While reading Wikipedia's page on decltype, I was curious about the statement,

Its [decltype's] primary intended use is in generic programming, where it is often difficult, or even impossible, to name types that depend on template parameters.

While I can understand the difficulty part of that statement, what is an example where there is a need to name a type that cannot be named under C++03?

EDIT: My point is that since everything in C++ has a declaration of types. Why would there ever be a case where it is impossible to name a type? Furthermore, aren't trait classes designed to yield type informations? Could trait classes be an alternative to decltype?