Bug in unapplySeq from scala-xml

Compiler error was found in scala-xml on 2.13.0-M5:

    18: error: error during expansion of this match (this is
        a scalac bug)
    The underlying error was: type mismatch;
     found   : Seq[Node] (in scala.collection)
     required: Seq[Node] (in scala.collection.immutable)
      def f(n: Node) = n match {

This was Lukas Rytz's minimal example that could reproduce:

abstract class Node extends NodeSeq {
  def label: String
  def child: scala.collection.Seq[Node]
}

object Elem {
  def unapplySeq(n: Node): Option[(String, collection.Seq[Node])] =
    Some((n.label, n.child))
}

abstract class NodeSeq extends collection.Seq[Node] {
  def iterator: Iterator[Node] = ???
  def apply(i: Int): Node = ???
  def length: Int = ???
}

object Test {
  def f(n: Node) = n match {
    case Elem(label, child @ _*) => child
  }
}

Author: ashawley

test/files/neg/t11102.check

@@ -0,0 +1,13 @@
+t11102.scala:5: error: error during expansion of this match (this is a scalac bug).
+The underlying error was: type mismatch;
+ found   : Seq[MutableCons] (in scala.collection.mutable)
+ required: Seq[MutableCons] (in scala.collection.immutable)
+  def f(x: MutableSeq) = x match {
+                           ^
+t11102.scala:8: error: error during expansion of this match (this is a scalac bug).
+The underlying error was: type mismatch;
+ found   : Seq[CollectionCons] (in scala.collection)
+ required: Seq[CollectionCons] (in scala.collection.immutable)
+  def f(x: CollectionSeq) = x match {
+                              ^
+two errors found

test/files/neg/t11102.scala

@@ -0,0 +1,117 @@
+object Test {
+  def f(x: ImmutableSeq) = x match {
+    case ImmutableCons(x, xs @ _*)  => xs
+  }
+  def f(x: MutableSeq) = x match {
+    case MutableCons(x, xs @ _*)    => xs
+  }
+  def f(x: CollectionSeq) = x match {
+    case CollectionCons(x, xs @ _*) => xs
+  }
+  def f(x: ScalaSeq) = x match {
+    case ScalaCons(x, xs @ _*)      => xs
+  }
+  def f(x: DefaultSeq) = x match {
+    case DefaultCons(x, xs @ _*)    => xs
+  }
+}
+
+/** 
+ * collection.immutable.Seq
+ */
+abstract class ImmutableSeq
+extends collection.immutable.Seq[Int]
+ with UnimplementedSeq
+
+object ImmutableCons {
+  def unapplySeq(x: ImmutableCons) =
+    Some((x.first, x.more))
+}
+
+abstract class ImmutableCons
+extends ImmutableSeq {
+  def first: Int
+  def more: collection.immutable.Seq[ImmutableCons]
+}
+
+/** 
+ * collection.mutable.Seq
+ */
+abstract class MutableSeq
+extends collection.mutable.Seq[Int]
+with UnimplementedSeq
+
+object MutableCons {
+  def unapplySeq(x: MutableCons) =
+    Some((x.first, x.more))                                 // !
+}
+
+abstract class MutableCons
+extends MutableSeq {
+  def first: Int
+  def more: collection.mutable.Seq[MutableCons]
+}
+
+/**
+ * collection.Seq
+ */
+abstract class CollectionSeq
+extends collection.Seq[Int]
+with UnimplementedSeq
+
+object CollectionCons {
+  def unapplySeq(x: CollectionCons) =
+    Some((x.first, x.more))                                 // !
+}
+
+abstract class CollectionCons
+extends CollectionSeq {
+  def first: Int
+  def more: collection.Seq[CollectionCons]
+}
+
+/**
+ * scala.Seq
+ */
+abstract class ScalaSeq
+extends collection.Seq[Int]
+with UnimplementedSeq
+
+object ScalaCons {
+  def unapplySeq(x: ScalaCons) =
+    Some((x.first, x.more))
+}
+
+abstract class ScalaCons
+extends ScalaSeq {
+  def first: Int
+  def more: scala.Seq[ScalaCons]
+}
+
+/**
+ * Seq
+ */
+abstract class DefaultSeq
+extends Seq[Int]
+with UnimplementedSeq
+
+object DefaultCons {
+  def unapplySeq(x: DefaultCons) =
+    Some((x.first, x.more))
+}
+
+abstract class DefaultCons
+extends DefaultSeq {
+  def first: Int
+  def more: Seq[DefaultCons]
+}
+
+/**
+ * Unimplemented sequence.
+ */
+trait UnimplementedSeq {
+  def iterator: Iterator[Int]           = ???
+  def apply(i: Int): Int                = ???
+  def length: Int                       = ???
+  def update(idx: Int, elem: Int): Unit = ???
+}

