[ 2010 Batavia ]

This is not a defect. Threre are a number of places in the standard where we allow implentations to choose their preferred technique, the most obvious example being the iterator/const_iterator types of set.

Typically, this means it is not portable to declare function overloads that differ only in their use of these types.

[ Resolution proposed in ballot comment ]

Require that it be a distinct class type.

Addresses US-112

What it means for high_resolution_clock to be a synonym is undefined. If it may or may not be a typedef, then certain classes of programs become unportable.