Don't optimize explicitly written isInstanceOf tests away.

Do it only for tests generated during pattern matching.

Fixes #14707.

See the issue for a rationale of the change.

Author: odersky

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

@@ -243,11 +243,8 @@ object TypeTestsCasts {
             else foundClasses.exists(check)
           end checkSensical
 
-          if (expr.tpe <:< testType)
-            if (expr.tpe.isNotNull) {
-              if (!inMatch) report.warning(TypeTestAlwaysSucceeds(expr.tpe, testType), tree.srcPos)
-              constant(expr, Literal(Constant(true)))
-            }
+          if (expr.tpe <:< testType) && inMatch then
+            if expr.tpe.isNotNull then constant(expr, Literal(Constant(true)))
             else expr.testNotNull
           else {
             val nestedCtx = ctx.fresh.setNewTyperState()

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"))