As it is, Seems like scaladoc could not got the parameter names when generating the document.
scala 2.13.0-M5> "hello".==(that = "world")
^
error: unknown parameter name: that
scala 2.13.0-M5> "hello".==(arg0 = "world")
^
error: unknown parameter name: arg0
scala 2.13.0-M5> "hello".==
final def ==(x$1: Any): Boolean
scala 2.13.0-M5> "hello".==(x$1 = "world")
^
error: unknown parameter name: x$1