Simplifies the C++11-style "-> decltype(...)" return-type deduction.
Note that you have to be careful about whether the function return type
is auto or decltype(auto). The difference is that bare auto
strips const and reference, just like lambda return type deduction. In
some cases that's what we want (or more likely, we know that the return
type is a value type), but whenever we're wrapping a templated function
which might return a reference, we need to be sure that the return type
No functional change.