Multiarg infix pattern has poor migration experience

[som-snytt]

Compiler version

3.1.3

Minimized code

object Main extends App {

  class Unapplyer {
    def unapplySeq(seq: Seq[Int]): Option[(Int, Seq[Int])] =
      if (seq.isEmpty) None else Some((seq.head, seq.tail))
  }
  val unapplyer = new Unapplyer

  Seq(1) match {
    case value unapplyer () => println(value)
  }
  Seq(42, 28, 17) match {
    case value unapplyer (x, y) => println(x + y)
  }
}

Output

-- Error: test/files/neg/t12110.scala:20:25 ----------------------------------------------------------------------------
20 |    case value unapplyer () => println(value)
   |                         ^^
   |                         Values of types Unit and Int cannot be compared with == or !=
-- [E008] Not Found Error: test/files/neg/t12110.scala:23:45 -----------------------------------------------------------
23 |    case value unapplyer (x, y) => println(x + y)
   |                                           ^^^
   |                             value + is not a member of Any, but could be made available as an extension method.
   |
   |                             One of the following imports might make progress towards fixing the problem:
   |
   |                               import math.Fractional.Implicits.infixFractionalOps
   |                               import math.Integral.Implicits.infixIntegralOps
   |                               import math.Numeric.Implicits.infixNumericOps
   |
2 errors found

Expectation

A less puzzling error message. Scala 2 -Xsource:3 fails to warn in the second, multiarg case.

I'm not sure the feature status is mentioned in the reference docs.

Via scala/bug#12110

The clean-up began at #4311