Fix #6336: Error on implicit conversions to AnyVal or AnyRef

Author: odersky

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

@@ -3140,7 +3140,9 @@ class Typer extends Namer
         if (isFullyDefined(wtp, force = ForceDegree.all) &&
             ctx.typerState.constraint.ne(prevConstraint)) readapt(tree)
         else err.typeMismatch(tree, pt, failure)
-      if (ctx.mode.is(Mode.ImplicitsEnabled) && tree.typeOpt.isValueType)
+      if ctx.mode.is(Mode.ImplicitsEnabled) && tree.typeOpt.isValueType then
+        if pt.isRef(defn.AnyValClass) || pt.isRef(defn.ObjectClass) then
+          ctx.error(em"the result of an implicit conversion must be more specific than $pt", tree.sourcePos)
         inferView(tree, pt) match {
           case SearchSuccess(found: ExtMethodApply, _, _) =>
             found // nothing to check or adapt for extension method applications

tests/neg/i6336.scala

@@ -0,0 +1,4 @@
+object Test {
+  val a: AnyVal = "foo" // error
+  val b: AnyRef = 1 // error
+}