No compilation errors on `xml.Elem` pattern matching with wrong number of arguments

[scabug]

There is no compilation error with wrong number of arguments for xml.Elem:

<root /> match { case Elem(prefix, lable, attribs, scope, minimizeEmpty, content @ _*) => ??? }

Method unapplySeq returns Option[(String, String, scala.xml.MetaData, scala.xml.NamespaceBinding, Seq[scala.xml.Node])].

It should be called unapply to produce a compilation error:

object ElemHelper {
  def unapply(n: Node) = Elem.unapplySeq(n)
}

<root /> match { case ElemHelper(prefix, lable, attribs, scope, minimizeEmpty, content @ _*) => ??? }

<console>:15: error: wrong number of arguments for object ElemHelper

See this SO question for error case. http://stackoverflow.com/q/17357359/406435

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-7623?orig=1
Reporter: Senia Popugaev (senia)
Affected Versions: 2.10.1, 2.11.0-M3
See #6524

[scabug]

@som-snytt said:
s/wrong number of args/too many args/

You get an error if there are not enough args, since the varargs don't align. That is true in general.

The Elem object serves as an extractor for Nodes, which have no notion of minimizeEmpty.

(That extractor tripped someone else up recently, so maybe that's a design issue.)

It might make a nice Abide rule to warn about this sort of thing.

Or maybe a compiler lint option to warn about "splitting" a varargs. Is that a general issue? Maybe only when mixing args and varargs.

scala> val r = "(a*)(b*)(c*)".r
r: scala.util.matching.Regex = (a*)(b*)(c*)

scala> "abc" match { case r(as, rest @ _*) => as }  // dangerous?
res5: String = a

scala> "abc" match { case r(as, _, _) => as }
res6: String = a

[scabug]

@som-snytt said:
Maybe scala/scala#4216