Explain and test that applyDynamic and selectDynamic can have using clauses

Author: odersky

docs/docs/reference/changed-features/structural-types-spec.md

@@ -43,7 +43,7 @@ def applyDynamic(name: String, ctags: ClassTag[?]*)(args: Any*): T
 ```
 Both versions are passed the actual arguments in the `args` parameter. The second version takes in addition a vararg argument of class tags that identify the method's parameter classes. Such an argument is needed
 if `applyDynamic` is implemented using Java reflection, but it could be
-useful in other cases as well.
+useful in other cases as well. `selectDynamic` and `applyDynamic` can also take additional context parameters in using clauses. These are resolved in the normal way at the callsite.
 
 Given a value `v` of type `C { Rs }`, where `C` is a class reference
 and `Rs` are structural refinement declarations, and given `v.a` of type `U`, we consider three distinct cases:

tests/run/structural-contextual.scala

@@ -0,0 +1,28 @@
+trait Resolver:
+  def resolve(label: String): Any
+
+class ResolvingSelectable extends Selectable:
+  def selectDynamic(label: String)(using r: Resolver): Any =
+    r.resolve(label)
+  def applyDynamic(label: String)(args: Any*)(using r: Resolver): Any =
+    r.resolve(label).asInstanceOf[Any => Any].apply(args.head)
+
+type Person = ResolvingSelectable {
+  val name: String
+  val age: Int
+  def likes(other: String): Boolean
+}
+
+
+@main def Test =
+  given Resolver:
+    def resolve(label: String) = label match
+      case "name" => "Emma"
+      case "age" => 8
+      case "likes" => (food: String) => food == "Cake"
+
+  val emma = ResolvingSelectable().asInstanceOf[Person]
+
+  assert(emma.name == "Emma")
+  assert(emma.age == 8)
+  assert(emma.likes("Cake"))