I mean, this isn't the only case of broken parametricity I've come across in C++ codebases. Though it's the only one in the stdlib.

I know vector<bool> is controversial (and I understand it can't be "fixed" now), I was just surprised that something breaking parametricity so blatantly was ever agreed upon in the first place -- in more functional languages parametricity is kinda much more jealously guarded. Bit of a culture shock there, is all :)