Humans, of course.
That does not guarantee, however, that humans will be able to fully grasp and fully understand the working code that such a system would produce.
“able to fully grasp and fully understand the working code that such a system would produce.”
Couldn’t be worse than the piles of working code out there now produced by humans.
Seriously, a computer producing its own code sounds like a dog chasing its tail.
I wrote some code a long time ago that generated code. Purpose was to expand a loop into a liner string of statements in order to eliminate the loop overhead.