I'm not working on this, but I have some concerns.

Perhaps we should provide them with alias templates with non-_Ugly names, like

template<int N> // or size_t which is consistent with the type of N deduced from T[N]
using bit_int = _BitInt(N);

template<int N>
using bit_uint = unsigned _BitInt(N);

where the keyword _BitInt doesn't need to go into the C++ standard.

Also, it may be desired to deduce N from _BitInt(N) or unsigned _BitInt(N) (otherwise, it would become hard to implement std::is_integral), which can be achieved by expanding [temp.deduct.type].

F.v.S.

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