From 6f7f7ae136b41710c0859fda143bc5c2a40322f3 Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Fri, 18 Mar 2022 12:46:58 +0100
Subject: [PATCH 1/2] Don't trust @unchecked types when optimizing type tests

Some type tests are elided or replaced with non null-checks if we can show
that the expression's type is a subtype of the test type. But that can be done
safely only if the expression's type does not come from an `@unchecked` cast,
inserted by pattern matching or the user.

Fixes #14705
---
 .../tools/dotc/transform/TypeTestsCasts.scala | 18 +++++++++++++++++-
 tests/neg/tuplePatDef.scala                   |  4 ++++
 tests/pos/t2613.scala                         |  2 +-
 tests/pos/tuplePatDef.scala                   |  4 ----
 tests/run/i14693.scala                        |  9 +++++++++
 tests/run/i14705.scala                        | 19 +++++++++++++++++++
 6 files changed, 50 insertions(+), 6 deletions(-)
 create mode 100644 tests/neg/tuplePatDef.scala
 delete mode 100644 tests/pos/tuplePatDef.scala
 create mode 100644 tests/run/i14693.scala
 create mode 100644 tests/run/i14705.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala b/compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala
index 0b880afdd995..7d1740b2b581 100644
--- a/compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala
+++ b/compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala
@@ -243,7 +243,23 @@ object TypeTestsCasts {
             else foundClasses.exists(check)
           end checkSensical
 
-          if (expr.tpe <:< testType)
+          def hasUncheckedPrefix(tpe: Type): Boolean = tpe.stripped match
+            case tpe: TermRef =>
+              tpe.symbol.info.hasAnnotation(defn.UncheckedAnnot)
+              || hasUncheckedPrefix(tpe.prefix)
+            case tpe: TypeRef =>
+              tpe.symbol.hasAnnotation(defn.UncheckedAnnot)
+              || hasUncheckedPrefix(tpe.prefix)
+            case tpe: AndOrType =>
+              hasUncheckedPrefix(tpe.tp1) || hasUncheckedPrefix(tpe.tp2)
+            case tpe: AppliedType =>
+              hasUncheckedPrefix(tpe.tycon) || tpe.args.exists(hasUncheckedPrefix)
+            case tpe: TypeProxy =>
+              hasUncheckedPrefix(tpe.underlying)
+            case _ =>
+              false
+
+          if expr.tpe <:< testType && !hasUncheckedPrefix(expr.tpe) then
             if (expr.tpe.isNotNull) {
               if (!inMatch) report.warning(TypeTestAlwaysSucceeds(expr.tpe, testType), tree.srcPos)
               constant(expr, Literal(Constant(true)))
diff --git a/tests/neg/tuplePatDef.scala b/tests/neg/tuplePatDef.scala
new file mode 100644
index 000000000000..03aa4bcd15e7
--- /dev/null
+++ b/tests/neg/tuplePatDef.scala
@@ -0,0 +1,4 @@
+
+object Test {
+  val (x,y): (String, Int) = null // error: unreachable
+}
diff --git a/tests/pos/t2613.scala b/tests/pos/t2613.scala
index 17ebe2d7e9bf..2540ef12df7b 100644
--- a/tests/pos/t2613.scala
+++ b/tests/pos/t2613.scala
@@ -7,5 +7,5 @@ object Test {
 
   type M = MyRelation[_ <: Row, _ <: MyRelation[_, _]]
 
-  val (x,y): (String, M) = null
+  val (x,y): (String, M) = null : Any
 }
diff --git a/tests/pos/tuplePatDef.scala b/tests/pos/tuplePatDef.scala
deleted file mode 100644
index 22f8f8e7d6ed..000000000000
--- a/tests/pos/tuplePatDef.scala
+++ /dev/null
@@ -1,4 +0,0 @@
-
-object Test {
-  val (x,y): (String, Int) = null
-}
diff --git a/tests/run/i14693.scala b/tests/run/i14693.scala
new file mode 100644
index 000000000000..52d7ca6de4a6
--- /dev/null
+++ b/tests/run/i14693.scala
@@ -0,0 +1,9 @@
+object Test {
+  val a: Array[Long] = Array(1L)
+
+  def test(x: Any) = x match {
+    case Array(i: Long) => println("Success!")
+    case _              => println("Failure!") }
+
+  def main(args: Array[String]): Unit = test(a)
+}
\ No newline at end of file
diff --git a/tests/run/i14705.scala b/tests/run/i14705.scala
new file mode 100644
index 000000000000..1ee4f6c7752b
--- /dev/null
+++ b/tests/run/i14705.scala
@@ -0,0 +1,19 @@
+trait Fruit
+case class Apple() extends Fruit
+case class Orange() extends Fruit
+
+case class Box[C](fruit: C) extends Fruit
+
+val apple = Box(fruit = Apple())
+val orange = Box(fruit = Orange())
+
+
+val result = List(apple, orange).map {
+  case appleBox: Box[Apple] @unchecked if appleBox.fruit.isInstanceOf[Apple] => //contains apple
+    "apple"
+  case _ =>
+    "orange"
+}
+
+@main def Test =
+  assert(result == List("apple", "orange"))

From dafc69a2a3c86a6151268897f1c7fe9db8d07aaf Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Fri, 18 Mar 2022 20:10:06 +0100
Subject: [PATCH 2/2] Drop unrelated test

---
 tests/run/i14693.scala | 9 ---------
 1 file changed, 9 deletions(-)
 delete mode 100644 tests/run/i14693.scala

diff --git a/tests/run/i14693.scala b/tests/run/i14693.scala
deleted file mode 100644
index 52d7ca6de4a6..000000000000
--- a/tests/run/i14693.scala
+++ /dev/null
@@ -1,9 +0,0 @@
-object Test {
-  val a: Array[Long] = Array(1L)
-
-  def test(x: Any) = x match {
-    case Array(i: Long) => println("Success!")
-    case _              => println("Failure!") }
-
-  def main(args: Array[String]): Unit = test(a)
-}
\ No newline at end of file