SI-6355 SI-7059 it is possible to overload applyDynamic

As our discussion at https://issues.scala-lang.org/browse/SI-6355 shows,
it looks like it is possible to overload applyDynamic, even though a
straightforward way is closed. This commit codifies the pattern proposed
by @paulp and makes sure that it doesn’t break in the future.

test/files/neg/t6355a.check

@@ -0,0 +1,7 @@
+t6355a.scala:12: error: implementation restriction: applyDynamic cannot be overloaded except by methods with different numbers of type parameters, e.g. applyDynamic[T1](method: String)(arg: T1) and applyDynamic[T1, T2](method: String)(arg1: T1, arg2: T2)
+  def applyDynamic(name: String)(x: Int): Int = 2
+      ^
+t6355a.scala:18: error: implementation restriction: applyDynamic cannot be overloaded except by methods with different numbers of type parameters, e.g. applyDynamic[T1](method: String)(arg: T1) and applyDynamic[T1, T2](method: String)(arg1: T1, arg2: T2)
+  def applyDynamic[T1, T2](name: String)(x: String, y: T1, z: T2): Int = 3
+      ^
+two errors found

test/files/neg/t6355b.check

@@ -0,0 +1,11 @@
+t6355b.scala:14: error: value applyDynamic is not a member of A
+error after rewriting to x.<applyDynamic: error>("bippy")
+possible cause: maybe a wrong Dynamic method signature?
+    println(x.bippy(42))
+            ^
+t6355b.scala:15: error: value applyDynamic is not a member of A
+error after rewriting to x.<applyDynamic: error>("bippy")
+possible cause: maybe a wrong Dynamic method signature?
+    println(x.bippy("42"))
+            ^
+two errors found

test/files/neg/t6355b.scala

@@ -0,0 +1,17 @@
+import scala.language.dynamics
+
+class A extends Dynamic {
+  def selectDynamic(method: String): B = new B(method)
+}
+class B(method: String) {
+  def apply(x: Int) = s"$method(x: Int) called with x = $x"
+  def apply(x: String) = s"""$method(x: String) called with x = "$x""""
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val x = new A
+    println(x.bippy(42))
+    println(x.bippy("42"))
+  }
+}

test/files/run/t6355.check

@@ -0,0 +1,2 @@
+bippy(x: Int) called with x = 42
+bippy(x: String) called with x = "42"

test/files/run/t6355.scala

@@ -0,0 +1,17 @@
+import scala.language.dynamics
+
+class A extends Dynamic {
+  def applyDynamic(method: String): B = new B(method)
+}
+class B(method: String) {
+  def apply(x: Int) = s"$method(x: Int) called with x = $x"
+  def apply(x: String) = s"""$method(x: String) called with x = "$x""""
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val x = new A
+    println(x.bippy(42))
+    println(x.bippy("42"))
+  }
+}