum = std::async( std::launch::async, [&vecTail]
        {
            return concurrency::parallel_reduce( std::begin(vecTail),
                                                 std::end(vecTail),
                                                 static_cast<T>(0) );
        } );
  

I only see some possible stylistic/reusability improvements:

• You use decltype(std::declval<T>() + std::declval<T>()) five times in the function (plus the one in the return type). You could probably typedef that to improve readability. I don't know how you would call it, but I am pretty sure that you can find a meaningful name.

• By the way, are there many types for which decltype(std::declval<T>() + std::declval<T>) is different from T while still being amp compatible? If it is the case, that may break some other pieces of your code anyway (I didn't check though; that's a wild guess).

• Whenever possible, try to use std::begin and std::end instead of the member functions. It helps to write reusable code; you might someday want to extract parts of your function to write smaller and more generic functions that could work on old C arrays.

• By default, if you don't specify the rank of an array_view, it is one. Therefore, you actually don't have to specify it since you always use 1.

• Your lines are really long. As I said in the comments, you might want to split some of them to improve readability:

    futTailSum = std::async( std::launch::async, [&vecTail]
        {
            return concurrency::parallel_reduce( std::begin(vecTail),
                                                 std::end(vecTail),
                                                 static_cast<T>(0) );
        } );