Add ClassTag.apply warning

Author: nicolasstucki

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -797,9 +797,12 @@ class Typer extends Namer
           case _ =>
             None
       }
-      withTag(defn.TypeTestClass.typeRef.appliedTo(pt, tref))
+      val tag = withTag(defn.TypeTestClass.typeRef.appliedTo(pt, tref))
         .orElse(withTag(defn.ClassTagClass.typeRef.appliedTo(tref)))
         .getOrElse(tree)
+      if tag.symbol.owner == defn.ClassTagClass && !config.Feature.sourceVersion.isAtLeast(config.SourceVersion.`3.1`) then
+        report.warning("Use of ClassTag for type testing may be unsound. Consider using `reflect.Typable` instead.", tree.srcPos)
+      tag
     case _ => tree
   }
 

tests/neg-custom-args/fatal-warnings/classtag-typetest/3_0-migration.scala

@@ -0,0 +1,6 @@
+import scala.language.`3.0-migration`
+import scala.reflect.ClassTag
+
+def f3_0m[T: ClassTag](x: Any): Unit =
+    x match
+      case _: T => // error

tests/neg-custom-args/fatal-warnings/classtag-typetest/3_0.scala

@@ -0,0 +1,6 @@
+import scala.language.`3.0`
+import scala.reflect.ClassTag
+
+def f3_0[T: ClassTag](x: Any): Unit =
+  x match
+    case _: T => // error

tests/neg-custom-args/fatal-warnings/classtag-typetest/3_1-migration.scala

@@ -0,0 +1,6 @@
+import scala.language.`3.1-migration`
+import scala.reflect.ClassTag
+
+def f3_1m[T: ClassTag](x: Any): Unit =
+  x match
+    case _: T =>

tests/neg-custom-args/fatal-warnings/classtag-typetest/3_1.scala

@@ -0,0 +1,6 @@
+import scala.language.`3.1`
+import scala.reflect.ClassTag
+
+def f3_1[T: ClassTag](x: Any): Unit =
+  x match
+    case _: T =>

tests/pos/classtag-typetest/3_0-migration.scala

@@ -0,0 +1,6 @@
+import scala.language.`3.0-migration`
+import scala.reflect.ClassTag
+
+def f3_0m[T: ClassTag](x: Any): Unit =
+    x match
+      case _: T =>

tests/pos/classtag-typetest/3_0.scala

@@ -0,0 +1,6 @@
+import scala.language.`3.0`
+import scala.reflect.ClassTag
+
+def f3_0[T: ClassTag](x: Any): Unit =
+  x match
+    case _: T =>

tests/pos/classtag-typetest/3_1-migration.scala

@@ -0,0 +1,6 @@
+import scala.language.`3.1-migration`
+import scala.reflect.ClassTag
+
+def f3_1m[T: ClassTag](x: Any): Unit =
+  x match
+    case _: T =>

tests/pos/classtag-typetest/3_1.scala

@@ -0,0 +1,6 @@
+import scala.language.`3.1`
+import scala.reflect.ClassTag
+
+def f3_1[T: ClassTag](x: Any): Unit =
+  x match
+    case _: T =>