Add section on named type arguments

Author: odersky

docs/docs/reference/named-typeargs.md

@@ -0,0 +1,25 @@
+---
+layout: doc-page
+title: "Named Type Arguments"
+---
+
+Type arguments of methods can now be named as well as by position. Example:
+
+
+    def construct[Elem, Coll[_]](xs: Elem*): Coll[Elem] = ???
+
+    val xs2 = construct[Coll = List, Elem = Int](1, 2, 3)
+    val xs3 = construct[Coll = List](1, 2, 3)
+
+Similar to a named value argument `(x = e)`, a named type argument
+`[X = T]` instantiates the type parameter `X` to the type `T`. Type
+arguments must be all named or un-named, mixtures of named and
+positional type arguments are not supported.
+
+The main benefit of named type arguments is that they allow some
+arguments to be omitted. Indeed, if type arguments are named, some
+arguments may be left out.  An example is the definition of `xs3`
+above. A missing type argument is inferred as usual by local type
+inference. The same is not true for positional arguments, which have
+to be given always for all type parameters.
+

docs/sidebar.yml

@@ -23,6 +23,8 @@ sidebar:
           url: docs/reference/implicit-by-name-parameters.htmldocs/reference/implicit-by-name-parameters.html
         - title: Auto Parameter Tupling
           url: docs/reference/auto-parameter-tupling.html
+        - title: Named Type Arguments
+          url: docs/reference/named-typeargs.html
     - title: Usage
       subsection:
         - title: sbt-projects

tests/pos/reference/named-typeargs.scala

@@ -0,0 +1,13 @@
+package namedTypeArgsR {
+
+object t1 {
+
+  def construct[Elem, Coll[_]](xs: Elem*): Coll[Elem] = ???
+
+  val xs3 = construct[Coll = List](1, 2, 3)
+
+  val xs2 = construct[Coll = List, Elem = Int](1, 2, 3)
+
+}
+
+}