Some[(A, Seq[B])] is not a valid result type of an unapplySeq

[allanrenucci]

This code snippet fails to compile with Dotty

class Test {
  class Foo(val name: String, val children: Int *)
  object Foo {
    def unapplySeq(f: Foo) = Some((f.name, f.children))
  }

  def foo(f: Foo) = f match {
    case Foo(name, ns @ _*) => 1
  }
}

[info] Running dotty.tools.dotc.Main -d out -language:Scala2 tests/allan/Test.scala -classpath dotty-library_2.12-0.4.0-bin-SNAPSHOT-nonbootstrapped.jar
-- Error: tests/allan/Test.scala:8:12 ------------------------------------------
8 |    case Foo(name, ns: _*) => 1
  |         ^^^^^^^^^^^^^^^^^
  |Some[(String, scala.collection.Seq[Int] @Repeated)] is not a valid result type of an unapplySeq method of an extractor

[odersky]

The code snippet now compiles with #3747 if the unapplySeq definition is replaced by unapply. Furthermore, the error message for the unapplySeq version suggests that one should use an unapply instead.

[Blaisorblade]

Given that the API in #3747 can't be used if you write the return type explicitly, I'm reopening this issue.

And given that we want cross-version Tasty to exist (at least for 3.0), and Tasty can't say that a method has a version-dependent user-written name (thankfully!), it appears we must support unapplySeq at least for 3.0.