I tried passing a reference to an element in std::vector<bool> and got a compiler error. Why is std::vector<bool> specialized differently?

std::vector<bool> is a historical space-optimization specialization that packs 8 booleans into each single byte! Because individual bits cannot have a memory address, operator[] returns a proxy object (std::vector<bool>::reference) instead of bool&. If you need a standard container of booleans with true references, use std::vector<uint8_t> or std::deque<bool> instead.

The proxy object breaks generic template code that expects T&. Good to know why it behaves that way!