Better error messages for illegal type tests

Author: odersky

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

@@ -206,19 +206,20 @@ object TypeTestsCasts {
            *  can be true in some cases. Issues a warning or an error otherwise.
            */
           def checkSensical(foundCls: Symbol)(using Context): Boolean =
+            def exprType = i"type ${expr.tpe.widen.stripAnnots}"
             if (!isCheckable(foundCls)) true
             else if (foundCls.isPrimitiveValueClass && !testCls.isPrimitiveValueClass) {
-              ctx.error("cannot test if value types are references", tree.sourcePos)
+              ctx.error(i"cannot test if value of $exprType is a reference of $testCls", tree.sourcePos)
               false
             }
             else if (!foundCls.derivesFrom(testCls)) {
               val unrelated = !testCls.derivesFrom(foundCls) && (
                 testCls.is(Final) || !testCls.is(Trait) && !foundCls.is(Trait)
               )
               if (foundCls.is(Final))
-                unreachable(i"type ${expr.tpe.widen} is not a subclass of $testCls")
+                unreachable(i"$exprType is not a subclass of $testCls")
               else if (unrelated)
-                unreachable(i"type ${expr.tpe.widen} and $testCls are unrelated")
+                unreachable(i"$exprType and $testCls are unrelated")
               else true
             }
             else true