Performance:
You already have #248 about a performance regression in flat_map. It can currently be twice as fast to use a struct { int key; mutable int value; } in a flat_set than a flat_map<int,int>. I would prioritize fixing known performance issues before looking for others, but I guess it is a worthy goal.
Yes, I previously was using type-punning to be able to use memcpy in flat_map when key and mapped_type were trivial, but compiler warnings and some suspicious aliasing issues were reported, so I had to roll-back. I'll need to revisit this, I'm not sure if other flatmap implementations (chromium, folly...) perform some type of type punning to achieve the speedup.
I have seen Abseil using a union of `pair<K,V>` and `pair<const K, V>` for flat_map nodes. However that had led to misoptimzation due to alias violations on some systems and GCC See my Issue there for a technical analysis from some time ago: https://github.com/abseil/abseil-cpp/issues/975<http://lists.boost.org/mailman/listinfo.cgi/boost> As usual: While type-punning with UB might work in most cases, there will be failures in some. And there the UB was non-obvious at all. Alex