I don't want exact-width types. I want an integral type that's at least N bits in width, which currently std::(u)int_leastN_t cannot provide for all widths. _BitInt is an interesting idea, however, does it work with arbitrary sizes and produces an integral type with at least N bits in width?
Does it also have a fast version for heavy computational data?

I want to further standardise these 2 cases.

Maybe once we have _BitInt, we could add std::bit_int_unpadded_t and std::bit_int_fast_t traits that give you the next larger _BitInt without padding, and the next larger _BitInt that is "fast".

I'm not so sure we want such traits for the standard integer types.