Section on trait parameters

Author: odersky

docs/docs/reference/trait-parameters.md

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+# Trait Parameters
+
+Dotty allows traits to have parameters, just like classes have parameters.
+
+```scala
+trait Greeting(val name: String) {
+  def msg = s"How are you, $name"
+}
+
+class C extends Greeting("Bob") {
+  println(msg)
+}
+```
+
+Arguments to a trait are evaluated immediately before the trait is initialized.
+
+One potential issue with trait parameters is how to prevent
+ambiguities. For instance, you might try to extend `Greeting` twice,
+with different parameters.
+
+```scala
+/*!*/ class D extends C with Greeting("Bill") // error: parameters passed twice
+```
+
+Should this print "Bob" or "Bill"? In fact this program is illegal,
+because it violates one of the following rules for trait parameters:
+
+ 1. If a class `C` extends a parameterized trait `T`, and its superclass does not, `C` _must_ pass arguments to `T`.
+
+ 2. If a class `C` extends a parameterized trait `T`, and its superclass does as well, `C` _may not`  pass arguments to `T`.
+
+ 3. Traits may never pass arguments to parent traits.
+
+Here's a trait extending the parameterized trait `Greeting`.
+
+```scala
+trait FormalGreeting extends Greeting {
+  override def msg = s"How do you do, $name"
+}
+```
+As is required, no arguments are passed to `Greeting`. However, this poses an issue
+when defining a class that extends `FormalGreeting`:
+
+```scala
+/*!*/ class E extends FormalGreeting // error: missing arguments for `Greeting`.
+```
+
+The correct way to write `E` is to extend both `Greeting` and
+`FormalGreeting` (in either order):
+
+```scala
+class E extends Greeting("Bob") with FormalGreeting
+```
+
+

tests/pos/reference/trait-parameters.scala

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+trait Greeting(val name: String) {
+  def msg = s"How are you, $name"
+}
+
+trait FormalGreeting extends Greeting {
+  override def msg = s"How do you do, $name"
+}
+
+class C extends Greeting("Bob") {
+  println(msg)
+}
+
+class D extends C with Greeting
+
+class E extends Greeting("Bob") with FormalGreeting
+
+// class D2 extends C with Greeting("Bill") // error
+
+