GCC already solved it:
https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html
The operations behave like C++ valarrays. Addition is defined as the addition of the corresponding elements of the operands. For example, in the code below, each of the 4 elements in a is added to the corresponding 4 elements in b and the resulting vector is stored in c.
Those type attributes are also used for the x86 intrinsics API, and they override default C behaviors like promotions and presumptions around aliasing (ironically they make type punning easier, though maybe it was just the few use cases I explored, and this isn't an area where I have alot of experience). C23 also gained the _BitInt type, which discards all the old promotion rules, which should help autovectorization.
I think ISPC is still the proper way to go. But these days everybody wants One Language to Rule Them All along with standard libraries for doing everything out-of-the-box. And while in principle ISPC's approach could be stitched into C or C++ in a fairly clean manner (perhaps with well-defined and enforced segregation of constructs to minimize complexity), it's just not gonna happen: C++ is too enamored with constructing libraries through deeply complex templated types (hammer, nail, yada yada), and C is just too conservative (though if GCC or clang went the distance with a full implementation, there's a good chance the C committee would adopt it).
