Add warning on != with same type fix isNullable.

Note that without the fix console tests will fail and without
the warning there is no way to test the fix.

Author: nicolasstucki

src/dotty/tools/dotc/core/TypeComparer.scala

@@ -527,7 +527,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
           }
           isSubType(hi1, tp2) || compareGADT
         case _ =>
-          def isNullable(tp: Type): Boolean = tp.dealias match {
+          def isNullable(tp: Type): Boolean = tp.widenDealias match {
             case tp: TypeRef => tp.symbol.isNullableClass
             case tp: RefinedOrRecType => isNullable(tp.parent)
             case AndType(tp1, tp2) => isNullable(tp1) && isNullable(tp2)

src/dotty/tools/dotc/transform/InterceptedMethods.scala

@@ -103,7 +103,13 @@ class InterceptedMethods extends MiniPhaseTransform {
       case Any_## =>
           poundPoundValue(qual)
       case Any_!= =>
-          qual.select(defn.Any_==).appliedToArgs(tree.args).select(defn.Boolean_!)
+        val lhs = qual.tpe
+        val rhs = tree.args.head.tpe
+        if (!(lhs <:< rhs || rhs <:< lhs)) {
+          ctx.warning(ex"comparing values of types ${lhs.widenDealias} and ${rhs.widenDealias} using `!=' will always yield true",
+              tree.pos)
+        }
+        qual.select(defn.Any_==).appliedToArgs(tree.args).select(defn.Boolean_!)
         /*
         /* else if (isPrimitiveValueClass(qual.tpe.typeSymbol)) {
             // todo: this is needed to support value classes