Fix subtyping of null and refined types.

Author: odersky

src/dotty/tools/dotc/core/TypeComparer.scala

@@ -777,8 +777,13 @@ class TypeComparer(initctx: Context) extends DotClass {
         case TypeBounds(lo1, hi1) =>
           isSubType(hi1, tp2)
         case _ =>
+          def isNullable(tp: Type): Boolean = tp.dealias match {
+            case tp: TypeRef => tp.symbol.isNullableClass
+            case RefinedType(parent, _) => isNullable(parent)
+            case _ => false
+          }
           (tp1.symbol eq NothingClass) && tp2.isInstanceOf[ValueType] ||
-          (tp1.symbol eq NullClass) && tp2.dealias.typeSymbol.isNullableClass
+          (tp1.symbol eq NullClass) && isNullable(tp2)
       }
     case tp1: SingletonType =>
       isNewSubType(tp1.underlying.widenExpr, tp2) || {

tests/pos/i262-null-subtyping.scala

@@ -0,0 +1,19 @@
+object O {
+  // This compiles
+  val a: { type T } = null;
+  val b: Any { type T } = null;
+
+  // This doesn't:
+  // found   : Null
+  // required: AnyRef{T}
+  val c: AnyRef { type T } = null;
+
+  class A
+  class B
+
+  val d: A & B = null
+  val e: A | B = null
+
+  val f: (A & B) { def toString: String } = null
+  val g: (A | B) { def toString: String } = null
+}