Cleanup commit

This commit cleans up a piece of code of #3774 (which is already merged).
It was suggested in review comments.

Author: odersky

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

@@ -375,6 +375,87 @@ object Inferencing {
   }
 }
 
+trait Inferencing { this: Typer =>
+  import Inferencing._
+  import tpd._
+
+  /** Interpolate those undetermined type variables in the widened type of this tree
+   *  which are introduced by type application contained in the tree.
+   *  If such a variable appears covariantly in type `tp` or does not appear at all,
+   *  approximate it by its lower bound. Otherwise, if it appears contravariantly
+   *  in type `tp` approximate it by its upper bound.
+   *  @param ownedBy  if it is different from NoSymbol, all type variables owned by
+   *                  `ownedBy` qualify, independent of position.
+   *                  Without that second condition, it can be that certain variables escape
+   *                  interpolation, for instance when their tree was eta-lifted, so
+   *                  the typechecked tree is no longer the tree in which the variable
+   *                  was declared. A concrete example of this phenomenon can be
+   *                  observed when compiling core.TypeOps#asSeenFrom.
+   */
+  def interpolateUndetVars(tree: Tree, ownedBy: Symbol, pt: Type)(implicit ctx: Context): Unit = {
+    val constraint = ctx.typerState.constraint
+    val qualifies = (tvar: TypeVar) =>
+      (tree contains tvar.bindingTree) || ownedBy.exists && tvar.owner == ownedBy
+    def interpolate() = Stats.track("interpolateUndetVars") {
+      val tp = tree.tpe.widen
+      constr.println(s"interpolate undet vars in ${tp.show}, pos = ${tree.pos}, mode = ${ctx.mode}, undets = ${constraint.uninstVars map (tvar => s"${tvar.show}@${tvar.bindingTree.pos}")}")
+      constr.println(s"qualifying undet vars: ${constraint.uninstVars filter qualifies map (tvar => s"$tvar / ${tvar.show}")}, constraint: ${constraint.show}")
+
+      val vs = variances(tp, qualifies)
+      val hasUnreportedErrors = ctx.typerState.reporter match {
+        case r: StoreReporter if r.hasErrors => true
+        case _ => false
+      }
+
+      var isConstrained = tree.isInstanceOf[Apply] || tree.tpe.isInstanceOf[MethodOrPoly]
+
+      def ensureConstrained() = if (!isConstrained) {
+        isConstrained = true
+        constrainResult(tree.tpe, pt)
+      }
+
+      // Avoid interpolating variables if typerstate has unreported errors.
+      // Reason: The errors might reflect unsatisfiable constraints. In that
+      // case interpolating without taking account the constraints risks producing
+      // nonsensical types that then in turn produce incomprehensible errors.
+      // An example is in neg/i1240.scala. Without the condition in the next code line
+      // we get for
+      //
+      //      val y: List[List[String]] = List(List(1))
+      //
+      //     i1430.scala:5: error: type mismatch:
+      //     found   : Int(1)
+      //     required: Nothing
+      //     val y: List[List[String]] = List(List(1))
+      //                                           ^
+      // With the condition, we get the much more sensical:
+      //
+      //     i1430.scala:5: error: type mismatch:
+      //     found   : Int(1)
+      //     required: String
+      //     val y: List[List[String]] = List(List(1))
+      if (!hasUnreportedErrors)
+        vs foreachBinding { (tvar, v) =>
+          if (v != 0 && ctx.typerState.constraint.contains(tvar)) {
+            // previous interpolations could have already instantiated `tvar`
+            // through unification, that's why we have to check again whether `tvar`
+            // is contained in the current constraint.
+            typr.println(s"interpolate ${if (v == 1) "co" else "contra"}variant ${tvar.show} in ${tp.show}")
+            ensureConstrained()
+            tvar.instantiate(fromBelow = v == 1)
+          }
+        }
+      for (tvar <- constraint.uninstVars)
+        if (!(vs contains tvar) && qualifies(tvar)) {
+          typr.println(s"instantiating non-occurring ${tvar.show} in ${tp.show} / $tp")
+          ensureConstrained()
+          tvar.instantiate(fromBelow = true)
+        }
+    }
+    if (constraint.uninstVars exists qualifies) interpolate()
+  }
+}
+
 /** An enumeration controlling the degree of forcing in "is-dully-defined" checks. */
 @sharable object ForceDegree {
   class Value(val appliesTo: TypeVar => Boolean, val minimizeAll: Boolean)