> Turns out, these two are equivalent in practice

Not in the x86-64 SysV ABI they aren’t. The arguments will be passed in registers (yes, even the variadic ones), so how your compiler will interpret 1[&a] is anybody’s guess. (For me, x86_64-unknown-linux-gnu-g++ -O2 yields, essentially, return a+a+a+a; which is certainly an interpretation. I’m also getting strange results from i686-unknown-linux-gnu-g++ -O2, but my x87 assembly is rusty enough that I don’t really get what’s going on there.)

K&R syntax is -1 char, if you are in C:

    double solve(double a,double b,double c,double d){return a+b+c+d;}
    double solve(double a...){return a+1[&a]+2[&a]+3[&a];}
    double solve(a,b,c,d)double a,c,b,d;{return a+b+c+d;}

    double solve(double a[]) {
      return 0[a] + 1[a] + 2[a] + 3[a];
    }

    solve((double[]){1, 2, 3, 4});

The cast in the invocation can be macro-ed away. And the best thing is, the actual stack layout and data movement/shuffling is pretty much identical to the approach with <stdargs.h>, and with no UB or compiler intrinsics.