From e0bd31dab6a119978cf0baa14ae3d6714e43dc67 Mon Sep 17 00:00:00 2001
From: EnzeXing <enzexing12@gmail.com>
Date: Sat, 5 Jul 2025 20:54:59 -0400
Subject: [PATCH] fix error in product-sequence match

---
 .../tools/dotc/transform/init/Objects.scala   |  3 ++
 .../unapplySeq-product-sequence-match.scala   | 29 +++++++++++++++++++
 2 files changed, 32 insertions(+)
 create mode 100644 tests/init-global/pos/unapplySeq-product-sequence-match.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/init/Objects.scala b/compiler/src/dotty/tools/dotc/transform/init/Objects.scala
index dfa685f8aaa9..92262d528487 100644
--- a/compiler/src/dotty/tools/dotc/transform/init/Objects.scala
+++ b/compiler/src/dotty/tools/dotc/transform/init/Objects.scala
@@ -1804,6 +1804,9 @@ class Objects(using Context @constructorOnly):
             val seqPats = pats.drop(selectors.length - 1)
             val toSeqRes = call(resToMatch, selectors.last, Nil, resultTp, superType = NoType, needResolve = true)
             val toSeqResTp = resultTp.memberInfo(selectors.last).finalResultType
+            elemTp = unapplySeqTypeElemTp(toSeqResTp)
+            // elemTp must conform to the signature in sequence match
+            assert(elemTp.exists, "Product sequence match fails on " + pat + " since last element type of product is " + toSeqResTp)
             evalSeqPatterns(toSeqRes, toSeqResTp, elemTp, seqPats)
           end if
           // TODO: refactor the code of product sequence match, avoid passing NoType to parameter elemTp in evalSeqPatterns
diff --git a/tests/init-global/pos/unapplySeq-product-sequence-match.scala b/tests/init-global/pos/unapplySeq-product-sequence-match.scala
new file mode 100644
index 000000000000..2105fb63dc6e
--- /dev/null
+++ b/tests/init-global/pos/unapplySeq-product-sequence-match.scala
@@ -0,0 +1,29 @@
+trait Node {
+  val prefix = 5
+  val child = Array(3)
+}
+
+class SpecialNode extends Node
+
+class Group extends Node
+
+class C extends Node
+
+object Elem {
+  def apply(prefix: Int, children: Int*) = new C
+  def unapplySeq(n: Node) = n match {
+    case _: SpecialNode | _: Group => None
+    case _                         => Some((n.prefix, n.child.toSeq))
+  }
+}
+
+object O {
+  def updateNode(node: Node): Node =
+    node match {
+      case Elem(prefix, children @ _*) =>
+        Elem(prefix, children*)
+      case other => other
+    }
+
+  val a = updateNode(new Group)
+}