Fix erroneous isInstanceOf test in compiler

Author: odersky

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -131,7 +131,7 @@ enum ErrorMessageID extends java.lang.Enum[ErrorMessageID]:
     DoubleDefinitionID,
     MatchCaseOnlyNullWarningID,
     ImportRenamedTwiceID,
-    TypeTestAlwaysSucceedsID,
+    TypeTestAlwaysDivergesID,
     TermMemberNeedsNeedsResultTypeForImplicitSearchID,
     ClassCannotExtendEnumID,
     ValueClassParameterMayNotBeCallByNameID,

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2177,13 +2177,9 @@ import transform.SymUtils._
     def explain = ""
   }
 
-  class TypeTestAlwaysSucceeds(scrutTp: Type, testTp: Type)(using Context) extends SyntaxMsg(TypeTestAlwaysSucceedsID) {
-    def msg = {
-      val addendum =
-        if (scrutTp != testTp) s" is a subtype of ${testTp.show}"
-        else " is the same as the tested type"
-      s"The highlighted type test will always succeed since the scrutinee type ${scrutTp.show}" + addendum
-    }
+  class TypeTestAlwaysDiverges(scrutTp: Type, testTp: Type)(using Context) extends SyntaxMsg(TypeTestAlwaysDivergesID) {
+    def msg =
+      s"This type test will never return a result since the scrutinee type ${scrutTp.show} does not contain any value."
     def explain = ""
   }
 

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

@@ -418,7 +418,7 @@ class TreeChecker extends Phase with SymTransformer {
     }
 
     override def typedSuper(tree: untpd.Super, pt: Type)(using Context): Tree =
-      assert(tree.qual.tpe.isInstanceOf[ThisType], i"expect prefix of Super to be This, actual = ${tree.qual}")
+      assert(tree.qual.typeOpt.isInstanceOf[ThisType], i"expect prefix of Super to be This, actual = ${tree.qual}")
       super.typedSuper(tree, pt)
 
     override def typedTyped(tree: untpd.Typed, pt: Type)(using Context): Tree =