std.container.RedBlackTree versus C++ std::set

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> I am starting to understand the difficulty with ref being too 
> restrictive.  Some kind of "const ref" in the language would 
> have helped indeed.  But it would probably cause some 
> allocation difficulties for actual constants... or worse?

Search for 'auto ref' or right after 'rvalue references' and read 
some of the discussions. It's instructive and then you can 
understand the whole topic maybe a little more.
No thread comes to a solution and unfortunately the answer of 
Walter and Andrei are very very thin on the subject.