Fix #2973: Check variances of class parents

Author: odersky

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

@@ -1499,10 +1499,10 @@ class Typer extends Namer
       checkNoDoubleDefs(cls)
       val impl1 = cpy.Template(impl)(constr1, parents1, self1, body1)
         .withType(dummy.termRef)
-      checkVariance(impl1)
       if (!cls.is(AbstractOrTrait) && !ctx.isAfterTyper)
         checkRealizableBounds(cls, cdef.namePos)
       val cdef1 = assignType(cpy.TypeDef(cdef)(name, impl1), cls)
+      checkVariance(cdef1)
       if (ctx.phase.isTyper && cdef1.tpe.derivesFrom(defn.DynamicClass) && !ctx.dynamicsEnabled) {
         val isRequired = parents1.exists(_.tpe.isRef(defn.DynamicClass))
         ctx.featureWarning(nme.dynamics.toString, "extension of type scala.Dynamic", isScala2Feature = true,

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

@@ -93,8 +93,8 @@ class VarianceChecker()(implicit ctx: Context) {
           this(status, tp.resultType) // params will be checked in their TypeDef or ValDef nodes.
         case AnnotatedType(_, annot) if annot.symbol == defn.UncheckedVarianceAnnot =>
           status
-        //case tp: ClassInfo =>
-        //  ???  not clear what to do here yet. presumably, it's all checked at local typedefs
+        case tp: ClassInfo =>
+          foldOver(status, tp.classParents)
         case _ =>
           foldOver(status, tp)
       }
@@ -132,14 +132,16 @@ class VarianceChecker()(implicit ctx: Context) {
           ctx.debuglog(s"Skipping variance check of ${sym.showDcl}")
         case tree: TypeDef =>
           checkVariance(sym, tree.pos)
+          tree.rhs match {
+            case rhs: Template => traverseChildren(rhs)
+            case _ =>
+          }
         case tree: ValDef =>
           checkVariance(sym, tree.pos)
         case DefDef(_, tparams, vparamss, _, _) =>
           checkVariance(sym, tree.pos)
           tparams foreach traverse
           vparamss foreach (_ foreach traverse)
-        case Template(_, _, _, body) =>
-          traverseChildren(tree)
         case _ =>
       }
     }

tests/neg/i2973.scala

@@ -0,0 +1,2 @@
+class Foo[U]
+class Bar[-T] extends Foo[T] // error: contravariant type T occurs in invariant position