Improve error message for bad vararg with named arg

[som-snytt]

reproduction steps

Welcome to Scala 2.13.3 (OpenJDK 64-Bit Server VM, Java 15).
Type in expressions for evaluation. Or try :help.

scala 2.13.3> def f(i: Int, j: Int*) = i + j.sum
def f(i: Int, j: Int*): Int

scala 2.13.3> f(List(2, 3, 4): _*)
                             ^
              error: no `: _*` annotation allowed here
              (such annotations are only allowed in arguments to *-parameters)

scala 2.13.3> f(j = List(2, 3, 4): _*)
               ^
              error: when using named arguments, the vararg parameter has to be specified exactly once

problem

The vararg was specified exactly once.

scala 2.13.3> f(i = 42, j = List(2, 3, 4): _*)
val res2: Int = 51

scala 2.13.3> f(j = List(2, 3, 4): _*, i = 42)
val res3: Int = 51

Oh, were you asking, What would Dotty do?

Starting scala3 REPL...
scala> def f(i: Int, j: Int*) = i + j.sum
def f(i: Int, j: Int*): Int

scala> f(j = List(2, 3, 4): _*, i = 42)
1 |f(j = List(2, 3, 4): _*, i = 42)
  |                     ^
  |                     _* can be used only for last argument

scala> f(List(2, 3, 4): _*)
1 |f(List(2, 3, 4): _*)
  |  ^^^^^^^^^^^^^^^^^
  |Sequence argument type annotation `: _*` cannot be used here: the corresponding parameter has type Int which is not a repeated parameter type

scala> f(j = List(2, 3, 4): _*)
1 |f(j = List(2, 3, 4): _*)
  |^^^^^^^^^^^^^^^^^^^^^^^^
  |missing argument for parameter i of method f: (i: Int, j: Int*): Int

So Dotty does fumble what they call in the "default args" business a "swap job." However, "missing i" is as desired.

"Missing i" was also Polyphemus's problem.