Add tests

from https://gist.github.com/Blaisorblade/a0eebb6a4f35344e48c4c60dc2a14ce6

Author: nicolasstucki

tests/neg-custom-args/fatal-warnings/IsInstanceOfClassTag.scala

@@ -0,0 +1,24 @@
+import scala.reflect.ClassTag
+
+object IsInstanceOfClassTag {
+  def safeCast[T: ClassTag](x: Any): Option[T] = {
+    x match {
+      case x: T => Some(x) // TODO error: deprecation waring
+      case _ => None
+    }
+  }
+
+  def main(args: Array[String]): Unit = {
+    safeCast[List[String]](List[Int](1)) match {
+      case None =>
+      case Some(xs) =>
+        xs.head.substring(0)
+    }
+
+    safeCast[List[_]](List[Int](1)) match {
+      case None =>
+      case Some(xs) =>
+        xs.head.substring(0) // error
+    }
+  }
+}

tests/neg-custom-args/fatal-warnings/IsInstanceOfClassTag2.scala

@@ -0,0 +1,22 @@
+import scala.tasty.TypeTest
+
+object IsInstanceOfClassTag {
+  def safeCast[T](x: Any)(given TypeTest[Any, T]): Option[T] = {
+    x match {
+      case x: T => Some(x)
+      case _ => None
+    }
+  }
+
+  def main(args: Array[String]): Unit = {
+    safeCast[List[String]](List[Int](1)) match { // error
+      case None =>
+      case Some(xs) =>
+    }
+
+    safeCast[List[_]](List[Int](1)) match { // error
+      case None =>
+      case Some(xs) =>
+    }
+  }
+}