After a hint from @akukanov, it seems there is a simpler approach to the fix provided in #1527.
Instead of specifically avoiding instantiating std::vector<T> for types T which are not valid, we can merely use std::decay_t prior to our USM iterator checks.
This will prevent any ill-formed std::vector from being instantiated, and also should not damage our ability to detect any valid USM allocated std::vector::iterator.
Any std::vector::iterator type created from a decayed value type should not match the original iterator type if std::decay has an effect on the value type. If std::decay has an effect on the value type, then it is not a valid std::vector.

This should fix the problem same issue with simpler code, and not change our behavior.

(There is no need to rush this before the freeze, the result should be identical, this is just cleaner code)