Prioritise sequence-matches over product-sequence-matches (#19260)

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -360,13 +360,13 @@ object PatternMatcher {
                 .map(ref(unappResult).select(_))
               matchArgsPlan(selectors, args, onSuccess)
             }
+            else if (isUnapplySeq && unapplySeqTypeElemTp(unapp.tpe.widen.finalResultType).exists) {
+              unapplySeqPlan(unappResult, args)
+            }
             else if (isUnapplySeq && isProductSeqMatch(unapp.tpe.widen, args.length, unapp.srcPos)) {
               val arity = productArity(unapp.tpe.widen, unapp.srcPos)
               unapplyProductSeqPlan(unappResult, args, arity)
             }
-            else if (isUnapplySeq && unapplySeqTypeElemTp(unapp.tpe.widen.finalResultType).exists) {
-              unapplySeqPlan(unappResult, args)
-            }
             else if unappResult.info <:< defn.NonEmptyTupleTypeRef then
               val components = (0 until foldApplyTupleType(unappResult.denot.info).length).toList.map(tupleApp(_, ref(unappResult)))
               matchArgsPlan(components, args, onSuccess)

tests/pos/i19221.orig.scala

@@ -0,0 +1,17 @@
+object Test:
+  class Custom extends scala.Product1[String]:
+    def length: Int = ???
+    def apply(i: Int): Boolean = ???
+    def drop(n: Int): scala.Seq[Boolean] = ???
+    def toSeq: scala.Seq[Boolean] = ???
+
+    def canEqual(that: Any): Boolean = ???
+
+    val _1: String = ???
+    val _2: String = ???
+    val _3: Seq[String] = ???
+
+  object A:
+    def unapplySeq(i: Int): Custom = ???
+
+  val A(a, rest*) = 1

tests/pos/i19221.scala

@@ -0,0 +1,35 @@
+class T1
+
+class P1
+final class P2
+class P3
+
+class E1
+class E2 extends E1
+class E3 extends E1
+
+object VarExt:
+  def unapplySeq(t1: T1): U1 = new U1
+
+class U1 extends Product1[P1]:
+  def canEqual(that: Any): Boolean = ???
+
+  val _1: P1      = new P1
+  val _2: P2      = new P2
+  val _3: Seq[P3] = Seq(new P3)
+
+  def length: Int           = ???
+  def apply(i: Int): E1     = ???
+  def drop(n: Int): Seq[E2] = ???
+  def toSeq: Seq[E3]        = ???
+
+class Test:
+  def m1(t1: T1): Unit = t1 match
+    case VarExt(c1, cs*) => // CCE: class P1 cannot be cast to class E1
+      val e1: E1       = c1
+      val e1s: Seq[E1] = cs
+
+object Main:
+  def main(args: Array[String]): Unit =
+    new Test().m1(new T1)
+