Fix #4922: unapplySeq mustn't allow arrays

Author: Blaisorblade

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -101,7 +101,7 @@ object Applications {
 
     if (unapplyName == nme.unapplySeq) {
       if (unapplyResult derivesFrom defn.SeqClass) seqSelector :: Nil
-      else if (isGetMatch(unapplyResult, pos)) {
+      else if (isGetMatch(unapplyResult, pos) && getTp.derivesFrom(defn.SeqClass)) {
         val seqArg = getTp.elemType.hiBound
         if (seqArg.exists) args.map(Function.const(seqArg))
         else fail

tests/neg/i4922.scala

@@ -0,0 +1,15 @@
+class SQ {
+  val isEmpty = false
+  val get = Array(1)
+}
+object A {
+  def unapplySeq(a: Int): SQ = new SQ
+}
+
+object Main {
+  def main(args: Array[String]): Unit = {
+    val seq: Seq[Int] = 2 match {
+      case A(xs: _*) => xs // error // error
+    }
+  }
+}