It's also a "language API break" (?) -- at the moment, `a - b` actually
compiles and yields ptrdiff_t, not float. (But if you actually
evaluate it, it's UB https://eel.is/c++draft/expr#add-5.3 .)
But that's not what the standard says, that's just what most compilers have decided to do. If it's already UB then why work to keep it stable?
If the platform doesn't have SIMD then the compiler can just turn it into a loop. I think that was the plan for the existing SIMD proposal too. The language change itself isn't that big of a change either. Using math operations on arrays was never possible (other than how arrays decay to pointers that can be treated like integers, but that's UB), so this is just an addition to the language. I think it would also be pretty easy for compilers to implement since it's just mixing concepts that they already support.