Introducing a new keyword means that all existing code that uses the new keyword as an identifier will break. That's why the `constexpr` keyword is deliberately weird.
It's also why they reuse keywords like `using`.
Re:Proposal to add constexpr to C [pdf]
by kevin_thibedeau on Saturday October 16, 2021 @02:50PM
(#28890348)
They could just use the reserved keyword naming like the C committee does.
They could have just used "pure" like in D, but this is C++, so that would be too sane and obvious.