TypeAssigner#avoid: preserve type parameters if possible

We need to use `baseTypeWithArgs` to replace `Bar` by its supertype
`Foo` in the type `Bar[Int]`.

Fixes scala#741.

Author: smarter

src/dotty/tools/dotc/typer/TypeAssigner.scala

@@ -88,13 +88,15 @@ trait TypeAssigner {
             case _ =>
               mapOver(tp)
           }
-        case tp: RefinedType =>
-          val tp1 @ RefinedType(parent1, _) = mapOver(tp)
-          if (tp1.refinedInfo.existsPart(toAvoid) && variance > 0) {
-            typr.println(s"dropping refinement from $tp1")
+        case tp: RefinedType if variance > 0 =>
+          val parent1 = apply(tp.parent)
+          if (tp.refinedInfo.existsPart(toAvoid)) {
+            typr.println(s"dropping refinement from $tp")
             parent1
-          }
-          else tp1
+          } else if (parent1 != tp.parent) {
+            tp.baseTypeWithArgs(parent1.classSymbol)
+          } else
+            tp
         case tp: TypeVar if ctx.typerState.constraint.contains(tp) =>
           val lo = ctx.typerState.constraint.fullLowerBound(tp.origin)
           val lo1 = avoid(lo, symsToAvoid)

tests/pos/escapingRefs.scala

@@ -1,3 +1,5 @@
+class Foo[A]
+
 class Outer {
   class Inner
 }
@@ -8,5 +10,10 @@ object Test {
       val o = new Outer
       new o.Inner
     }
+
+    val b = {
+      class Bar[B] extends Foo[B]
+      new Bar[Int]
+    }
   }
 }