fix #11583: skip typarams of right assoc ext meth

[bishabosha]

skip the first type parameter list of a right associative extension method before checking for a single non given parameter
fixes #11583

[rjolly]

The fix is broken. The test compiles, but not into what you want:

extension (s: String)
  def :*:[T <: Tuple](that: T) : String *: T = ???

def res = "foo":*:(1, 2)
//             ^^^^^^^^^
//      value :*: is not a member of (Int, Int).
//      An extension method was tried, but could not be fully constructed:

//          :*:[T](Tuple2.apply[Int, Int](1, 2))    failed with

//              Found:    (Int, Int)
//              Required: String

def res = (1, 2):*:"foo"
// compiles

[neko-kai]

@rjolly
Are extension methods supposed to be exempt from reversal on operators ending with :? Since a :+: b is desugared to b.:+:(a) (in Scala 2 at least) that would lead to the behavior in your code sample.

[rjolly]

a :+: b is desugared to :+:(b)(a) and resugared to a :+: b (so to speak).