test/files/pos/t11102.scala

@@ -0,0 +1,117 @@
+object Test {
+  def f(x: ImmutableSeq) = x match {
+    case ImmutableCons(x, xs @ _*)  => xs
+  }
+  def f(x: MutableSeq) = x match {
+    case MutableCons(x, xs @ _*)    => xs
+  }
+  def f(x: CollectionSeq) = x match {
+    case CollectionCons(x, xs @ _*) => xs
+  }
+  def f(x: ScalaSeq) = x match {
+    case ScalaCons(x, xs @ _*)      => xs
+  }
+  def f(x: DefaultSeq) = x match {
+    case DefaultCons(x, xs @ _*)    => xs
+  }
+}
+
+/** 
+ * collection.immutable.Seq
+ */
+abstract class ImmutableSeq
+extends collection.immutable.Seq[Int]
+ with UnimplementedSeq
+
+object ImmutableCons {
+  def unapplySeq(x: ImmutableCons) =
+    Some((x.first, x.more))
+}
+
+abstract class ImmutableCons
+extends ImmutableSeq {
+  def first: Int
+  def more: collection.immutable.Seq[ImmutableCons]
+}
+
+/** 
+ * collection.mutable.Seq
+ */
+abstract class MutableSeq
+extends collection.mutable.Seq[Int]
+with UnimplementedSeq
+
+object MutableCons {
+  def unapplySeq(x: MutableCons) =
+    Some((x.first, x.more.toSeq))                           // !
+}
+
+abstract class MutableCons
+extends MutableSeq {
+  def first: Int
+  def more: collection.mutable.Seq[MutableCons]
+}
+
+/**
+ * collection.Seq
+ */
+abstract class CollectionSeq
+extends collection.Seq[Int]
+with UnimplementedSeq
+
+object CollectionCons {
+  def unapplySeq(x: CollectionCons) =
+    Some((x.first, x.more.toSeq))                           // !
+}
+
+abstract class CollectionCons
+extends CollectionSeq {
+  def first: Int
+  def more: collection.Seq[CollectionCons]
+}
+
+/**
+ * scala.Seq
+ */
+abstract class ScalaSeq
+extends collection.Seq[Int]
+with UnimplementedSeq
+
+object ScalaCons {
+  def unapplySeq(x: ScalaCons) =
+    Some((x.first, x.more))
+}
+
+abstract class ScalaCons
+extends ScalaSeq {
+  def first: Int
+  def more: scala.Seq[ScalaCons]
+}
+
+/**
+ * Seq
+ */
+abstract class DefaultSeq
+extends Seq[Int]
+with UnimplementedSeq
+
+object DefaultCons {
+  def unapplySeq(x: DefaultCons) =
+    Some((x.first, x.more))
+}
+
+abstract class DefaultCons
+extends DefaultSeq {
+  def first: Int
+  def more: Seq[DefaultCons]
+}
+
+/**
+ * Unimplemented sequence.
+ */
+trait UnimplementedSeq {
+  def iterator: Iterator[Int]           = ???
+  def apply(i: Int): Int                = ???
+  def length: Int                       = ???
+  def update(idx: Int, elem: Int): Unit = ???
+}