Document TypeBox and move to library/src

Author: odersky

library/src-bootstrapped/scala/internal/TypeBox.scala

@@ -0,0 +1,11 @@
+package scala.internal
+
+/** A type for skolems that are generated during capture conversion. Capture conversion
+ *  narrows the type of a tree whose type has wildcard arguments. A typical situation
+ *  is a tree `t` of type `C[_ >: L <: U]` and an expected type `C[X]` where `X` is an
+ *  instantiatable type variable. To be able to instantiate `X`, we cast the tree to type
+ *  `X[$n.CAP]` where `$n` is a fresh skolem type with underlying type `TypeBox[L, U]`.
+ */
+final abstract class TypeBox[-L <: U, +U] {
+  type CAP >: L <: U
+}