note that default arguments are still acceptable in using parameters

locally {
    extension (x: String)(using s: String = "default value")
      def f1: String = x.reverse
    
    "my string".f1
  }

we may also prohibit by-name parameters in extensions,
because while even a regular argument to a by-name parameter might be surprising,
such as bar(foo()),
it's much more unexpected that foo().bar() might result in calling bar() without calling foo()

Discussing this with @mbovel, we believe we should put these checks in Refchecks
(As opposed to in the parser)

You can find our work over there:
#17321

I left some TODOs to point at what still needs to be done