gensub（）可以接受函数调用作为第二个参数吗？

Inspired by this code-golf challenge, I thought it would be easy to achieve the end result with awk. The algorithm would be: convert everything to lowercase, then get only vowels to uppercase.

我以为这样可以

awk '{$0=tolower($0);print gensub(/[aeiou]/,toupper("&"),"g")}'
> HeLlO
> hello

但它所做的只是以小写形式回显输入。然后，我开始摆弄它：

$ awk '{$0=tolower($0);print gensub(/[aeiou]/,"&"toupper("&"),"g")}'
> HeLlO
> heelloo

I was about to conclude that maybe the parser was reading "&""&", since awk can be very forgiving at times. But then, I tried with other functions:

$ awk '{$0=tolower($0);print gensub(/[aeiou]/,strtonum("&"),"g")}'
> HeLlO
> h0ll0

因此，总而言之，我缺少什么？