SI-6663: don't ignore type parameter on selectDynamic invocation

Fix mkInvoke to handle selectDynamic calls of the form

  new C.foo[T].xyz

or

  new C.foo[T].xyz :U

(where C extends Dynamic)

Without this patch, the type parameter was silently ignored, and
possibly inferred to a different.

This patch fixes mkInvoke to handle these cases, where ctxTree has
the form Select(TypeApply(fun, targs), nme) or Typed(...)

Author: jannic

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -3935,9 +3935,14 @@ trait Typers extends Modes with Adaptations with Tags {
             case t: ValOrDefDef => t.rhs
             case t              => t
           }
-          val (outer, explicitTargs) = cxTree1 match {
+          val cxTree2 = cxTree1 match {
+            case Typed(t, tpe)  => t // ignore outer type annotation
+            case t              => t
+          }
+          val (outer, explicitTargs) = cxTree2 match {
             case TypeApply(fun, targs)              => (fun, targs)
             case Apply(TypeApply(fun, targs), args) => (Apply(fun, args), targs)
+            case Select(TypeApply(fun, targs), nme) => (Select(fun, nme), targs)
             case t                                  => (t, Nil)
           }
           def hasNamedArg(as: List[Tree]) = as.collectFirst{case AssignOrNamedArg(lhs, rhs) =>}.nonEmpty

test/files/neg/t6663.check

@@ -0,0 +1,6 @@
+t6663.scala:16: error: type mismatch;
+ found   : String
+ required: Int
+  var v = new C(42).foo[String].get :Int
+                                ^
+one error found

test/files/neg/t6663.scala

@@ -0,0 +1,19 @@
+import language.dynamics
+
+class C(v: Any) extends Dynamic {
+  def selectDynamic[T](n: String): Option[T] = Option(v.asInstanceOf[T])
+  def applyDynamic[T](n: String)(): Option[T] = Option(v.asInstanceOf[T])
+}
+
+object Test extends App {
+  // this should be converted to
+  // C(42).selectDynamic[String]("foo").get
+  // causing a compile error.
+
+  // but, before fixing SI-6663, became
+  // C(42).selectDynamic("foo").get, ignoring
+  // the [String] type parameter
+  var v = new C(42).foo[String].get :Int
+  println(v)
+}
+

test/files/run/t6663.check

@@ -0,0 +1 @@
+42

test/files/run/t6663.scala

@@ -0,0 +1,17 @@
+import language.dynamics
+
+class C(v: Any) extends Dynamic {
+  def selectDynamic[T](n: String): Option[T] = Option(v.asInstanceOf[T])
+  def applyDynamic[T](n: String)(): Option[T] = Option(v.asInstanceOf[T])
+}
+
+object Test extends App {
+  // this should be converted to
+  // C(42).selectDynamic[Int]("foo").get
+  // but, before fixing SI-6663, became
+  // C(42).selectDynamic[Nothing]("foo").get
+  // leading to a ClassCastException
+  var v = new C(42).foo[Int].get
+  println(v)
+}
+