Fix widen in AndType

Author: noti0na1

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1208,6 +1208,7 @@ object Types {
         if tp.isOverloaded then tp else tp.underlying.widen
       case tp: SingletonType => tp.underlying.widen
       case tp: ExprType => tp.resultType.widen
+      case tp: AndType => tp.derivedAndType(tp.tp1.widen, tp.tp2.widen)
       case tp =>
         val tp1 = tp.stripped
         if tp1 eq tp then tp

tests/explicit-nulls/pos/i13197.scala

@@ -0,0 +1,7 @@
+trait Bar:
+  def b: String | Null
+
+class Foo(a: String = "", b: String)
+
+object Foo:
+  def foo(bar: Bar) = Foo(b = bar.b.nn)