Range-for-loops and std::vector

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Because std::vector<bool> is not a container !

std::vector<T>‘s iterators usually dereference to a T&, which you can bind to your own auto&.

std::vector<bool>, however, packs its bools together inside integers, so you need a proxy to do the bit-masking when accessing them. Thus, its iterators return a Proxy.
And since the returned Proxy is an prvalue (a temporary), it cannot bind to an lvalue reference such as auto&.

The solution : use auto&&, which will correctly collapse into an lvalue reference if given one, or bind and maintain the temporary alive if it’s given a proxy.

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