Re:a 'few' rough edges (Score 1)

If those are the motivations, we should have an ImmutableList class without an add function, and derive the List with an add function from it (in C++, probably by private inheritance, that is, inheritance of implementation). Only the List class with the (working) add should publically inherit from the interface with an add function.

This is a bad solution. If List inherits from ImmutableList, this implies that List is an ImmutableList. Since that is not the case, this is obviously a bad design.

Also, your solution would be impossible to implement if you don't have control over the List class. For example, the List class could be a part of a component you can't modify, or even a part of your programming language.