From 445195fed196655593c3971ffd377cae9484ac92 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Patryk=20Zieli=C5=84ski?= <pzielinski@virtuslab.com>
Date: Sun, 26 Oct 2025 13:47:40 +0100
Subject: [PATCH] treat `x*` pattern arg as a Seq of the element type.

---
 .../src/dotty/tools/dotc/transform/patmat/Space.scala     | 7 ++++++-
 tests/warn/i23459.check                                   | 6 ++++++
 tests/warn/i23459.scala                                   | 8 ++++++++
 tests/warn/i23459b.check                                  | 6 ++++++
 tests/warn/i23459b.scala                                  | 8 ++++++++
 5 files changed, 34 insertions(+), 1 deletion(-)
 create mode 100644 tests/warn/i23459.check
 create mode 100644 tests/warn/i23459.scala
 create mode 100644 tests/warn/i23459b.check
 create mode 100644 tests/warn/i23459b.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
index 2a0cbbdbf006..446fd4780acd 100644
--- a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
+++ b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
@@ -382,7 +382,12 @@ object SpaceEngine {
       project(pat)
 
     case Typed(_, tpt) =>
-      Typ(erase(tpt.tpe.stripAnnots, isValue = true, isTyped = true), decomposed = false)
+      val tptTpe = 
+        if isWildcardStarArg(pat) then 
+          tpt.tpe.translateFromRepeated(toArray = false)
+        else 
+          tpt.tpe
+      Typ(erase(tptTpe.stripAnnots, isValue = true, isTyped = true), decomposed = false)
 
     case This(_) =>
       Typ(pat.tpe.stripAnnots, decomposed = false)
diff --git a/tests/warn/i23459.check b/tests/warn/i23459.check
new file mode 100644
index 000000000000..bbe9bf5a4ec7
--- /dev/null
+++ b/tests/warn/i23459.check
@@ -0,0 +1,6 @@
+-- [E092] Pattern Match Unchecked Warning: tests/warn/i23459.scala:7:14 ------------------------------------------------
+7 |    case Test(x*) => println(x) // warn
+  |              ^
+  |the type test for Seq[Int]* cannot be checked at runtime because its type arguments can't be determined from Seq[Int]
+  |
+  | longer explanation available when compiling with `-explain`
diff --git a/tests/warn/i23459.scala b/tests/warn/i23459.scala
new file mode 100644
index 000000000000..af6a01e2ebce
--- /dev/null
+++ b/tests/warn/i23459.scala
@@ -0,0 +1,8 @@
+case class Test()
+object Test:
+  def unapply(t: Test): Some[Seq[Int]] = Some(Seq(1, 2))
+
+@main def run(): Unit =
+  Test() match
+    case Test(x*) => println(x) // warn
+
diff --git a/tests/warn/i23459b.check b/tests/warn/i23459b.check
new file mode 100644
index 000000000000..cd82f2005906
--- /dev/null
+++ b/tests/warn/i23459b.check
@@ -0,0 +1,6 @@
+-- [E092] Pattern Match Unchecked Warning: tests/warn/i23459b.scala:7:14 -----------------------------------------------
+7 |    case Test(_*) => () // warn
+  |              ^
+  |the type test for Seq[Int]* cannot be checked at runtime because its type arguments can't be determined from Seq[Int]
+  |
+  | longer explanation available when compiling with `-explain`
diff --git a/tests/warn/i23459b.scala b/tests/warn/i23459b.scala
new file mode 100644
index 000000000000..792afafe038e
--- /dev/null
+++ b/tests/warn/i23459b.scala
@@ -0,0 +1,8 @@
+case class Test()
+object Test:
+  def unapply(t: Test): Some[Seq[Int]] = Some(Seq(1, 2))
+
+@main def run(): Unit =
+  Test() match
+    case Test(_*) => () // warn
+