Refine lub calculation.

When a reasonable lub cannot be arrived at by direct means, use the
bounds of the lub inputs to derive bounds for the lub rather than giving
up. Closes SI-4938, review by moors.

Author: paulp

src/compiler/scala/reflect/internal/Types.scala

@@ -5822,10 +5822,18 @@ A type's typeSymbol should never be inspected directly.
           }
         } else {
           val args = (sym.typeParams, argss.transpose).zipped map { (tparam, as) =>
-              if (depth == 0)
-                if (tparam.variance == variance) AnyClass.tpe
+              if (depth == 0) {
+                if (tparam.variance == variance) {
+                  // Take the intersection of the upper bounds of the type parameters
+                  // rather than falling all the way back to "Any", otherwise we end up not
+                  // conforming to bounds.
+                  val bounds0 = sym.typeParams map (_.info.bounds.hi) filterNot (_.typeSymbol == AnyClass)
+                  if (bounds0.isEmpty) AnyClass.tpe
+                  else intersectionType(bounds0)
+                }
                 else if (tparam.variance == -variance) NothingClass.tpe
                 else NoType
+              }
               else {
                 if (tparam.variance == variance) lub(as, decr(depth))
                 else if (tparam.variance == -variance) glb(as, decr(depth))

test/files/pos/t4938.scala

@@ -0,0 +1,4 @@
+class A {
+  import scala.collection.mutable._
+  val xs = List(Set(), Seq())
+}