1 files changed, 138 insertions, 0 deletions
diff --git a/src/dotty/tools/dotc/typer/VarianceChecker.scala b/src/dotty/tools/dotc/typer/VarianceChecker.scala
new file mode 100644
index 000000000..36310767a
--- /dev/null
+++ b/src/dotty/tools/dotc/typer/VarianceChecker.scala
@@ -0,0 +1,138 @@
+package dotty.tools.dotc
+package typer
+
+import dotty.tools.dotc.ast.{ Trees, tpd }
+import core._
+import Types._, Contexts._, Flags._, Symbols._, Annotations._, Trees._, NameOps._
+import Decorators._
+import Variances._
+
+object VarianceChecker {
+  private case class VarianceError(tvar: Symbol, required: Variance)
+  def check(tree: tpd.Tree)(implicit ctx: Context) =
+    new VarianceChecker()(ctx).Traverser.traverse(tree)
+}
+
+/** See comments at scala.reflect.internal.Variance.
+ */
+class VarianceChecker()(implicit ctx: Context) {
+  import VarianceChecker._
+  import tpd._
+
+  private object Validator extends TypeAccumulator[Option[VarianceError]] {
+    private var base: Symbol = _
+
+    /** Is no variance checking needed within definition of `base`? */
+    def ignoreVarianceIn(base: Symbol): Boolean = (
+         base.isTerm
+      || base.is(Package)
+      || base.is(Local)
+    )
+
+    /** The variance of a symbol occurrence of `tvar` seen at the level of the definition of `base`.
+     *  The search proceeds from `base` to the owner of `tvar`.
+     *  Initially the state is covariant, but it might change along the search.
+     */
+    def relativeVariance(tvar: Symbol, base: Symbol, v: Variance = Covariant): Variance = /*ctx.traceIndented(i"relative variance of $tvar wrt $base, so far: $v")*/ {
+      if (base == tvar.owner) v
+      else if ((base is Param) && base.owner.isTerm)
+        relativeVariance(tvar, paramOuter(base.owner), flip(v))
+      else if (ignoreVarianceIn(base.owner)) Bivariant
+      else if (base.isAliasType) relativeVariance(tvar, base.owner, Invariant)
+      else relativeVariance(tvar, base.owner, v)
+    }
+
+    /** The next level to take into account when determining the
+     *  relative variance with a method parameter as base. The method
+     *  is always skipped. If the method is a constructor, we also skip
+     *  its class owner, because constructors are not checked for variance
+     *  relative to the type parameters of their own class. On the other
+     *  hand constructors do count for checking the variance of type parameters
+     *  of enclosing classes. I believe the Scala 2 rules are too lenient in
+     *  that respect.
+     */
+    private def paramOuter(meth: Symbol) =
+      if (meth.isConstructor) meth.owner.owner else meth.owner
+
+    /** Check variance of abstract type `tvar` when referred from `base`. */
+    private def checkVarianceOfSymbol(tvar: Symbol): Option[VarianceError] = {
+      val relative = relativeVariance(tvar, base)
+      if (relative == Bivariant) None
+      else {
+        val required = compose(relative, this.variance)
+        def tvar_s = s"$tvar (${varianceString(tvar.flags)} ${tvar.showLocated})"
+        def base_s = s"$base in ${base.owner}" + (if (base.owner.isClass) "" else " in " + base.owner.enclosingClass)
+        ctx.log(s"verifying $tvar_s is ${varianceString(required)} at $base_s")
+        ctx.log(s"relative variance: ${varianceString(relative)}")
+        ctx.log(s"current variance: ${this.variance}")
+        ctx.log(s"owner chain: ${base.ownersIterator.toList}")
+        if (tvar is required) None
+        else Some(VarianceError(tvar, required))
+      }
+    }
+
+    /** For PolyTypes, type parameters are skipped because they are defined
+     *  explicitly (their TypeDefs will be passed here.) For MethodTypes, the
+     *  same is true of the parameters (ValDefs).
+     */
+    def apply(status: Option[VarianceError], tp: Type): Option[VarianceError] = ctx.traceIndented(s"variance checking $tp of $base at $variance") {
+      if (status.isDefined) status
+      else tp match {
+        case tp: TypeRef =>
+          val sym = tp.symbol
+          if (sym.variance != 0 && base.isContainedIn(sym.owner)) checkVarianceOfSymbol(sym)
+          else if (sym.isAliasType) this(status, sym.info)
+          else foldOver(status, tp)
+        case tp: MethodType =>
+          this(status, tp.resultType) // params will be checked in their TypeDef nodes.
+        case tp: PolyType =>
+          this(status, tp.resultType) // params will be checked in their ValDef nodes.
+        case AnnotatedType(annot, _) if annot.symbol == defn.UncheckedVarianceAnnot =>
+          status
+        //case tp: ClassInfo =>
+        //  ???  not clear what to do here yet. presumably, it's all checked at local typedefs
+        case _ =>
+          foldOver(status, tp)
+      }
+    }
+
+    def validateDefinition(base: Symbol): Option[VarianceError] = {
+      val saved = this.base
+      this.base = base
+      try apply(None, base.info)
+      finally this.base = saved
+    }
+  }
+
+  private object Traverser extends TreeTraverser {
+    def checkVariance(sym: Symbol) = Validator.validateDefinition(sym) match {
+      case Some(VarianceError(tvar, required)) =>
+        ctx.error(
+          i"${varianceString(tvar.flags)} $tvar occurs in ${varianceString(required)} position in type ${sym.info} of $sym",
+          sym.pos)
+      case None =>
+    }
+
+    override def traverse(tree: Tree) = {
+      def sym = tree.symbol
+      // No variance check for private/protected[this] methods/values.
+      def skip = !sym.exists || sym.is(Local)
+      tree match {
+        case defn: MemberDef if skip =>
+          ctx.debuglog(s"Skipping variance check of ${sym.showDcl}")
+        case tree: TypeDef =>
+          checkVariance(sym)
+          foldOver((), tree)
+        case tree: ValDef =>
+          checkVariance(sym)
+        case DefDef(_, _, tparams, vparamss, _, _) =>
+          checkVariance(sym)
+          tparams foreach traverse
+          vparamss foreach (_ foreach traverse)
+        case Template(_, _, _, body) =>
+          foldOver((), tree)
+        case _ =>
+      }
+    }
+  }
+}