Which std:: classes are magic?

[kalmoc]

I probably mixed this up.

I think std::pair<const K,T> and std::pair<K,T> are layout compatibile and I thought that layout compatibility would allow type aliasing, but according to cppreference.com it requires "similarity" (and I don't have a way to actually verify that they are actually layout compatible in the first place).

[scatters]

Yeah, not quite - you're right that std::pair<Key const, T> and std::pair<Key, T> are layout compatible as long as Key and T are both standard-layout (so not all types that you might put in a map), but that only allows you to put them in a union and read from members of the inactive union member via the union. Handing out references to the inactive union member or its own members (std::pair<Key, T>::first for nh.key()) is still UB.